Plimsoll lines - Question

[SophiaRivera007]

I have read about plimsoll lines, while loading a ship, care is taken it should not be overloaded. Its load depends on density of the see in which it floats. the ship will shink if its load exceeds.

 

Now my question is how to indicate depth up to which a ship can shink? And at what density value? which ensures stabilty and safety of

the ship.

Edited January 2, 2017 by SophiaRivera007

[Sriman Dutta]

All these things are based on Archimedes' Principle, which states that when a solid body is immersed in a fluid, then it experiences an upward thrust which is equal to the weight of the volume of the fluid displaced by the immersed part of the solid body.

 

The depth upto which a ship can be sinked is determined by some small calculations based on the above principle.

[imatfaal]

The modern loadline - the descendent of the plimsoll line is quite complex at first glance but easy if you look hard.

 

There are two sections - a roundel with a horizontal line through it with two letters (AB, LR, NV etc.). The letters tell you which Authority has classified the ship as safe and measured her safe load (American Bureau of Shipping, Lloyds Register, Det Norske Veritas etc.) The Horizontal line tells you where the sea will come up to when the ship is in a safely fully-loaded situation in salt water and in the summer. This is known as the Summer draft - this is the line from which the others are calculated.

 

Beside the roundel will be a branched symbol - this shows the level of the sea when the ship is in a safely fully-loaded condition in other sea conditions. All the other marks are calculated such that if a vessel is loaded to her summer draft and she goes into other situations (with a different density water) then the sea will come up to the other marks - and vice versa.

 

The Marks are as follows - on the left of the tree at the very top is TF for tropical fresh; this water is warm and has no salt in it. Below that is F Fresh water; no salt but not warm. On the right of the tree at the top is T for Tropical, and going downwards S Summer, W Winter, and the lowest line WNA Winter North Atlantic. Tropical fresh water is the least dense - it is warm and has no salt in it. Winter North Atlantic is the most - it is very cold and salty.

 

If the vessel is fully loaded in Arctic waters (ie the water is at the winter north atlantic marks WNA) when she gets to the warmer waters she will be slightly lower in the water (such that the water now reaches her Summer marks, if she then made a river passage into say Manaus she would be much lower in the water such that she was now riding with her Tropical Fresh Marks at the waterline.

 

The summer loadline is calculated by the shipyard which built the ship, the designers who laid the plans, the owners, and most importantly a group called the Classification Society. The Classification Society (the "Class") is an internationally recognized organisation who vet plans for ship, check the build process, survey the new ships for quality, and regularly inspect and survey ships during their trading life. The other loadlines are calculated using simple physics and agreed standards for the density and temperature of water in various parts of the world.

 

You ask about the stability of the ship - this has very little to do with the loadline; the stability depends on the overall design and most importantly the cargo plan - ie where the mass is. Basically, you have two centres of flotation; the centre of gravity and the centre of buoyancy - ideally you want the centre of gravity to be a little below the centre of buoyancy. This area is the purview of the Master Mariner (again with recommendations and rules from the Class) - a badly loaded ship will easily sink or break apart at the first sign of bad weather or adverse sea conditions

[Sriman Dutta]

thanks for the info imatfaal

[Ophiolite]

Slightly off-topic, but remaining on the subject of ship loss, I find that most people are unaware of the number of ships lost on a regular basis. These attract less attention than air-crashes, since most of the vessels are cargo ships. Not many of us travel routinely by cargo ship.

 

Illustrative of the issue is this on-line Guardian article from 2015, "Large ship losses at lowest rate in decade, report shows". And that new low rate, for 2014? "Only 75 went down at sea during 2014 but losing bigger ships could could cost (up to) $1bn each."

 

The article also notes, "Between 2004 and 2014, 1,271 ships have been lost."

[imatfaal]

Slightly off-topic, but remaining on the subject of ship loss, I find that most people are unaware of the number of ships lost on a regular basis. These attract less attention than air-crashes, since most of the vessels are cargo ships. Not many of us travel routinely by cargo ship.

 

Illustrative of the issue is this on-line Guardian article from 2015, "Large ship losses at lowest rate in decade, report shows". And that new low rate, for 2014? "Only 75 went down at sea during 2014 but losing bigger ships could could cost (up to) $1bn each."

 

The article also notes, "Between 2004 and 2014, 1,271 ships have been lost."

 

Indeed. Samuel Plimsoll saved countless lives and we wish all safety measures were so easy to implement and so simple for coastguards etc to check.

 

Your figure of 75 ships will - tragically - be only the large ocean going vessels which are flagged in good flag states and classed reputably. Around the world the ghosts of overloaded Newcastle Coal Carriers still haunt us; big ships on the international trade routes are well organised, the crews unionised, and overall they are safe; but small coastal traders, fishing boats, and passenger ferries can still be death-traps and sink on a regular basis.

 

We raise a glass to Samuel Plimsoll - they even named a shoe after him (true)!

[HB of CJ]

The load or cargo must be arranged not to create areas of increased stress on the fabric of the vessel. It must be evenly loaded. The cargo must not shift. The center of balance must be within designed limits of the ships construction. If the center of gravity is too low, the ship will roll and pitch. If the center of gravity is too high, the ship might tip over. Consumables such as fuel oil usage must be considered. The static loads are easy to figure out. It is the dynamic loads like passengers moving about the ship that sometimes can create concerns.

 

Winter North Atlantic is usually the lightest laden. Stormy seas. Other oceans and seasons usually less but not always. Typhoons and Hurricanes during seasons are exceptions. Salt water is denser: the ship rides higher. Fresh water less dense; the ship rides lower. A good Engineer always try to keep 2 steps ahead of possible conditions. Sometimes he fails.

[SophiaRivera007]

The modern loadline - the descendent of the plimsoll line is quite complex at first glance but easy if you look hard.

 

There are two sections - a roundel with a horizontal line through it with two letters (AB, LR, NV etc.). The letters tell you which Authority has classified the ship as safe and measured her safe load (American Bureau of Shipping, Lloyds Register, Det Norske Veritas etc.) The Horizontal line tells you where the sea will come up to when the ship is in a safely fully-loaded situation in salt water and in the summer. This is known as the Summer draft - this is the line from which the others are calculated.

 

Beside the roundel will be a branched symbol - this shows the level of the sea when the ship is in a safely fully-loaded condition in other sea conditions. All the other marks are calculated such that if a vessel is loaded to her summer draft and she goes into other situations (with a different density water) then the sea will come up to the other marks - and vice versa.

 

The Marks are as follows - on the left of the tree at the very top is TF for tropical fresh; this water is warm and has no salt in it. Below that is F Fresh water; no salt but not warm. On the right of the tree at the top is T for Tropical, and going downwards S Summer, W Winter, and the lowest line WNA Winter North Atlantic. Tropical fresh water is the least dense - it is warm and has no salt in it. Winter North Atlantic is the most - it is very cold and salty.

 

If the vessel is fully loaded in Arctic waters (ie the water is at the winter north atlantic marks WNA) when she gets to the warmer waters she will be slightly lower in the water (such that the water now reaches her Summer marks, if she then made a river passage into say Manaus she would be much lower in the water such that she was now riding with her Tropical Fresh Marks at the waterline.

 

The summer loadline is calculated by the shipyard which built the ship, the designers who laid the plans, the owners, and most importantly a group called the Classification Society. The Classification Society (the "Class") is an internationally recognized organisation who vet plans for ship, check the build process, survey the new ships for quality, and regularly inspect and survey ships during their trading life. The other loadlines are calculated using simple physics and agreed standards for the density and temperature of water in various parts of the world.

 

You ask about the stability of the ship - this has very little to do with the loadline; the stability depends on the overall design and most importantly the cargo plan - ie where the mass is. Basically, you have two centres of flotation; the centre of gravity and the centre of buoyancy - ideally you want the centre of gravity to be a little below the centre of buoyancy. This area is the purview of the Master Mariner (again with recommendations and rules from the Class) - a badly loaded ship will easily sink or break apart at the first sign of bad weather or adverse sea conditions

Thank you so much for guide me.