Marine Navigation Archives - Marine Insight

What is Stowage Factor?

Subhodeep Ghosh — Mon, 19 Feb 2024 10:12:31 +0000

The freight valuation of any cargo vessel directly depends on how much cargo it can carry. Be it a containership, a bulker, or a tanker, all ship owners and operators aim to maximise the cargo occupancy in the ship holds, optimising other factors such as flotation, stability, design strength, service guidelines, and so on. The greater the occupancy of a vessel’s cargo spaces, the higher the profits from a voyage between two successive ports of call.

However, for a given space designated for cargo, very seldom is a space fully occupied. In other words, for instance, in a perfectly cuboidal cargo tank in some vessel, 100% of it cannot be ideally filled. Why?

This depends on two factors, the first one being the most important.

Type of cargo
The design and disposition of the cargo space

Dealing with the second factor first, the arrangement of the cargo space is crucial. For example, a hexagonal or diamond-shaped tank can accommodate a lesser volume of a certain type of cargo than a perfectly cuboidal tank but is effective for another type of cargo.

Moreover, other factors also come into play, like the disposition of stiffening members, piping, electrical connections, equipment, ducting and ventilation, etc., that interfere with the available space for cargo.

Furthermore, for a given vessel designed to transport a certain type of cargo, the available cargo space, though maximised during loading, is not filled to the brim owing to several reasons like leaving some clearance at the top to cater for ventilation and prevent build-up of high air pressure in fully confined volumes, reduce flammability risks and overflow for liquid cargo carriers like tankers, and other operational reasons.

However, the most important factor that dictates the disposition of cargo in a space or hold is the nature of the cargo itself. This is measured in terms of the stowage factor, which is a very important term in the marine field.

The stowage factor is an expression that determines how much usable space one tonne of cargo, by weight, can occupy. Note the term usable space again. This considers the designated cargo spaces or holds in a vessel only, the aggregation of which relates to the common term, net tonnage, and the maximum freight or cargo weight a ship can carry. The stowage factor is measured in cubic metres/ton or cubic feet/tonnes.

Hence, if we say that a specific type of cargo has a stowage factor of X metre cubes per tonne, 1 tonne occupies X metre cubes of volume, assuming ideal conditions.

Now, these ideal conditions mean a variety of things like surface roughness, exclusion of damages and defects, more or less expected weather conditions, the compartment or hold free of any other content, and so on. However, like all other real-world scenarios, none of these are always 100% idealistic, and the stowage factors designated for various types of cargo are simply the nearest approximation based on first principles and ignoring any minimum deviance and errors.

A higher stowage factor means that the particular cargo content requires more space to be stowed as compared to the same weight of another cargo with a lower stowage factor.

Now, contrary to the cargo X above, if we have another cargo with a stowage factor of Y lesser than X, this essentially means that Y can occupy lesser space for a given unit weight of cargo and, hence, can be filled in greater amounts as compared to X in the same cargo space, assuming that the deadweight fraction contributed by Y if the hold is completely occupied does not affect the design limits of the vessel in terms of weight.

Hence, for all practical purposes, a lower stowage factor always equates to greater utilisation of cargo space (and hence net tonnage). This translates to maximising profits due to the greater occupancy of cargo spaces in a single voyage. Similarly, a higher SF cargo can be stowed in lower amounts in a single voyage.

We often have to deal with freight containing high SF cargo content, which is an important contributing factor in terms of market pricing. The cost of imported wood is much higher than coal or iron ore.

The stowage factor depends on primarily two factors:

Density
Nature of the material

As we can understand, density is directly related to the specific gravity and, hence, the settlement of a particular material in a space. Heavier materials, with higher densities, tend to settle more, and when heaped, like in cargo tanks or holds in ships, have their average centre of gravity lower as compared to lighter ones.

For example, iron ore has a much higher density than coal or sugar and hence has a lower stowage factor, therefore being able to occupy less volume for the same weight when heaped in the same space. Likewise, they can be stowed in greater amounts in the hold or compartment than coal or sugar, which fills the same space but has a much lower weight.

Thus, the same vessel can carry more iron ore than coal or sugar, considering the design limits of the vessel permit it. The nature of the material content also plays an important role.

As obvious, granular or finer materials tend to occupy lesser volume and, hence, have a lower stowage factor. Coarser materials are the opposite, with much of their occupied volumes considering gaps and free space. Imagine the simple example of filling stone chips and sand in the same jar.

When you fill the jar in both cases and weigh them individually, you will find that the latter weighs more than the former, even though the stone has a higher unit density. Similarly, this is the concept for the stowage factor. When you fill 10 tonnes, for example, of iron ore and large stone chips in the same vessel, you shall find that in the case of the former, there is still a considerable amount of space after emptying the contents as compared to the latter, despite their densities being not much dissimilar.

Thus, the interplay of both these factors determines the stowage factor. Because of their properties, liquid cargo, like petroleum products, has a much lower stowage factor than bulk cargo.

Some common stowage factor values are 0.4-0.5 cu. Metres per tonne for iron ore, 1.2-1.4 cu. Metres for coal, 0.3-0.6 cu. Metres for rolled steel, 2.5 cu. Metres for wood, etc.

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Disclaimer: The author’s views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used in the article, have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendations on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight.

What is Stowage Factor? appeared first on Marine Insight - The Maritime Industry Guide

What is an Emergency Wreck Marking Buoy?

Subhodeep Ghosh — Thu, 05 Oct 2023 04:53:42 +0000

In our previous articles, we learned about safe water marks and cardinal marks used at sea for a litany of purposes. We saw how each mark is designed to identify itself as a unique symbol suited for a specific indication, mostly a hazard or risk.

In this article, we shall discuss one specific type of sea mark known as the emergency wreck marking buoy.

What is an emergency wreck marking buoy?

As the name suggests, an emergency wreck marking buoy is used to identify and mark wreckages at sea, at least temporarily.

Though, for all practical purposes, we mean shipwrecks or flotsams while speaking of wreckages at sea, these kinds of marks are also used to mark other critical points of interest like damaged civil or offshore structures at sea, abnormally discovered natural formations like new sandbanks or reefs, some obstruction created due to a ship accident like fallen containers or stone chips, large debris, or even wrecks of other bodies like aircraft or submarines.

The primary objective of these marks is to immediately cordon off affected areas at sea from moving traffic and reduce the chances of a further collision, especially during the night or low visibility.

The need for these emergency wreck marking buoys to be exclusively used during such occasions gained prominence in the wake of the 2002 accident of the car carrier vessel Tricolour whose wreck further collided with three other passing vessels successively within a few days.

The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), the regulatory body responsible for proposing guidelines for navigational aids and signals, laid out the application for such special marks that became effective in 2005.

As per the current requirements, it is now mandatory to dispose of these markings in the way of any newly discovered wreckages and should be kept in place until:

The wreck is now well-identified and properly circulated to navigators and seafarers using the route.
In the case of a submerged wreck, the wreck has been comprehensively inspected, and all its details have been made well aware.
The wreck has been permanently cordoned off by some means, and some kind of permanent marking has been provided, especially in natural formations or wrecks that have been fully or partially difficult to remove.
The wreck has been salvaged.

For all practical purposes, these emergency wreck-marking buoys stay in place for not more than 3 to 4 days. They are often replaced by other kinds of marks, like cardinal marks, in case they are not salvaged. Henceforth, it can be said that emergency wreck marking buoys are temporary means to mark an affected area like a wreck.

As per IALA guidelines, the design, construction and disposition of emergency wreck marking buoys are also unique like other kinds of sea marks. They are essentially pillars or spar buoys that remain afloat in water. Their colour coding is mainly characteristic of alternate yellow and blue stripes.

The number of such stripes depends on the size. The size of these marks varies based on the kind of wreckage but needs to be above a certain minimum requirement. Mostly they are conical at the top and have a flat circular base at the bottom (spar buoys) or are slender (pillar buoys) but can be of other forms.

Usually, at the topmost tip or apex, they have a cross mark. Sometimes, the word “WRECK” is also imprinted on it for convenience.

During the dark, they have a unique lighting system. They have a flashlight/beacon fitted on them that emits blue and yellow light flashes at regular intervals. Blue and yellow light flashes for a short duration of 1 second, and the interval between two successive flashlights is usually around 0.5 seconds.

The disposition and number of these buoy marks depend on the type of wreck and expanse. For wrecks or debris spread over a large area, more such marks are used and arranged to aid navigators in the best visual manner possible.

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Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used in the article, have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendations on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight.

What is an Emergency Wreck Marking Buoy? appeared first on Marine Insight - The Maritime Industry Guide

What are Isogonic Lines?

Subhodeep Ghosh — Mon, 25 Sep 2023 05:58:45 +0000

One of the most crucial cornerstones of the maritime sector is the realm of navigation. Irrespective of the vessel’s type, size, service, and so on, any marine traffic is rendered incomplete without the very important aspect of navigation and directionality.

In endless swathes of open seas or oceans, a very strong foundation of direction and real-time location of the vessel is indispensable to a seafarer.

In the aviation sector, it is equally important for a pilot to control the flight in the vast skies. Unlike roads, you do not have the advantage of looking at all the familiar landmarks and roads, or in today’s times, you solely rely on mobile application-based apps on your smartphones!

As ships are much older than planes, directionality in the earlier days was challenging. The very first mariners and sailors relied on the position of celestial bodies like the sun, moon, and stars to navigate their way through the vast and hostile seas in primitive wooden vessels.

However, soon after, the incredible advent of compasses came into the picture around the 11th and 12th centuries in both Eastern world and Western world countries; and this changed the maritime industry forever.

We are all familiar with magnetism and compasses since our school days. The compass utilises the Earth’s basic magnetism theory, where the Earth itself has an inherent magnetic field.

Currently, the earth’s magnetic field lines are from the southern hemisphere to the northern hemisphere. So, whenever you hold a compass, the end of the needle marked magnetic north point essentially in a direction opposite to the earth’s actual magnetic field, from south to north.

Thus, we can also say that the compass’s magnetic needle (north magnetic pole) points more or less towards the geographic north of the earth, which is nothing but the extreme tip of the North Pole, and the other end towards the geographic south (South Pole). Now, carefully note the term more or less.

The earth’s geographic north-south orientation is aligned with the geocentric meridian, and the centre of rotation of the earth’s axis lies approximately 10 degrees away or askew or offset from the north-south alignment of the earth’s magnetic field. This is because of the geomagnetism dynamics, leading to this small shift every thousand years or so.

According to scientists and experts, from a time of about over a thousand years ago, roughly the same time the early compasses became handy of mankind, to the present day, the earth’s magnetic field orientation HAS indeed undergone a small change or diversion more away from the geographic north-south orientation. However, once again, this change is not very dramatic. As of the current day, the magnetic north pole lies somewhere in the Canadian Arctic region.

Please note that the change in magnetic north-south orientation is not to be confused with a change in magnetic polarity or magnetic reversal. This is where the entire magnetic field changes its polarity; that is when the current direction of the magnetic field will become north to south from north to south.

According to scientific data, this happens at an average time of at least a few lakh years. So, even if on a pessimistic note, when we consider the next magnetic reversal to happen, humans, in all probabilities, may not see it!

Magnetic declination

Now, when we hold a magnetic compass at any random place on the earth’s surface, does the magnetic north pointer of the needle show the same deflection as someplace else?

The answer is a no, even if we strangely expect it to be yes.

This difference and angle between the magnetic north and the geographic north are known as the magnetic declination.

The reason behind this effect can be explained in the following points and descriptions:

The direction of the magnetic north is aligned with the horizontal component of the field intensity. So, for level places like the sea surface, the angle is less as the intensity vector of the magnetic field is more planar. However, for very high altitudes like high mountain angles and so on, the value of this declination is different. Hence, the altitude and topography of the place is crucial.
The secular variation of the magnetic field. This is a time-variant change in the magnetic field over a given area. This depends from location to location. In some places, the intensity is more, and in some, the intensity of this change is less. So, the variation of the magnetic field over London in the last few centuries may be different from that of Mumbai.
The crucial point in the introductory section about the difference in magnetic and geographic orientation also contributes to this declination.
Spatial variation in earth’s magnetic field and rapid dynamics in places due to changes in geology and underground mineral distributions.
The complicated and erratic nature of the earth’s magnetic field is highly unpredictable and changing, encompassing various complex geological and physical phenomena beyond the scope of discussion.

However, multiple places or locations may have the same degree of magnetic declination. The locus or the imaginary line joining these locations is known as an isogonic line. See the figure below.

They have a lot of importance in the marine field regarding navigation. Mariners mostly use isogonic charts that have a collection of several isogonic lines.

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Disclaimer: The author’s views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or, recommendations on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight.

What are Isogonic Lines? appeared first on Marine Insight - The Maritime Industry Guide

What is Fendering?

Subhodeep Ghosh — Fri, 18 Aug 2023 08:47:21 +0000

Fenders are devices or entities used as guards for a vessel against impact with a jetty, dock, quay, berth, or any other ship close to a shore, preventing resultant structural damage.

Fenders act as cushions or absorbers that reduce the impact of mutual contact by absorbing kinetic energy through elastic action.

All vessels need to berth at their designated points. During this process, the ship must position itself at a definite location to serve its purpose based on the availability of space and other factors in the jetty, port, dock, pier, quay, etc.

Hence, there is a high chance of direct collision or impact of the vessel with the structure, even though the ship’s speed may be very low.

Later after the vessel is stationed suitably, the risk of contact is still highly prevalent. Though the vessel’s speed is zero after docking or berthing, the nature of the water is highly dynamic.

Thus, even a floating stationary vessel comes in contact with the shore structure now and then by the continuous inherent motions due to water currents, tides, and waves.

Moreover, at a jetty, pier, dock, berth, or quay, there are multiple vessels catering to their operations. Thus, there is also a fair chance of direct contact between the individual ships.

Furthermore, when a vessel is near a port or harbour with great traffic, the risks associated with a collision with another vessel are also high.

Also, two vessels can be berthed to each other for various utilities like bunkering or cargo transfer in what is known as Ship-to-Ship berthing (STB).

This also creates a similar situation, where two vessels are likely to collide continuously. Therefore, the question arises: Can there be any means of protection similar to bumper and mudguards in vehicles or buffers in rail cars? The simple answer is fenders.

Remember those tyre-like things on the sides of the hull and the jetty platform when you first went for your pleasure boat ride?

Those are nothing but simple forms of fendering again. From small speedboats to large cargo carriers, all vessels must have a fendering system to protect their hull structure and any shore structure or other ships from localised collisions or impacts.

However, due to the momentum of the vessel (that further depends on its size) and often continuity, the effect of these kinds of impacts may be significant enough to damage the land structure or cause high localised stress concentrations that can be enough to be called structural damage and even a localised failure for the vessel itself or any other vessel.

Factors for deciding the selection of Fenders

A type of fender suitable for one purpose may render completely inefficient for another. Hence, based on the type of vessel and the purpose.

Thus, the selection and disposal of fenders for a particular location and operation are based upon many factors.

The vessels under consideration: This is the most important factor considered. A fendering arrangement that caters to a fisherman’s wharf is completely ineffective against large cargo ships in a port. Moreover, the vessel’s type, size, and design are also crucial. For example, arch fenders are appropriate for small and medium-sized vessels.

Again, for instance, bulk carriers and general cargo ships need to be berthed right next to the jetty or quay with minimal clearance to facilitate efficient cargo transfer by maximum outreach of cranage. Similarly, passenger vessels require the same to allow safe and convenient boarding and de-boarding of passengers.

Thus, large flat fenders of suitable size and shape that are rigid and require minimum clearance gap are mostly chosen.

Modern systems also incorporate advanced types like parallel motion, sliding, or retractable extrude types that not only are capable of absorbing high values of vessel momentum over prolonged periods but also can adjust themselves accordingly based on the applied forces such that the margin of clearance between the vessel and the quay is minimal. Berthing energy is directly related to the vessel type.

Structure and Environment: The shore structure and environment are also crucial. The conditions in which the jetty, quay, pier, etc., are subjected, like the tidal levels, wave factors, currents, and so on, are determinants of the risks of collision and the forces during the interaction.

Likewise, the type and configuration of the structure are taken into account. For instance, open pile jetties that are common for deepwater operations, load-sensitive, and have limited face area for fendering require large and highly efficient fenders that can, in lesser numbers, not only cater to high loads from large vessels but also be able to work under variable external conditions like high tides or large wave loads.

The berthing configuration and approach: Vessels can be berthed in different configurations at shore based on their requirements and design. Side berthing is the most common mode, followed by end berthing (bow or stern). There are other rare types, like a dolphin or lock berthing. Hence, when a vessel must be berthed by bow or aft, the berthing arrangement on both the ship and structure differs from when the berthing procedure is sideways.

This factor is again closely related to our first point, vessel type. For instance, when a vessel has a bulbous bow and must be berthed by the forward end, the fendering differs from a vessel without a bulbous bow. Other important factors include the approach velocity (again related to the second point of environmental conditions; rough seas increase velocities), approach angle, etc.

Types, Design, and Arrangement of Fenders

Fenders are usually disposed in a single line at more or less regular intervals at regions with the most likely influence of interactions.

On a quay wall, pier, or jetty, they are distributed at the outermost edge where there is the likelihood of contact with the vessel hull. On ships or boats, the fenders are disposed on the side shell in areas close to the waterline and the deck edge.

We know from our knowledge of conventional hull forms that when the vessel is being berthed, the land structure is most likely to strike the lower side shell part of the hull, as shown near the bilge region.

Similarly, for a ship-to-ship interaction, the most likely part of being involved is the upper part of the side shell near the deck edge, as shown.

Thus, the fenders are accordingly disposed of as required. However, for all practical purposes, fendering is optional in large seagoing ships in voyages as there is no chance of a low-scale impact in the deep sea, and fendering unnecessarily interferes with weight, stability and speed.

Types Of Fenders

Fenders can differ widely in size, shape, type, and design.

Based on shapes, some common types of fenders are:

Cylindrical fenders
Spherical
Square fenders
Corner fenders
Circular fenders
Cone fenders
Doughnut fenders
Arch fenders
D-fenders

Cylindrical fenders

Corner fenders

Depending on their mobility, there can be:

Fixed fenders
Floating fenders

As the name suggests, fixed fenders are fixed to a structure like a vessel or a land platform. Floating fenders are suspended on the water and allowed to float while acting as a buffer between two bodies, like vessels or vessels and a fixed structure.

Based on the construction and design, fenders can again be broadly categorised as:

Flat fenders
Pneumatic fenders
Foam fenders

Flat fenders are only fitted in land structures like jetties, piers, or quays. They are mostly of rubber and have a high index of rigidity. They do not compress much and thus are suitable to sustain low momentum impacts. They are mostly circular, doughnut, square or D shape.

As they are mostly land-based, they are also mostly fixed fenders. Pneumatic fenders are mostly used between vessels but are often used on land when a bigger ship is involved.

These are larger and have compressed air filled in them. Thus, they can absorb a high energy value without deflecting much and have higher flexibility, something apt for situations like berthing between two floating vessels having significant degrees of freedom or for berthing large ships with a sizeable oncoming momentum involved. They are primarily cylindrical or spherical.

They are mainly under the category of floating fenders, as they can float because of their pneumatic nature. Foam fenders are similar to pneumatic fenders and can float because of their construction. They have an inner foam core and an outer shell of synthetic polymers or elastomers. Another added advantage of foam polymers is that they cannot deflate when punctured.

Nowadays, mechanised fenders that are adjustable and retractable based on loading pressures are also common. Leg-type, extruded, sliding and parallel motion fenders are some important ones.

Design factors

Berthing energy: This is the most crucial parameter for fendering for designing fenders. Berthing energy is the kinetic energy of the impact load when it is transferred from a vessel to a berth or between two successive vessels.

Now, as we know, the kinetic energy of a body is measured as ½ X mass X velocity square (1/2 X m X v2 ). When a vessel is imposed on a fixed structure like a berth, this mass is simply the vessel’s displacement. However, when two vessels interact, the effective mass is given by M1 X M2/ (M1+M2), where M1 and M2 are the masses of the two bodies.

Fender spacing: The interval between 2 successive fenders. This depends on the type of vessels, the environment, and the berthing types.

Fender Contact: This is the force shared by each fender. This is again related to the berthing configuration and the vessel type. As we discussed above, for instance, when a vessel is side berthed, the forces on the fenders Are more or less uniform compared to when it is at an angle dolphin berthing or berthing between two ships when the entire loading cycle is highly dynamic.

Materials

As discussed above, the material for fenders depends on their type. For example, the inner core is foam in foam fenders, and the outer core is elastomers. Flat fenders typically use polyethene, rubber, and sometimes steel additives to improve strength and rigidity.

Pneumatic ones use plain rubber (like tyres) and monomers. PVC is also a very commonly used material. Mechanised fendering uses mostly steel with rubber paddings at the points of contact. The material is selected based on strength and operational requirements.

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What is Fendering? appeared first on Marine Insight - The Maritime Industry Guide

What Are Cardinal Marks?

Subhodeep Ghosh — Mon, 10 Jul 2023 05:48:43 +0000

In the previous article, we learnt about seamarks and their importance. Sea marks are a system of conventions and reference markings used in the sea to guide a navigator or seafarer in specific areas requiring special attention. They are analogous to road signs we see on roads.

We briefly touched upon the safe water marks and how they are significant in indicating safe and navigable waters by their characteristic system of markings. This article will briefly overview another kind of seamark known as cardinal marks.

Cardinal marks are a crucial system of markings similar to safe watermarks but are essentially direction-based. Their name is derived from the mathematical concept of cardinality, or in simple words, grouping.

Hence, the cardinal marks are grouped or instead organised into four distinct categories of markings, often represented as quadrants based on directionality as per the compass rule:

North Cardinal
East Cardinal
South Cardinal
West Cardinal

The use of these cardinal marks is mainly for:

Guiding a vessel through safe and deepest waters
Indication of safe waters in case of a hazard, danger, or obstruction
Indicate the presence of some interfering landmass, sharp turns, bends, junctions, and so on.

The cardinals indicate the safe, navigable waterways relative to their absolute location in terms of geographical direction, as seen by a seafarer when approaching an area of attention from any given direction.

Each of the four types of cardinal marks mentioned above is distinct or unique and thus aids in identifying the exact location of avoidance and, therefore, the navigable water to ply through.

Let me start with an example. There is a shipwreck or an accident site at a particular place. Due to fog and reduced visibility, a ship approaching the site from a specific direction does not know the location.

However, the navigator or captain from the helm manages to spot a cardinal mark pertaining to a definite direction, for instance, the west. This means that the hazard mainly lies eastwards, and the vessel needs to make a hard turn towards the west of the mark to ply through safely.

Assuming the vessel’s compass or navigational mechanism is fully functional, the navigator turns the rudder towards the west so that the vessel’s heading is aligned with the west compass direction and the ship can pass safely.

Similarly, a trawler in a river is unaware that a sharp and precarious turn is lying ahead. However, an east cardinal mark can be seen. This means that the waters east of the mark are safe, and the vessel needs to turn eastwards to avoid collision with the land.

This example makes it even clearer. As per historical accounts and reports and depictions in books and movies, the fated Titanic encountered the infamous iceberg from her starboard side or right of her heading direction.

For simplicity, assuming a linear voyage path in a westward heading as per its course from Britain to New York, the iceberg was situated northward as per geographical compass coordinates.

So, in an imaginary situation, if hypothetically someone had placed a south cardinal at some miles distance before the iceberg, the vessel could have made a southward or a portside turn away from the berg in prior and saved itself!

For all practical purposes, in localised sites like the shipwreck example above, cardinal marks are placed in all directions around the area of attention. So, the vessel mentioned in the above example can also take a hard turn eastwards when it spots an eastern cardinal to avoid obstruction.

After that, the vessel is likely to spot a north or south cardinal depending on its direction of approach, and this means that the waters north or south of the accident site are free of any risks, and the vessel can move ahead unhindered. The following figure depicts a generic representation of cardinal marks. The north side, or the north cardinal, is aligned with the geographic north.

Like safe water marks, the cardinal marks are also under the guidelines and regulations of the International Associations of Lighthouse Authorities or IALA.

For all practical purposes, cardinal marks are average-sized floating-type marks like buoys. They are mostly pillar-like, spar-like or tapering in shape. They are 2-3 metres in diameter but may weigh up to 5 or 6 tonnes (a large SUV or mini-truck). They are moored or tethered to the seafloor by cables or tethers.

The four cardinal marking systems are distinct in colour coding and lighting arrangement. As we already know, the colour coding system plays the marking role during the day and the lighting during the dark.

Above the main structure, there is another pointed mark known as the top mark, defined in terms of small black arrows (similar to a weatherman’s cock) that are oriented in 4 different ways depending on the direction cardinality. The light or beam is usually towards the top-end of the structure close to the top marks.

Let us now see how these four marks are different from one another.

North Cardinal

As we know, they represent the safe, navigable waterway relative north of its position. Here, essentially, the lower and wider base of the structure is yellow, and the narrower top part is black. The top marks are black, and both successively point upward towards the sky. Refer above diagram. At night, the beam continuously emits flashes at very short intervals, like a car indicator lamp.

East Cardinal

They indicate safe waters east of its location. Here the colour coding is slightly more complicated. The broader base or lower part of the structure is black, the lower part of the upper or narrower structure is yellow, and the upper part of the upper structure is black again.

The colour coding can be remembered as B-Y-B for black-yellow-black. The top arrows are fully black and point opposite to one another such that their apexes are away from each other, similar to two interposed cones or a diamond shape.

The beam emits three short flashes at night every 10 or 15 seconds. When they flash every 10 seconds, they are known as Very Quick Flashes; when they flash every 15 seconds, they are quick flashes.

South Cardinal

They indicate safe waters south of its position. The lower part is black, and the upper is yellow, the reverse of the north cardinal, when going from bottom to top. The top marks are precisely opposite to that of the north cardinal, with both pointing downwards.

The light patterns are complicated here. Six short flashes are followed by a long flash every 10 to 15 seconds (6+1). Like above, the time interval decides whether they are quick or very quick types.

West Cardinal

Lastly, the west cardinal marks indicate that the water westwards is safer. The broader base or lower part of the structure is yellow, the lower part of the upper or narrower structure is black, and the upper part of the upper structure is yellow again. The colour coding can be remembered as Y-B-Y for yellow-black-yellow. The top mark arrows again point in the opposite ways, but this time, towards one another, like a wineglass or a sand clock.

The lighting arrangement flashes nine times during the dark every 10 or 15 seconds.

While using reference, the marks are easily detected and identified during the day or with clear visibility; at night, it becomes even more critical, especially for south and west marks, as they are similar and can be easily confused. Thus, careful observation, preferably multiple times, is crucial.

The distinct lighting pattern can also be easily identified and remembered in the following way. Suppose you are standing on the road. A person to your east is basically at your 3 o’clock position, or the hands of the clock during 3 pm or am.

So, that’s the number of times the light flashes for an east cardinal! Similarly, a person right behind you is at your 6 o’clock position, and the one at your west is at 9 o’clock. So, likewise, at the number of flashes for the respective cardinals.

Similarly, other than the standard colour coding, the cardinal marks are characteristic of retroreflectors that enhance appearance by reflecting off incident light on the marks.

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What are Sea Water Marks And Safe Water Marks?

Subhodeep Ghosh — Mon, 19 Jun 2023 07:09:25 +0000

At the beginning of this article, it is important to first learn about the term ‘sea marks’.

So what are sea marks?

Sea marks are physical indicators for navigational aid and reference. These indicators or references can be used for a wide range of purposes, as follows:

Marking passages, channels, canals, or other water bodies of specific interest or reason. Examples include enemy or restricted territories, defence or administrative or governmental areas.
Marking coasts, shorelines, islands, rocky formations, or some form of obstructions, both natural and artificial.
Demarcation and identification of hazardous areas or water-based regions associated with danger or potential risks. For example, shoals, reefs, barriers, or other geographical or topological features. Other examples may include artificial causes like enemy regions planted with underwater mines, shipwrecks, or submerged structures.
Indicating areas of high maritime traffic and congestion, like near large ports or terminals or artificial channels, for the passage of many vessels regularly.
Marking water bodies based on high or low drafts, tidal conditions, icing, or land restrictions.
Identification of safe, risk-free, and navigable areas.

Sea marks can be best compared to road signs that we encounter now and then. They are of paramount importance for sailors and navigators. There can be many kinds of sea marks. However, we can broadly categorise them into the following:

Fixed
Floating

Fixed structures are those that are fixed on the ground. They may either be fixed onto the seabed or to the landmass if it is on a shore or anywhere in the vicinity of a water body. The lighthouses, about which we had been enthused since childhood, are itself nothing but a classic sea mark!

Its primary purpose is to convey the presence of a landmass or coast to a seafarer and, conversely, keep track of the oncoming maritime traffic.

For those attached to the seafloor, they may range from a small, tubular, single-leg structure attached to the seafloor that is used to mark a specific point or location to larger structures similar to offshore oil structures that may include an intricate combination of lighting, as well as radio or sonic signals for some specific purpose like defence or to mark and safeguard a major coastal facility.

Floating structures are usually buoys that remain afloat in a specific location and are typically tethered or anchored such that it does not drift away. These smaller structures serve a similar purpose as being discussed.

They are often used in multiple numbers for purposes like forming a guided route for a vessel near a coast or port, demarcating an unnavigable zone, or acting as separators for safe navigation in busy areas of high maritime traffic.

Now, other than these, there can be seamarks based on the following features or combinations of them:

Having lighting and illumination
Having acoustic features
Having specialised features like radio signals
Having no significant features

The first type is the most common. These can be detected because of their illuminative features. They are usually equipped with lighting features like beacons, floodlights, foglamps, flashers, etc.

However, they may also be passively illuminating, like having reflectors. In the second category, there is an audio or an acoustic feature like horns. They are meant to be detected based on their sound signals instead of visual presence.

A lighthouse or other fixed structures like towers are usually equipped with audio and visual detection features in the form of large light beacons and foghorns.

Modern sea markers, including some latest constructions of lighthouses, are often equipped with advanced technologies like radio transmission and satellite or telecommunication capabilities.

Some seamarks are devoid of any features. They include buoys or structures only meant to be visually detected during the daytime. They are known as daymarks.

They cannot be detected during night-time. The International Association of Lighthouse Authorities, or the IALA, is the main governing body that gives regulations and guidelines about the design, disposal, usage, and operation of these sea marks. The application of these regulations can be broadly classified into two regional categories:

Region A: For every other region in the world other than North and South America, Japan, Korea, and the Philippines.
Region B: North and South America, Japan, Korea, and the Philippines.

Safe Water Marks

Now, seamarks can be of various types and kinds. They can be of varying designs, looks, colours, shapes, forms, and configurations coded for their respective purpose as designated by IALA existing guidelines.

Safe water marks are one such kind. They are seamarks used to indicate navigable waters in and about their place of disposition. These safe watermarks are used to identify the following: fairway, midchannel, end of channel, and landfall.

Fairways are usually used to denote the entrance or approach of a navigable path and often for channels, ports, or estuaries. Midchannel indicates a middle region of a safe navigable channel, an unhindered waterway thoroughfare for the given route, in simple terms.

Similarly, safe watermarks are also used to mark the end of a navigable waterway or channel. Landfall marks basically indicate the presence of a landmass in the approaching vicinity from the point of view of a seafarer or navigator.

Safe water marks are usually colour-coded in red and white. For all practical purposes, they are floating marks like buoys, but fixed structures are also there. They are either spherical (for buoys), spar/ tapering pillar-like (floating or fixed), or slender tubular structures (mainly fixed ones). Spherical shape-like marks are usually used in midchannel indicators.

Safe water marks are also notably distinguished by the presence of a red ball-like sphere at the top apex, similar to one on Christmas trees or Santa’s red cap.

The lighting pattern for these types of marks is also unique. At night, safe water marks emit long flashes at every 10 seconds.

Safe water marks are also often used in underpass waterways beneath bridges as a mark to indicate the safe passage of vessels with a certain maximum height as deemed for that particular waterway.

In the next article, we are going to have a brief look into cardinal marks.

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Effects Of Wind On Ship Handling

Subhodeep Ghosh — Fri, 16 Jun 2023 06:56:15 +0000

When we speak about ships, the most prominent force that comes to our mind is that offered by the surrounding water body. Delving deeper into the effects of water, the forces which act on a floating vessel can be further subdivided into various types like hydrostatic, hydrodynamic, and those kinds of loading stemming from wave action, also known as wave loads.

However, it is worth saying that the wind also significantly affects a floating vessel, though that is far, far lesser than those from water.

When a ship floats in the open, unbridled seas, there is the action of wind from every direction acting on the vessel.

The intensity of the wind depends on the local climatic conditions and may vary from a calm breeze to fiery gale storms. So, when a ship is designed, other than the principally acting hydrodynamic loads, the effects of wind are also considered.

The collective time-variant effects of wind action on a vessel bring about what we know as the wind resistance of the vessel.

Before we delve further into the detailed effects of wind action, it is important to understand how the wind acts on a floating vessel. So, the effect of wind resistance on any vessel depends on three major factors:

The nature and intensity of the wind, as mentioned above
The extent of the area on which the wind forces act
The directional characteristics of wind

Windage Area, Wind forces, and Wind Pressure

Concentrate on the second point. On any floating vessel, the wind acts primarily on the exposed area. Exposed area means the surfaces of the vessel that are directly under the influence of wind action. So, when you see a vessel floating on the sea at a specific draft, what areas are exposed to the wind?

As expected, everything which is above the waterline or which is not submerged. This includes the superstructure/deckhouse and the part of the main hull above the waterline, the extent of which is also known as the freeboard in technical terms. So, the greater the area subjected to wind action, the greater the effects.

In technical terms, this area is also known as the Windage Area. So, windage area is the sum of all areas when any view of a vessel is projected on a plane. The figure below clearly describes everything described so far.

What is the maximum area on any vessel where the wind can act? The answer lies in the way we perceive it in floating conditions.

As expected, the highest area on which the wind comes from that specific direction can act is the lateral area, or in other words, the profile view of the ship. The reason is simple. In any ship, the length is always more than the breadth. So, when viewed along the length or from a transverse or lateral direction, the surface is always more than when viewed in a longitudinal direction or from the aft or front.

For all practical purposes, the face area of the superstructure or deckhouse can be easily determined from the first principles as they mostly have straight edges and no curves.

On the other hand, determining the lateral projected area of the superstructure or deckhouse is slightly complicated as they often have curvatures characteristic to the hull form.

However, using design drawings, they can be estimated. For fuller-form vessels with lesser curvatures, like tankers, the approximate part of the main hull contributing to the windage area can be calculated as:

Length Overall (LOA) X Depth of the vessel (D) – Length between perpendiculars (LBP) X Average Draft (T).

It can be said that larger vessels with more windage area suffer a greater influence of wind action. Of course, for such vessels with high values of displacement (and thus inertia), the resultant effect is far less as compared to a lighter vessel under the same conditions. However, the type and design of the vessel are also important.

For example, a large passenger cruise ship with a high and broad superstructure will be more vulnerable to wind action as compared to a loaded tanker or bulker with a small deckhouse/superstructure and a lesser exposed area of the hull as well under high displacement.

At this point, it can also be said that a given vessel having a higher amount of loading, that is, having a higher displacement, has a reduced freeboard (due to higher immersion) and thus lower windage area as compared to when it is under a lighter load condition, that is having a lower value of displacement. Hence, lower windage area coupled with greater inertia (due to increased displacements) leads to significantly much lower effects of wind action in the case of higher loading.

All classification rules for loads take this windage area and the resultant action of winds under consideration.

Accurate determination of the wind force is a complicated task as in sea conditions, the wind forces can be highly non-uniform and erratic in nature. However, for all practical purposes, the wind force can be approximately calculated (in tonnes per square metre) as per the empirical relation:

W X V2 /18000

Where W is the windage area in square meters as determined from above, and V is the velocity component of the wind in the direction of action on the given windage area. That is, when a random sea wind is acting on a vessel from any arbitrary direction, the component of velocity in the lateral direction is perpendicular to the vessel’s length if we are interested in finding the force acting on the profile of the vessel and the component of the velocity in the longitudinal direction if we are interested in finding the wind forces on the front or aft end of the vessel.

For a beam wind, almost the entire value of the total wind forces is assumed to be acting on the vessel from its sides with no component in the longitudinal direction. Similarly, for a typical headwind condition, the entirety of the wind-induced forces is acting in the fore-aft or longitudinal direction with nearly no component in the transverse or lateral direction.

Wind Pressures

Along with the windage area, it is crucial to understand the effects of wind forces in a vertical sense, that is, how a wind force acting on the vessel from the sides can influence the loads. It is fair to consider that the effects of wind on a given exposed region of a vessel can be deduced from the vertical height above the baseline.

For a floating vessel, the baseline can be considered the line about which the resultant moment acts from the component of wind force produced. Henceforth, the lever or moment arm can be taken as the distance between the baseline or bottom of the ship and the centroid of the area over which the wind is taken to act.

So, for a given area A on the superstructure, VCG alludes to the vertical centre of gravity of the given area above the baseline, which in this case is the same as the moment arm or lever for the moment from wind forces.

And the net moment from the wind action is given as the product of the vector component of the wind force acting on the area (point load acting on the centroid of the area) multiplied by the VCG or vertical moment. Thus, for higher superstructures, the wind action on the upper regions is more pronounced as compared to the lower ones due to higher moments caused.

The pressure distribution also varies accordingly, with the gradient decreasing from top to bottom. The pressure for a given area under wind action can be calculated simply by dividing the net value of wind force acting divided by the area (P=F/A).

One very important consideration, in this case, is that the wind has a uniform distribution of loading over the area, which acts for simplicity of estimations. Though in real scenarios, the nature of a blowing wind is highly random and over any given area, its intensity varies from point to point.

After we have discussed the windage area and wind pressures, it is now important to learn more about how the directionality of the wind can affect a floating vessel. But before that is important to know about the types of wind-based on their direction.

Types of Winds and Wind Action on planar turning

The types of wind-based on the direction can be categorised as follows:

Headwind: This is the wind which acts in a direction opposite to the vessel’s heading. As they interfere with the vessel’s surge, it produces the highest level of wind resistance to the vessel.
Aft wind: They also act in a longitudinal direction but from the aft direction of the vessel. As they are concurrent with the vessel’s heading, they constructively interfere with the surge headway and may also bring about increasing the speed of the vessel without the expense of propulsive power, something very desirable.
Beam winds: They act in a direction perpendicular to the vessel’s length and, thus, headway. The resultant forces affect the vessel’s surge as they tend to drive the vessel in a lateral or sideways direction, also influencing the manoeuvring problems of the vessel. Suppose the vessel has a significantly high windage area as described. In that case, they produce large degrees of resistance, which may exceed that produced by an equivalent intensity headwind due to the forces acting on the profile.
Oblique winds: These winds flowing from any arbitrary direction are most common. They act in both the longitudinal as well as transverse directions. For estimating the effects of the wind on the vessel along a particular direction, they can be resolved into respective components and combined with the windage area as described above.

Now, while we know about the different directions of wind, it is crucial to understand how it affects the vessel in terms of manoeuvring.

Recall that previously we had discussed how the wind force creates a moment at any given area. So, while we take the entire windage area into consideration, the net result of the wind forces can be considered acting on a centroidal point known as the Centre of effort of the wind, often denoted as W. In other words, this W is the weighted average of all the centres of action of the wind forces.

Now, also recall that all kinds of turning effects of the vessel are based on the pivot point of the vessel, P.

This pivot point, P, is forward of the midship and close to the bow when the vessel is moving ahead, and vice-versa when the vessel is moving astern. When the vessel is at rest, the pivot point is more or less close to the midship for all practical purposes.

So, the interplay of the pivot point with this centre of effort affects the turning tendency of the vessel based on the intensity of the wind and the current displacement of the vessel, of course. The physics of turning is based on the lever WP, which is the distance between these two points.

Though there can be several cases for consideration, for now, in this article, we consider a few simplistic cases.

When the vessel is at rest, and there is pure beam wind: In this case, as the vessel is at rest, the pivot point can be considered at midships. For beam wind cases, in a longitudinal sense, the centre of effort will also be near midships only. So, it can be said that both W and P are close to each other, and thus, the lever or moment arm for turning, WP, is very small or almost negligible. However, if the wind forces are significant and the vessel’s displacement is not sufficient to fully resist the wind forces, there can be a lateral drift of the vessel in the direction of the wind. So, for vessels at rest and having beam winds, there is no tendency for turning the vessel but can be a tendency to drift sideways.

When the vessel is moving ahead, and there is beam wind: When a vessel is surging ahead, the pivot point is skewed towards the bow. Considering a uniform flow of wind, the centre of effort can be considered close to the midship again. So, this separation between these two points creates a turning lever that causes the vessel to rotate.

When the vessel is moving astern, and there is beam wind: This is the reverse case, and for the same orientation of the vessel and wind direction, the turning sense is opposite.

For headwinds and aft winds, as discussed above, the winds can only constructively or destructively interfere with the vessel’s linear motion. Since the wind force vector is concurrent with the ship’s centerline, there is no turning moment created.

There can be other complicated cases as well in various combinations depending on wind direction and vessel orientation. The effects of wind when the vessel is not on a level waterline and has a trim forward or aft are complex and are omitted from discussion in this article.

Wind forces are of good importance when the berthing of the vessel is taken into consideration.

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Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendations on any course of action to be followed by the reader.

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What is ODAS or Ocean Data Acquisition Systems?

Bhupinder Marriya — Fri, 02 Jun 2023 11:20:58 +0000

Meteorological stations worldwide require access to sea conditions and marine environments to provide weather forecasts and warnings. This data is provided to the meteorological stations through a network of ODAS (Ocean Data Acquisition Systems), ship weather observations, coastal radars, and satellites.

Ship weather observation reports are typically available only along major shipping routes and not worldwide. However, due to increasing workloads and busy schedules, it has become increasingly difficult for ship staff to participate in these reporting activities and maintain the meteorological observation schedules.

The quality of the data transmitted also varies from ship to ship. Ships generally tend to avoid areas with rough weather and rough seas where weather observations are most needed.

Satellite imagery offers a comprehensive view of weather patterns as it can capture a wide area. However, its effectiveness depends on factors such as the satellite type and the positioning of the weather in relation to the satellite. Nevertheless, there are still challenges in obtaining data for all areas and acquiring it promptly. Therefore, while satellite imagery is beneficial, it also has limitations to consider.

Coastal radar serves as a valuable tool for detecting approaching precipitation and severe weather near land. However, its effectiveness is constrained by a limited range and, in certain locations, by topographical factors.

To provide accurate, fast, and real-time data to the meteorological stations, equipment called ODAS (Ocean Data Acquisition Systems) is mounted on buoys, structures, platforms, and unmanned light vessels.

Mariners use this weather information and forecast for passage planning and weather monitoring throughout their routes. This information contributes to the safety of navigation and helps to avoid potential dangers like storms or rough seas. Mariners can choose the safest routes for their vessels based on this information.

What is an ODAS (Ocean Data Acquisition Systems) Buoy?

An ODAS (Ocean Data Acquisition Systems) buoy is an automated buoy used for collecting and transmitting meteorological, scientific, and oceanographic data in real time. This data is transmitted to shore via geostationary or polar-orbiting satellites. Some ODAS (Ocean Data Acquisition Systems) transmit the data through HF or line-of-sight UHF links to shore.

To provide position coordinates or locate these buoys, they are equipped with GPS (Global Positioning System). ODAS buoys can be deployed either through mooring or as free-floating devices. In the case of mooring, the buoy is anchored to the seafloor using conventional mooring techniques. They are found to be more effective and reliable sources for providing live data to meteorological stations.

The ODAS buoy is not an aid to navigation but is defined as a type of special buoy in the IALA Maritime Buoyage System. The IALA Maritime Buoyage System is a nautical publication 735 published by the UKHO. The IALA Maritime Buoyage System is divided into two regions: Region A, and Region B. Region A includes countries such as India, Australia, Gulf countries, and European Union countries. Region B includes countries like North and South America, Japan, Korea, and the Philippines. The marking for special buoys is the same in both regions.

Special buoys are used to indicate a special area or feature, the nature of which is apparent from reference to a chart, admiralty sailing directions, or notices to mariners.

The colour yellow is assigned for special buoys. The shape of the buoy is optional as long as it does not conflict with the shapes used for lateral or safe water buoys.

For example, an outfall buoy situated on the port side of a channel can have a can-shaped form, but it should not be conical. The top mark of this buoy is a single yellow cross in the shape of an “X,” but it is also optional. The special buoys are lettered to indicate their purpose. The light used for special buoys, when fitted, must be yellow and may have any rhythmic pattern not used for white lights. In the case of an ODAS buoy, the rhythmic pattern of the light is five flashes in a group every 20 seconds.

To distinguish the unlit buoy at night, retroreflectors are used to reflect the light from the buoy’s back. There are two codes used for the retroreflectors’ marking: the standard code and the comprehensive code. In a specified area, only one code can be used, and the code in use can be found in the Admiralty Sailing Directions. The marking of the special buoy retroreflector is the same in both codes: one yellow band, an “X,” or a symbol is used.

What is the data transmitted by the ODAS buoy?

The data transmitted by ODAS (Ocean Data Acquisition Systems) is of great importance for meteorological, oceanographic, and scientific research purposes. This data is also crucial for coastal areas, particularly due to the large number of people residing near the coast. It helps in monitoring severe weather situations that pose a threat to life and property along the coast. The ODAS (Ocean Data Acquisition Systems) transmits the following information:

Wind speed and direction
Wind gusts
Wave height, direction, and period
Air Humidity
Air temperature
SST (Sea Surface Temperature)
Precipitation
Visibility
Solar radiation
Oceanic currents
Atmospheric pressure
Sea State
Position coordinates with transmitted data

Now, as we are near the end, I believe you have grasped the fundamentals of Ocean Data Acquisition Systems. To summarise, ODAS (Ocean Data Acquisition Systems) play a vital role in transmitting real-time, fast, and accurate meteorological and oceanographic data to maritime stations and for scientific research purposes. Meteorological stations utilise this data to provide weather forecasts and warnings crucial for navigators, the fishing industry, recreational boating, and drilling activities.

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What are Lubber Lines?

Subhodeep Ghosh — Mon, 29 May 2023 06:11:19 +0000

For ages, the magnetic compass has been considered the most essential device or instrument for navigation in the vast stretches of the seas.

In today’s time, advanced mechanisms like GPS or digitised systems using high-end satellite data help in navigation and keep the vessel communicable from literally everywhere. The compass is still present in almost all vessels as a standby and still upholds its importance.

Now, reiterating the simple aspects, the magnet or associated devices as the magnetometer works on the basic principle of geomagnetism, showing the directions based on the earth’s inherent magnetic field.

Thus, when the compass needle points somewhat towards the north, your vessel is headed towards the northern direction.

Now, while analysing compasses, some form of reference is crucial in determining the direction with respect to the vessel’s heading. For this reason, the use of lubber lines comes into the picture.

Lubber lines are calibrated marks inside the dial or the binnacle of a compass that shows the direction of the vessel’s centreline, that is, the foe-aft orientation of the vessel. They appear as a thin line or a mark or a strip aligned with the vessel’s direction of heading.

Now for all practical purposes, as the compass or any other form of navigation instrument is located towards the front side of the vessel on the bridge, the lubber line is mostly straight when viewed on the device.

Lubber lines essentially act as a reference for navigation and give the resultant angle of the vessel’s course of heading and any given direction.

For example, in a compass, when the lubber line makes an angle of about 45 degrees concerning the North direction, the vessel is headed in a direction about 45 degrees offset from the north. The lubber line also alludes to the line of 0 degrees with the vessel’s heading.

There may also be other additional lubber lines at intervals of 45 degrees from the main one in many cases. In modern devices like GPS or radar navigation charts, the lubber line is often displayed digitally on the console or screen.

The term is derived from the nautical term lubber meaning a sailor.

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What are Foul Anchors?

Subhodeep Ghosh — Tue, 09 May 2023 04:56:26 +0000

The anchors are as old as the ships themselves. They are those age-old devices used to haul a floating vessel to the ground, like the seabed or seafloor, when the vessel is required to be halted or stationed at some location for some requirement.

Anchors come in different shapes, sizes, types, and builds. Depending on the vessel size and type, the size and weight of the anchor vary accordingly.

The anchors are usually stowed in an enclosure of the main hull, usually known as the anchor pocket.

When needed, they are lowered into the seafloor with the help of a chain-cable mechanism operated from onboard.

After the anchor is lowered into the seabed, it settles down by virtue of gravity into the undersea floor.

This creates a firm grip on the seabed, and the vessel is fixed to its location by the inertial weight of the anchor coupled with the fixity it creates on the seafloor.

Though in modern times, technologies like Dynamic Positioning Systems or DPS have gained popularity, anchor still remains very common amongst vessels.

And for all vessels that use technologies like DPS, an anchor is still kept on board as a reliable backup source when any of these systems fail. Hence, the importance of anchors is indispensable.

As an intrinsic part of the anchor systems, the anchor chains also have a great deal of importance. They should be sturdy, have strong connections and strength, and resist high loads.

What do we mean by foul anchor?

Foul anchors are those where the chain (or the rope for ancient ships) becomes entangled about the entire structure, or the anchor gets enmeshed by some obstruction underneath the sea level.

The shank is the central vertical structure of the anchor. The crown is the lower part that embeds into the seafloor.

Flukes are at the ends of the fluke and further help in the settling process of the anchor. The topmost point of the anchor (atop the shank), where the chain or rope is attached, is known as the ring or hook.

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In the first case of an anchor fouling, the rope or wire becomes entangled or entwined about the whole anchor structure.

This can be best visualised by this classic example. All of us must have, at some point in our lives, ridden or still ride a bicycle.

And inevitably, nearly all of us must have experienced our chain being derailed from its slot over the paddle wheel.

If we continued for even a few moments after this debacle, the entire chain would get entangled miserably, and we would have a hard time getting things back to normalcy!

Similarly, for a chain-release mechanism like the anchor chain system, there is often this risk of entanglement due to inaccurate action, erratic motions of the vessel, operational errors, or just unbalanced forces.

Hence, in these events, the chain gets entangled about the anchor, initially about the shank and later about the crown or base. This poses a great deal of risk to the functionality of the anchor system as a whole, rendering it ineffective.

The second case of fouling of the anchor is in the event of any kind of obstruction or impediment underneath the sea surface. These can be because of barnacles, seaweed, cacti, moss, fern, and all other kinds of aquatic or marine vegetation.

Similarly, there can be any kind of natural or geographical obstructions or some kind of artificial ones like wreckage parts or some structure.

Fouling of anchors is a very cumbersome event, and recovering or reorienting the anchor back to the normal configuration is challenging. As the chains are quite heavy, any complex form of twist is almost impossible to unentangle by human effort. The common practice is moving the vessel in different ways to change the alignment of the entangling rope or alter the state till it loosens up and becomes straight and taut gain.

This may involve moving the vessel back and forth or, in some cases, manoeuvre the vessel in different manners and suitably varying the engine power till it reaches a suitable position conducive to the fouled anchor.

In worst cases, the only option is cutting the tangled chains back on shore using welding or different cutting methods and thereafter, refitting the chain system.

For fouling by other external means, the first technique of vessel motions is primarily used. Else, often the obstruction is removed by external intervention using underwater divers or other machinery-based means.

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The Strait Of Dover – The Busiest Shipping Route In The World

Shamseer Mambra — Wed, 26 Apr 2023 04:52:18 +0000

The Dover Strait, or the Strait of Dover, is a vital maritime route in the Western European region located at the narrowest part of the English Channel. It has been a strategic route since ancient times, and the Romans called it the Fretum Gallicum or Fretum Britannicum or Fretum Morinorum.

It is the strait that separates the English Channel and the North Sea and a boundary between Great Britain and France, or the European Continent.

Beneath the Dover Strait is an undersea Channel Tunnel enabling trains and vehicles to travel from Southern England to Northern France, decreasing maritime traffic and congestion in the strait that handles about 400 commercial vessels daily. This is one of the longest underwater tunnels in the world, with an average depth of 40 m.

The 50.45-kilometre-long Channel Tunnel was opened in 1994 between the UK and France. The underwater rail tunnel, one of the biggest engineering projects ever undertaken in the UK, connects Folkestone in UK and Coquelles in France.

Electricity is supplied to the trains, the tunnels and lightning and drainage pumps by 2 160MW substations located on both sides of the tunnel. If one station faces any issues, the second one can provide electricity to the whole system.

The Dover Strait has a width of 18-25 miles, while the depth of the strait is between 120 to 180 feet. Across the Dover Strait, the shortest distance is 20.7 miles, from the South Foreland to Cap Gris Nez, a cape near Calais in France.

Since the width of the strait is less, it is possible to see the opposite coastline of both countries with the naked eye on a clear day. One can easily spot the White Cliffs of Dover from the french shoreline and the buildings on both sides of the coasts and also city lights which look beautiful, as described in Matthew Arnold’s poem ‘Dover Beach’.

Dover is known for White Cliffs, which comprises a part of the coastline lined with abrupt cliff formations made of white chalk. A similar geologic formation can be seen across the channel at Cap Blanc-Nez, indicating that both coasts were once joined together.

How was the Dover Strait Formed?

Geologists suggest that the Strait was created half a million years ago by erosion through high-magnitude floods of a land bridge that connected Great Britain and mainland Europe. Researchers claim that water started cascading over the narrow strip of land and eventually damaged the natural bridge thousands of years later in a two-stage process.

Sub-bottom data records show sediment-infilled depressions incised into bedrock. These indicate initial erosion of the Dover Strait by lake overspills, plunge pool erosions by waterfalls and finally, dam breaching.

Also, before the opening of the Dover Strait, Britain was connected to Europe through a structural ridge that went from southeast England to northwest France. Hence, the geographic insularity of the UK from continental Europe is a result of high interglacial sea levels, leading to marine flooding of shallow shelf regions of the English Channel and the North Sea.

A new study said the first breach must have happened about 450,000 years ago, creating a smaller channel compared to the existing one. The second one probably happened hundreds of thousands of years later, creating a catastrophic breach separating Britain from France completely.

During the first breach, the ice-dammed lake in the southern part of the North Sea flowed, breaking the Weals-Artois chalk range in a major erosion and flood event. Later, the Thames and Scheldt also flowed into the English Channel through the gap. However, Meuse and Rhine still continued their northward flow.

In the second flood event, which occurred around 225,000 years ago, the Meuse and Rhine were ice-dammed into a lake which broke its barriers. Both flood events led to the creation of huge flood channels into the dry bed of the English Channel, like the Channeled Scablands or Wabash River in the USA.

The Lobourg Strait or the Lobourg Channel is a prominent feature of the strait’s seabed that runs six kilometres wide on a north-north-east axis. It is close to the French coast than the English Coast; it goes along the Varne sandbank, where it plunges to 68 metres at its deepest.

Since the exact period of these events remains uncertain, the Geologists from the UK, Belgium and France are planning to drill into the seafloor in order to retrieve samples from the plunge pool sediments, hoping to determine the precise time.

Importance of Dover Strait in World Maritime Map

The Strait of Dover, which is considered to be the busiest maritime route in the world, has been a mainstay of the European shipping network for several years now. In spite of its narrowness, the Strait’s geographic location is quite distinct.

Both for vessels wanting to cross the English harbours and enter European harbours and for those entering the North and the Baltic Sea through the English Channel, passing over the Dover Strait is unavoidable.

Statistically, it is estimated that the Dover Strait sees the passage of around 400 ships on an everyday basis. The vessels that transit through the Strait of Dover are not only cargo-carrying ships but also Voyager and specific Ro/Ro ferryboats.

Hence, safety in the strait’s waters is taken seriously, with HM Coastguard maintaining a 24-hour watch over the channel and implementing a strict regime of shipping lanes.

The harbour of Dover on the strait’s British side and the harbour of Calais on its French side are two of the world’s most engaged harbours located along the Strait. Since it is a major transportation link between the two countries and also part of a busy shipping route, traffic safety has become a critical issue in the water in recent times.

And the significance of the Strait has been further emphasised with several necessary protocols set up in order to enable a safer and more secure passage through the strait. In addition, the Dover Strait is also referred to as the Strait of Calais on account of its significance to the French maritime domain.

Alongside being a crucial marine entryway, the Dover Strait is also a popular recreational swimming location, especially for swimmers wanting to cross the English Channel.

The shortest distance from the strait is from the South Foreland, 20.6 miles northeast of Dover, in Kent, England, to Cap Gris Nez, which is a cape close to Calais in Pas-de-Calais, France. Also, between these points is the most popular route for cross-channel swimmers.

Maritime Protocols and Regulations in the Strait of Dover

Being the busiest international seaway in the world, several important and noteworthy regulations have been established to aid the passage of vessels through the strait during the past four decades. Due to its narrowness, the Strait had witnessed various accidents every year in the earlier days.

As part of the introduction of the Collision Regulations of 1960, the strait comes under full radar surveillance and also operates a Traffic Separation Scheme (TSS), with which two lanes run through the strait for inward and outward-bound traffic in order to avoid collisions.

Introduced in 1967, the Traffic Separation Scheme in Dover was the first International Maritime Organisation (IMO) approved TSS in the world. The Collision Regulations was later replaced with International Regulations for Preventing Collisions at Sea, which was adopted as a convention of the IMO in October 1972. The new law amended the existing regulations with more strict navigation rules to prevent collisions in the water.

In addition, the CNIS (Channel Navigation Information Service), which was established in 1972, runs for the fulfilment of the aforesaid objective. The CNIS helps supervise the maritime traffic crossing through the Strait by way of a full-day, round-the-clock radar system surveying and radio channel feasibility.

Jointly operated by the UK and France from the Dover Maritime Rescue Co-ordination Centre (MRCC) in the UK and France’s CROSS Gris Nez, CNIS is assigned to keep the Dover Strait TSS under observation in addition to monitoring the flow of traffic. In the case of any ship not following the stipulated guidelines while crossing the Strait, the Channel Navigation Information Service is also authorised to report the lapse and take any measures required as a set-off.

The Strait of Dover also falls under the category of a ‘mandatory reporting zone.’ Under Pas de Calais/Dover Strait report or CALDOVREP, the vessels with GRTs over 300 tonnes transiting these zones are mandated to announce their details and specifications.

The information required to provide includes the name of the ship, call sign, IMO identification number and MMSI number, position in latitude and longitude, the draught of the ship, course and speed of the vessel, route information, details about the hazardous cargo and IMO class and quantity, among others.

The vessels entering the Strait through its Southwestern entryway are required to announce their details to the British Coast Guard at the Dover harbour. Similarly, ships that use its Northeastern entryway to pass through the strait are required to announce their details to the French coastal authorities at the Cape of Gris Nez.

However, ferries operating in the Strait of Dover need not take part fully in the scheme but only need to advise the Dover Coastguard or CROSS Gris Nez about their departure.

Naval Battles and The Dover Strait

The Strait of Dover has played a significant role in several historic naval battles in the region. While the first notable one would be connected with the Spanish Armada, the most significant involvement is in the Battle of Dover Strait, which happened in October 1916 during the First World War.

The Battle of Dover Strait saw five groups of German torpedo boats entering the Strait and attacking a different section of the shipping in the channel, and also attempting to disrupt the Dover Barrage.

Though the German torpedo boats were challenged by the British naval ships, they could destroy one British destroyer, a transport, and several drifters before withdrawing from the scene.

The Strait was also the scene of the second Battle of the Dover Strait that happened during the First World War itself. In April 1917, two groups of German torpedo boats raided the Dover Strait as they did a year ago in a bid to attack Royal Navy ships patrolling in the Strait.

Two Royal Navy destroyers, HMS Broke and Swift, engaged with the torpedo boats and were able to sink two of the German torpedo boats after suffering damages themselves. Moreover, the Strait of Dover marked its role in the history of the Second World War as it witnessed the Battle of Dunkirk.

As portrayed in Christopher Nolan’s latest war film, Dunkirk, the battle marks the fight between British allies and Nazi Germany, and also the defence and evacuation of British and Allied forces in June 1940.

Marine Life In the Dover Strait

Apart from its economic importance, the Strait of Dover also boasts a rich and varied maritime ecology. Having a depth of 120 to 180 feet, the Dover Strait has a succession of rocky areas, sandy flats and sub-aqueous dunes underneath, creating a rich marine environment.

Many underwater species comfortably find shelter in the Strait since the strong currents of the Channel run slowly around the rocky areas. According to ecologists, the water is clearer in these areas, helping a variety of algae to grow significantly.

Moreover, since the Dover Strait makes a bridge between the Atlantic Ocean and the southern part of the North Sea, it has become a transition zone for the species from both oceans.

The rich marine life in the strait has led to various environmental stipulations being imposed in order to ensure the continuity of the Strait’s ecological richness and variety. In addition to these, further, continued efforts are also being undertaken both by British and French authorities to nurture the environmental diversity of the locale.

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What is Transverse Thrust in Ships?

Subhodeep Ghosh — Thu, 16 Mar 2023 03:30:49 +0000

Thrust alludes to the force that pushes or spurs a body to move in a particular direction. Regarding ships, Thrust can be defined as the propulsive force that drives the vessel through the water against the resistive forces, mostly hydrodynamic.

What provides the Thrust to a ship?

The answer is – the propeller or the propulsive device associated with the vessel.

How?

Again, in simple words, the Thrust is derived from the propeller’s action that is caused due to the rotational motion of the blades, which is translated into linear motion. More specifically, the torque stemming from the rotational motion of the blades is converted into Thrust that produces a large fraction of dynamic force to the surrounding water medium, which in turn, impels the vessel due to the resultant reaction (Newton’s third law).

Moreover, there is also an interplay of hydrodynamic principles, including the pressure differentials about the blades, which affect the flow oncoming to the propeller. The propeller is driven by the continuous power supply from the main engine, where electrical and mechanical power is converted into rotational motion through a linkage of intermediate members like the crankshaft, intermediate shaft, stern tube shaft, and gearbox.

For unconventional modes of propulsion like propulsors and so on, the mechanism of power supply and transmission may be different. Still, the underlying principle remains the same: imparting a force on the adjoining water medium, which causes the body to move in the desired direction.

Now, for all practical purposes, we are familiar with the concept that the propeller or propulsor action causes the vessel to move either forward or astern (due to reverse or astern thrust) direction. This is mostly true, but the physics of action is a bit more complicated than that.

The force which drives the vessel in a particular direction is essentially the axial or longitudinal component of the resultant Thrust derived from propeller action on the water.

Returning to the basics of propulsion, it is important to know that the overall Thrust produced due to the propeller action on water is multi-directional in nature, like most other physical forces caused due to external means. However, the axial component of force is the predominant component that triggers the vessel to move ahead or astern, making the net effect of motion in a linear sense.

The smaller component of this Thrust, which is the one acting in the direction perpendicular to the vessel’s motion (the y-component when perceived in a conventional reference coordinate system), is what we call the transverse Thrust.

The effect of this component is not very significant in the resultant motion of the vessel but can be somewhat found from the initial tendencies of motion of the vessel. In a more technical sense, this transverse Thrust can be attributed to the collective interaction between the hull, propeller, and, up to some extent, the rudder.

A Detailed look into Transverse Thrust

Let us look at this interesting phenomenon in detail. For our better understanding, we take the simplest case of a conventional single-screw vessel with a right-handed propeller, that is, the clockwise motion of the blades, when viewed from the aft, causes the vessel to move in a forward direction, that is, in the direction of its bow. Conversely, from the same position of reference, a left-hand motion or a counterclockwise motion of the same set of blades causes the astern Thrust or the tendency of the vessel to move aftward. Now, with regard to transverse Thrust, two cases arise:

1. The vessel is in forward motion

When the vessel moves in a forward direction, which is the most common case, the propeller taken for consideration moves in a clockwise or a righthand sense when viewed from behind. Due to the action of the blade forces in the slipstream of the propeller, there is a high degree of pressure on the starboard side (as the principal action of the propeller blades is towards the right or clockwise direction).

Furthermore, during the initial stages, when the engine power is high, but the resultant speed of the vessel is low, that is, the vessel is gradually accelerating, and the axial Thrust is still not very high enough, the transverse component of the Thrust is more pronounced. This induces the stern side to turn towards the starboard. This means the bow now turns in the anti-clockwise direction, which is towards the port. This entire couple takes place at the pivot point of the vessel at that time.

Now, as we know, for all practical purposes, for conventional vessels moving ahead, the pivot point is located towards the bow (1/3rd to 1/4th distance of length from the bow) and vice-versa. When a ship is at rest, the pivot point is more or less centred towards the midships.

Look at the below figure. The moment arm or the linear distance between the point of action and the pivot point is way larger in this case with respect to the stern as compared to the bow.

Hence, as per the coupling equation for balance:

Fb X Db + Fs X Ds = M

Where the suffixes b and s stand for the bow and stern, respectively. Fb essentially means the force component or the transverse Thrust acting on the front end or bow. Similarly, Fs stands for the thrust component acting on the aft end or the stern. Db and Ds are the distances from the point of action from the pivot point for the bow and stern, respectively. M is the resultant or net unbalanced moment.

Once again, PP, as shown in the figure, stands for the pivot point or the moment centre where the net coupling action is taking place.

Hence, from the first principles, another moment is created at the bow end. From Newton’s third law again, the transverse Thrust at the bow is equal to that at the stern, which means Fb=Fs. Now, as per the coupling equation, as the value of Ds > Db, that is, the moment arm or lever, as described, is much more at the stern as compared to the bow, the moment acting in the bow direction, that is Fb X Db, is smaller as compared to that of the moment at the stern, the difference being the significantly larger moment arm. So, for a given moment resultant moment M, the product Fb X Db, the moment at the bow, is smaller than the moment at the stern, Fs X Ds.

Moreover, at this juncture, the action of the rudder also comes into play. When the vessel is moving ahead, the rudder is directly in the wash of the propeller action. Simplifying this hydrodynamic term, the rudder is under the influence of the propeller-induced water flow, which is predominantly backward in the axial sense.

Thus, the stream of water flow acting from the propeller motion, which creates the forward Thrust, is overridden or compensated by the rudder directly in its wake. So, if the rudder angle is suitably and steadily maintained, it proves effective in cancelling the transverse thrust effects at the stern. Now, the lesser this value of initial transverse Thrust generated at the aft, the lesser the resultant moment, and this translate to a further reduced force at the bow, tending to turn it portside even lesser.

Thus, for all practical purposes, it can be said that, FORTUNATELY, the effects of transverse Thrust when the vessel is moving forward are not significant and can be ignored if it does not prove to be very much interfering with the vessel’s heading and overall propulsive efficiency.

2. The vessel is in a backward motion

Now, what happens in the reverse case? As expected, the opposite phenomenon. For the same propeller, the flow dynamics and pressure patterns are reversed, and the transverse Thrust is created at the stern in the opposite direction, that is, port in our case.

Critically, for reverse motions, the pivot point of the vessel is now centred close to the stern region. Again, refer to the below figure. For an initial transverse thrust directed towards the port at the stern for our right-handed propeller, the bow now tends to turn in the clockwise sense, that is, towards the starboard to complete the moment couple. The position of the point of action of this couple, that is, the pivot point, is now very crucial. Now, Db > Ds, which means the moment arm from the bow is greater as compared to from the stern, thanks to the location of the pivot point.

So, taking individual moments, the product Fb X Db is higher at the bow in this case.

Furthermore, the rudder action is not fully effective in this scenario; thus, the effects are increased further. In other words, the steering effects of the rudder are not sufficient enough to suppress the turning action of the transverse thrust component arising from the interaction of the propeller-induced flow and the hull.

Also, due to the hydrodynamics of the flow and the propeller, during an astern move, the pressure build-up on the starboard aft ward part of the hull is quite large. Hence, the transverse Thrust is quite large towards the port at the stern, and so is the moment produced. As Fb=Fs, the transverse Thrust at the bow is also higher proportionally during an astern move. This high value of Thrust, coupled with the larger moment arm or lever, creates a significantly high value of turning moment at the bow region (towards the starboard in our case).

Henceforth, for all practical purposes, when a vessel is going astern, the effects of transverse Thrust are higher. Thus, there is a significant tendency for the vessel’s heading to turn or drift sideways (towards starboard for a conventional right-handed propeller).

Factors affecting Transverse Thrust and ways to reduce it

Other than the two important cases described above, other factors affect the value or magnitude of the transverse Thrust irrespective of the direction of motion. As already mentioned above, the value of the transverse Thrust is the highest when a vessel is at low speeds or starting from rest.

This is because, during these spans of time, the torque produced from the propeller action is more significantly expended in the transverse component of the Thrust as compared to the axial component, as the heading of the vessel is still at lesser speeds. Thus, at slow speeds, there is a higher tendency of the vessel to turn or change its heading as compared to steady higher speeds when there is a continuously high value of axial Thrust to move the vessel ahead or astern, overcoming the visible effects of transverse Thrust.

Therefore, the highest pronounced effects of the transverse Thrust are when the vessel is moving astern at low speeds.

For all practical purposes, for a vessel moving astern, the average propulsive power of the propeller consumed in the transverse Thrust for a conventional, sea-going commercial ship varies between 10-15 %.

The depth of the water also plays a crucial role. The effects of heading or turning due to transverse Thrust are more pronounced in shallow water than in deeper waters due to hydrodynamic effects on the propeller.

The weather and sea states also play a role. The maximum effects of the transverse Thrust can be seen in calm and undisturbed waters and conducive weather conditions. During rough conditions, the entire dynamics of the water and the erratic nature of the wind forces create a total state of disorder beyond the possible ways to predict and estimate the correct effects of transverse Thrust.

Transverse Thrust is sometimes an issue, especially during reverse turns, as mentioned, the heading can often be altered. So, other than applying higher power to attain higher speeds, the rudder angle is often altered accordingly by masters and navigators. For instance, when a vessel is reversing and is required to maintain a more or less fixed trajectory, the rudder angle is often given a hard turn towards the port side to cancel or compensate for the effects of turning starboard, as mentioned above for a right-handed propeller, often at the expense of higher power.

Uses of transverse Thrust

Transverse Thrust is not always a negative aspect and is often useful. During operations like anchoring or berthing, the transverse Thrust is often used to advantage by navigators and merchants by intentionally having an astern move. Often during operations like deep-sea cable-laying, transverse Thrust is often useful.

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What Do You Mean By Stopping Distance Of A Ship?

Subhodeep Ghosh — Mon, 13 Mar 2023 05:01:09 +0000

We know that a ship, unlike an automobile, a train, or an aircraft, does not have brakes to stop. The answer lies in the physics of the water and the manner in which a vessel interacts with it.

Hence, for a ship to come to a stop, it must first cease to apply its propulsors’ original sense of rotation (and hence resultant forward thrust) and, thereafter, cause the engines to produce a reverse or astern thrust.

This is often caused by changing the pitch of the propellers to a negative pitch and changing the propulsion such that the thrust now created is the forward direction instead of backward, which in turn, causes the reactionary forces to drive the vessel in the backward direction.

For all practical purposes, this is achieved by turning the propeller blades in a reverse way as compared to before. For other types of unconventional propulsors like Azimuth Thrusters, there are special ways like orienting the placement of the thrusters.

In a nutshell, for a ship, there is essentially a thrust reversal in the opposite direction of its heading that triggers the vessel to gradually come to a halt. Now, carefully note the word ‘gradually’. This is another crucial juncture where a vessel stands out considerably from other modes of transportation.

When we apply brakes to a car on a road or an aeroplane on a runway or a taxiway, the motion is immediately arrested and, within a very short amount of time, comes to a complete stop. The amount of time taken for the moving body to stop after the application of the brakes is known as the stoppage time, and the distance travelled by the body in the given direction in this span of time is k, known as the stopping distance.

Both this time and distance depend on three very important factors: i) the current speed of the body, ii) the size and weight of the body, and iii) the surface on which it moves. So, a small hatchback moving at an average speed will require far less amount of time and distance to stop as compared to a Boeing 777 landing on the runway.

Similarly, the surface is also a critical determinant, thanks to the physics of friction. A vehicle speeding on a wet road will require a more significant amount of time and distance to come to a standstill, unlike one trundling on a rough field.

Moreover, the manner in which the brakes are applied is also important. When you are driving on a rainy road at a very high speed, the brakes should be applied slowly and gradually, as any hard braking can lead to it toppling or dangerously skidding off.

However, for ships, thankfully, all these complexities are absent, and there are only two predominant factors that need to be considered:

The size and displacement of the vessel
The speed

Still, from the point of view of hydrodynamics, it is expected that a vessel moving in rough, choppy seas will have a much lesser stoppage or stopping time as compared to one moving in calm waters.

Hence, to summarise, the stopping time of a vessel is nothing but the average time required for the vessel to come to a halt after the application of the reverse thrust. The stopping distance is the distance traversed by the vessel during this interval of time.

Like any other moving body, this time and distance taken to come to a stop can be directly attributed to the inertial effects as described in Newton’s first law. The displacement and the speed of the vessel, combined, indicate the momentum of the vessel. As obvious, the time and distance required for a tanker or a bulk carrier to stop are much greater than a small trawler travelling at the same speed on the same sea surface.

The stopping distance, coupled with the stopping time, is measured as the overall stopping ability of the vessel. This stopping ability of a vessel is of very high importance to both the designer as well as the operator as this is a direct determinant of the margin of safety or the margin of error that can be accommodated at different instances of operation.

In other words, for a given vessel, the stopping distance decides the time or the minimum path at which a ship should be prepared for a halt when any destination or obstruction is a certain distance away.

The effectivity or the competence of a vessel’s stopping ability depends on the minimum stopping distance or the least stoppage time. For a large gas carrier, the master or the captain needs to apply the reverse thrust and work on all procedures in sequence to stall the various operations of the propulsion system.

At the same time, it enters a port or a harbour in a much more planned way as compared to a small ferry. Moreover, the instances of emergencies or exceptions are always taken into account. So, depending on the size and type of the vessel, the speeds at different points of operation take into consideration this stopping distance.

Hence, for a large tanker or bulker, there is always a strict limit to speeds at coastal waters during arrival or departure where there is not only proximity to landmass but also higher traffic leading to increased chances of collision in case of unrestrained motions.

Stopping Process and Sea Trials

Now, in technical terms, there can be two distinct phenomena in which a vessel can come to a stop, and the stopping distance can be defined further accordingly:

Inertia Stop: This is an older practice of stopping vessels which are seldom used for larger commercial or military vessels these days as there is a requirement for time adherence everywhere. Here the engine power supply is simply stopped, and the vessel comes to a gradual and slow halt by virtue of the hydrodynamic resistance offered by the surrounding water media. This method is very unconventional and so rarely practised by mariners these days. Moreover, due to increased maritime traffic all over, there is limited availability of berths or places in the port or jetty for a given vessel. This time-consuming method cannot be afforded. However, this practice is still widely followed for smaller vessels like fishing trawlers or low-powered ones like short-route passenger ferries. This considerably saves fuel by not only shutting off the engine power beforehand but also as reverse or hard aft ward thrust is neither technically nor economically feasible for them.

Crash Stop: This method is widely followed in most of the mainstream seagoing vessels. Here, as already discussed above, the engine is given a full reverse or astern thrust. This creates a component of momentum vector in the aft direction, which militates against the ensuing forward motion of the ship, bringing it to equilibrium once the hydrodynamic equilibrium is attained. In instances of crash stops, other than the vessel dimensions and the current speed, the characteristics of the propulsion machinery, including the propeller design, play a crucial role. In certain designs, the entire process may take just a few seconds, whereas, in others, it may take up to a few long minutes.

For all practical purposes, there are two broad stages in the crash-stop process of a vessel:

Stage 1 (Speed reduction and application of negative power): This stage comprises mainly the time from when the procedure to the astern thrust is initiated, which in ship technical terms is also known as the “full astern” command. During this stage, the engine power is temporarily cut off and then directed so as to produce a negative thrust in the reverse direction. This also involves a reversal of the operation of the propellers or propulsors in unison. As a result, the forward component of the motion is gradually diminished, and the acceleration is increased in the backward direction. In this way, the resultant torque attains a negative value.
Stage 2 (Gradual stop): This stage encompasses the time from which the negative torque acts to the time when the vessel ultimately comes to a final halt after attaining mechanical as well as hydrodynamic equilibrium, and all sources of supply can be cut off.

Now, a vessel at sea seldom remains in a perfectly straight line or a steadily fixed path. Due to the action of hydrodynamics, wind, waves, currents, and often the rudders themselves, they deviate from their initial course in varying amounts. So, while measuring the stopping distance, this effect is also taken into account. Look carefully at the following figure.

The vessel initially treads for a certain distance and then slightly deviates from its initial heading before it comes to a halt. The starting point is considered to be the time after which all the procedures related to vessel stoppage or the full astern power are initiated.

The head reach is the linear distance along the vessel’s initial direction vector with the endpoint parallel to the ship’s actual endpoint. It is also a measure of the shortest possible distance. Track Reach is the actual distance measured along the ship’s trajectory. This is always greater than the former. And lateral deviation is the transverse distance which measures the linear offset by which the vessel has digressed from its initial heading.

As per IMO requirements, the stopping capability of a vessel is also mandated to be assessed along with other sea trials. ITTC and other regulations have standard procedures for carrying out these tests, collectively known as Stopping Tests. These stopping tests are of various types but are somewhat similar. All of them measure the minimum distance and time the vessel requires before coming to a halt, along with other factors like engine power and sea conditions.

The aforesaid quantities are measured as well (head and track reach, lateral deviation). Mostly the stopping tests start with full ahead engine power and corresponding speed. Likewise, full astern power is given as well. For all practical purposes, the rudder is always kept in a neutral position while conducting the stopping trials.

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Understanding Vessel’s Hull Speed And Its Determination

Subhodeep Ghosh — Mon, 27 Feb 2023 06:26:33 +0000

Whenever we think of the term ‘hull speed’, these are the few things that immediately pop into our minds: It is the maximum rated speed of the vessel; it is the service or the design speed of the vessel; or it is the current speed at which the ship is cruising. These are not the case!

A hull speed is a different hydrodynamic concept and stems from the complex physics of the interplay of the vessel with the surrounding water medium.

Before continuing into this article, it is important to know that hull speed is quite a complicated topic that requires a deep understanding of hydrodynamics and ocean-level wave mechanics but can be somewhat simplified from its physics point of view for everyone’s understanding.

Moreover, we shall first discuss everything from the assumption of the displacement type hulls, which are the simplest and commonest vessels and relies on the basic Archimedes’ principle of displacement and buoyancy.

To begin with, as the vessel moves in water, it is encumbered by the continuous reaction of the water around it. In a technical sense, this is the hydrodynamic resistance suffered by the moving body and is often further divided as

i) frictional resistance originating from the viscous effects of the water with the hull and

ii) the wave resistance due to the various waves associated with the vessel’s movement and the existence of the waves in the water itself.

Thus, to sum it up, a moving vessel is inevitably associated with a system of wave patterns. These waves can be of various types and is a huge topic of hydrodynamics altogether, but for all practical purposes, a moving vessel broadly creates two wave patterns, also known as wave trains.

These are transverse and longitudinal. Transverse waves essentially move in a direction transverse to the vessel’s moving direction, and longitudinal ones traverse in a direction from bow to sternwards or from front to aft.

Now, these longitudinal wave systems are mostly triggered by the kinetic energy of the moving vessel, and thus, this increases quadratically with the vessel’s speed of advance. The transverse waves are less triggered by this kinetic energy as they have their component of motion completely perpendicular to the vessel’s advance.

Now, as this kinetic energy gets transferred to the water medium (ice effects are ignored and beyond the scope of this article), there are two things happenings:

The vessel is losing its energy of motion.
The waves are gaining energy from this transferred kinetic energy, and this is further hindering the forward motion of the vessel.

Thus, the vessel’s dynamics have to work up against these waves constantly. For a simple example, consider a simple and unpropelled floating body like a fisherman’s boat in choppy seawater. Imagine that it is being imparted to a large one-time force in the forward direction.

You will notice that the boat surges for some time in the direction of the applied force and then comes to a halt. This is because the kinetic energy gained momentarily is gradually attenuated due to the action of the waves, and the boat stops after attaining dynamic equilibrium.

Now, for a vessel with a constant power source like a ship, the forces continuously supplied by the engine work to overcome this loss of kinetic energy due to the waves, and the vessel continues to surge forward. Here the engines essentially compensate for the energy loss from the waves to maintain the vessel at the given speed. But this comes at the expense of an excess amount of power required and fuel consumed. Why?

The answer lies in the hydrodynamics associated with the waves themselves.

Reiterating what we have discussed above, the motion of a vessel in water leads to the formation of a system of linear and transverse waves that cause wave resistance and absorbs the kinetic energy of the moving vessel.

In technical language, this collective effect of waves is known as standing waves. These waves are time-varying oscillations where the peak amplitude, also known as the crests, changes with time but are not varying in space.

We omit further discussing these waves with the knowledge that they are constantly interacting with the vessel as it tends to surge ahead. From the point of view of the moving vessel, these patterns of waves are also collectively known as the wake.

The wake is responsible for this loss of dynamic energy of the vessel, and the latter needs to stave off these wake effects to move forward. However, due to the constant building energy from the transferred kinetic energy on these waves, the vessel faces further hindrance or resistance that needs to be overcome at the expense of a higher value of energy if it is to continue moving ahead.

From the nature of the mechanics of these waves, they move at a velocity of square root [(acceleration due to gravity X wavelength)/(2Xpi)].

It can be symbolically expressed as (gλ/2π)^(1/2)

Where g is the acceleration due to gravity (9.8m/s2), λ is the wavelength of the surface wave, and π is the mathematical constant pi which is 3.14.

Suppose a vessel has a length of L, and the created wavelength is λ. The number of crests and troughs along the hull of the vessel is L/λ. This is the length of the vessel divided by the wavelength. As seen from the profile, the first crest is always generated near the bow, known as the bow wave.

Now, two scenarios occur.

The vessel does not impart any further force, like the previously discussed fisherman’s boat example. In this case, the vessel experiences a loss in energy and comes to a gradual halt when the dynamic equilibrium is attained, that is, the dynamic energy of the passing wave train is equal to the initial kinetic energy of the vessel.
The vessel continues to supply energy like in powered ships. And after a certain point of time, its velocity becomes equal to the wave velocity.

In both cases, when there is this energy equilibrium or a kinetic equilibrium, the speed at which this occurs is known as the hull speed or critical speed. Now, at this value of speed, the wavelength becomes essentially equal to the length of the vessel. And the vessel’s speed becomes equal to the wave speed.

Conversely, the hull speed is the maximum speed or the limiting speed at which an unpowered vessel will not experience a loss in speed or at which a powered vessel will continue surging without an added expense of power. In the latter case, after exceeding this critical value of speed, the increase in wave resistance is now at an exponential rate. This translated to a very high expense of power to attain further increments in speed.

Look at the below graph. After the hull speed value, for a conventional monohull-powered sea-going ship, the energy expended to attain higher speeds now escalates exponentially instead of quadratic.

This can be simply explained by the physics of relative speed. As the vessel’s surge exceeds the speed of the waves, the relative velocity between the two becomes negative. The wave now cannot outrun the vessel’s speed tendency. As a result, there is a great deal of energy build-up at the bow region and many crests at the aft instead of crests.

In more languages of physics, the wave trains now become out-of-phase with the ones initially generated. Hydrodynamically, this leads to a dramatic increase in induced values of wave resistance. Moreover, due to the wave train build-up at the bow area, the vessel tends to lose buoyancy support at the stern. In technical terms, this is also known as squatting effects or squat.

So, in the simplest of terms, hull speed can alternatively be described as the maximum speed at which the vessel continues to accelerate or surge without facing significant losses or expenses in power.

Recall the expression of velocity mentioned above. As in the case of hull speed, the wave speed becomes equal to the vessel’s speed. The hull speed can be further mathematically expressed as: 1.34 times the square root of the vessel’s length, simplifying the above terms and replacing the wave speed term with the given vessel’s speed as both are equal.

So, hull speed can be numerically defined as: 1.34 X (L)^(0.5), where L is the overall length of the vessel.

For all practical purposes, commercial vessels, for a very long time, avoid this hull speed and keep their velocities within this limit unless necessary to save on high power consumption.

Now, some vessels, like defence crafts, often ply at speeds much greater than hull speeds due to their special design and high delivery of power from their powerful engines. Smaller boats like speedboats or some motor yachts often tend to avoid this hull speed obstruction by just ‘climbing up the waves’ built up in front and avoiding the bow waves themselves!

Planing and semi-displacement crafts, which uses the physics of lift action, are highly efficient in avoiding the hull speeds and their related effects.

These days, Froude Number, a dimensionless ratio of speed and length, is mostly used for estimating resistance effects and hull-water interaction.

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Understanding Vessel’s Hull Speed And Its Determination appeared first on Marine Insight - The Maritime Industry Guide

A Comprehensive Guide to Marine Sextant – Principles, Usage, and Maintenance

Shilavadra Bhattacharjee — Sun, 29 Jan 2023 04:29:50 +0000

The sextant is a valuable instrument used to determine the angle between the horizon and a celestial body like the Sun, Moon or Star. It is used in celestial navigation to find out the latitude and longitude.

Sextant derives its name from the Latin word’ sextus; or ‘one-sixth’, as the sextant’s arc spans 60° or one-sixth of a circle. Octans with 45° arcs were initially used to determine the latitude. However, Sextants were developed with wider arcs to calculate longitude from lunar observations. They replaced octants by the latter half of the 18th century.

It consists of an arc of a circle marked in degrees. It also has a movable radial arm pivoted at the circle’s centre. There is a telescope mounted to the framework, which is lined with the horizon.

A mirror is placed on the radial arm. It is moved or adjusted until the celestial body is reflected into a half-silvered mirror in line with the telescope and appears to coincide with the horizon through the telescope.

The angular distance of the celestial body or star above the horizon is read from the graduated arc of the sextant.

Mainly used at sea, the tool is so named because its arc is one-sixth of a circle – 60 degrees. It adheres to the principle of double reflection hence it can measure angles up to 120 degrees. Practically speaking, the arc of the sextant is a little over 60 degrees, and therefore the total angle measurable is about 130 degrees.

Sextant is an essential tool for celestial navigation and is also used by mariners to measure the angle between the horizon and a visible object (or two objects at sea).

Hold the sextant vertically and point it in the direction of the celestial body. See the horizon through an unsilvered part of the horizon mirror. Continue to move or adjust the index arm until the image of the star/sun, which has been reflected by the index mirror and then by the silvered portion of the horizon mirror, seems to rest on the horizon.

The altitude of the celestial body can be determined by reading from the scale on the arc of the sextant’s frame.

The sextant is used to measure the following:

Vertical Sextant Angle (VSA)
Horizontal Sextant Angle (HSA)
Altitudes

Brief History Of Sextant

A ship’s altitude above the horizon was related directly to the ship’s latitude. Mariners began to invent tools for measuring these factors to aid in navigation. One of the simplest was the kamal used by Arab navigators from the 6th century onwards.

A 2-inch long rectangle board was used. A string with evenly spaced knots was attached to it. This arrangement was called a kamal. The navigator held the string using his teeth and moved this board farther from his body, aligning its bottom edge with the horizon and the top with the object, generally the Polaris or the north star.

The number of knots between the mouth and the board gave an idea of the relative height. Although kamal was quite useful, it was not precise enough and, by the 13th century, gave way to the astrolabe and the mariner’s Quadrant.

The Quadrant was popular with Portuguese explorers that travelled south along the African coast to search for a route to the Orient.

When the seafarers reached close to the equator heading south, Polaris disappeared below the horizon. Hence, in the southern seas, mariners used another way to find their latitude. Per instructions from Prince Henry of Portugal, by 1480, Portuguese astronomers had found a way to determine the latitude using the position of the Sun when it moved north and south of the equator with changing seasons, what we now refer to as its declination.

To put it simply, the navigator could calculate the Altura or altitude and latitude by using his Quadrant to take the altitude of the Sun when it came to its highest altitude at local noon and then make a correction for the position of the sun north or south of the equator per the date.

Columbus used it extensively on his voyages to the New World. He marked off the latitudes of places he visited, such as Lisbon, Serra Leoa, Cabo Verde and other places he might have landed.

Also, it was common for navigators during those times to record the altitude of the Polaris in degrees at ports where they wished to return again. Hence, lists of alturas of many ports were published to guide the seafarers up and down the coasts of Africa and Europe.

Principle of the Sextant

When a ray of light is reflected by a plane mirror, the angle of the incident ray is equal to the angle of the reflected ray; when the incident ray, reflected ray and the normal lie on the same plane
When a ray of light suffers two successive reflections in the same plane by two plane mirrors, the angle between the incident ray and the reflected ray is twice the angle between the mirrors

Different Parts Of A Sextant

A sextant is shaped in the form of a sector (60 degrees or 1/6th of a circle). It is the reason the navigational instrument is called a Sextant (the Latin word for 1/6th is Sextans). The sector-shaped part is called the frame.

A horizontal mirror is attached to the frame, along with the index mirror, shade glasses (sunshades), telescope, graduated scale and a micrometre drum gauge.

How Does A Sextant Work And How To Use It?

Watch this video to understand how to use a sextant.

Navigation Sextant – Readings ON and OFF the arc

The normal graduations of the arc, to the left of zero, extending from 0 to 130 degrees, are referred to as ON the arc. To the right of 0 degrees, the graduations extend for a few degrees and are referred to as OFF the arc. When reading OFF the arc, graduations of the micrometer should be read in the reverse direction (59 as 1′, 55 as 1′ and so on).

Errors of the Sextant

The errors can be classified as

1. Adjustable Errors (adjustable onboard), and

2. Non-adjustable Errors (not adjustable onboard)

Adjustable Sextant Errors

The Perpendicularity error : This is caused when the index glass is not perpendicular to the plane of the instrument. To check for this, clamp the index bar about the middle of the arc, and holding the sextant horizontally, with the arc away from you, look obliquely into the index mirror till the arc of the sextant and its reflection on the index mirror is simultaneous. If in alignment, the error does not exist. If not, turn the adjustment screw at the back of the index glass until they are aligned.

Side Error: This is caused by the horizon glass not being perpendicular to the plane of the instrument. Clamp the index bar at 0 degrees 0.0′. Hold the sextant vertically and look at the heavenly body. Turn the micrometre one way and then the other while looking at the body. The reflected image of the body will move above and below the direct image and should pass exactly over it. If the reflected image passes to the left or right of the direct image, a side error exists. This error can be removed by turning the second adjustment screw (the top screw behind the horizon glass) until the true and reflected horizons appear in the same line.

Index Error: This is caused if the index mirror and the horizon glass are not exactly parallel to each other when the index is set at 0 degrees 0.0′. Basically, this is the difference between the optical zero of the sextant and its graduated zero, termed OFF the arc if the optical zero lies to the right of the graduated zero and termed ON the arc if the optical zero lies to the left of the graduated zero. There are three methods of obtaining the index error of a sextant:

A) By observing the horizon: Clamp the index at 0 deg 0.0′ and, holding the sextant vertical, look at the horizon. The reflected image and the direct image should appear in a perfect line. If not, turn the micrometer until they coincide exactly. The reading of the micrometre, ON or OFF the arc, gives the IE

B) By observing the star or planet: Clamp the index at 0 deg 0.0′ and holding the sextant vertical, look at the star/planet. The reflected and direct image must coincide. If not, turn the micrometer till they do. The reading of the micrometre, ON or OFF the arc, gives the IE

C) By observing the Sun: Set the index at about 32′ ON the arc. Hold the sextant vertically and look at the Sun, using shades. The reflected image of the Sun would appear below the direct image. Turn the micrometer until their closer limbs just touch. Note reading ON the arc.
Set the index at about 32′ OFF the arc and look at the Sun. The reflected image of the Sun would appear above the direct image. Turn the micrometer until their closer limbs just touch. Note reading OFF the arc.
The name of IE is the name of the reading having a higher numerical value.

The error of Collimation: This is due to the axis of the telescope not being parallel to the plane of the instrument. The telescope is attached to the sextant in such a manner that it cannot tilt. These modern sextants are, therefore, not provided with any collimating screws

Non-Adjustable Errors Of Sextant

Graduation Error: Due to the inaccurate graduation of the main scale on the arc or of the micrometre/vernier

Centring Error: Caused if the pivot of the index bar is not situated at the geometric centre of the arc. This can be caused due to a manufacturing defect or due to careless handling.

Shade Error: The shades should be so mounted that their glass surfaces are normal to the rays of light passing through them. If not, the distortion would result. The greater number of shades used, the greater the chances of distortion.

Optical Errors: Caused by prismatic errors of the mirrors or aberrations in the telescope lens

Wear on the rack and worm: This causes a backlash, leading to inconsistent errors. Wearing down of the worm can be due to lack of lubrication, the presence of dust particles, careless handling.

Dip

This is the angle at the observer between the plane of the observer’s sensible horizon and the direction of his visible horizon. A dip occurs because the observer is not at sea level. The value of the dip increases as the height of the eye of the observer increases. The values of dip are given on the cover page of the nautical almanac and in nautical tables (Nories) as a function of the height of the eye.

Pointers on the use of a sextant

Always check the errors before use
Focus the telescope while looking at the horizon and make a mark on the circumference of the stem
During use, hold the sextant steady. For this, stand with feet slightly apart for balance with hands holding the sextant steady
While observing the altitude of a celestial body, remember to swing the sextant to the other side; the body will appear to move along the arc. Measure the altitude at the lowest point on this arc
Stand as close as practicable to the centerline of the ship
Use appropriate dark shades while observing the Sun
If a backlash error exists, remember to rotate the micrometer in one direction only
Altitudes of stars and planets should be taken during twilight
Nighttime sextant observations should be avoided as far as practicable. The strong moonlight gives the illusion of a good horizon which is most probably false
While observing the HSA, set the index at zero, look at the object on the right through the telescope, gradually swing the index around and finish while facing the object on the left
When measuring VSA, look at the top of the object, set the index at zero and look at the top of the object. VSA = height of the object in meters
1852 X Tan VSA

Care and maintenance of a sextant

Do not put too much stress on the index bar when grasping a sextant
Never touch the arc. It will smear it. These aren’t oleophobic per se
Ensure that the worm and rack are clean
Coat worm and rack with Vaseline when not using it for too long
Mirrors, lenses and shades should be wiped clean with a soft cloth
After each use, gently wipe the index mirror, horizon glass
Put it in the box when not using it
Do not bump the sextant anywhere
Avoid exposure to sunlight
Keep sextant stowed away from direct sunlight, dampness, heaters or blowers

The sextant is an expensive, precision instrument which should be handled with utmost care.

Reference: Principles of Navigation by Capt. Joseph & Capt. Rewari, The Marine Sextant by Capt. H. Subramaniam

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A Comprehensive Guide to Marine Sextant – Principles, Usage, and Maintenance appeared first on Marine Insight - The Maritime Industry Guide