Big ship stability.

Appleyard

Appleyard

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Apropos the Costa Concordia accident,Someone mentioned that a large cruise type ship is less stable in shallow water .

I can't see how the depth of water makes any difference,the upthrust must surely be the same whether there is 1 metre or 100 metres under the keel.

Have I picked this up wrongly?
 
jamesmar said:
Apropos the Costa Concordia accident,Someone mentioned that a large cruise type ship is less stable in shallow water .

I can't see how the depth of water makes any difference,the upthrust must surely be the same whether there is 1 metre or 100 metres under the keel.

Have I picked this up wrongly?
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I understand that whilst under way at speed the automatic stabiliser fins use transfer the fwd motion into a corrective balancing force. Without speed under way that will be diminished or lost, and in shallow waters they are not at speed.
 
chewi said:
I understand that whilst under way atspeed the automatic stabiliser fins use transfer the fwd motion into a corrective balancing force. Without speed under way that will be diminished or lost. and in shallow waters they are not at speed.
Click to expand...

I think that maybe true but the depth of water would not really make any difference as I understand it.
 
Are you saying they continued at significant if not full speed? If not then surely their stability would be noticeably reduced?
 
From today's Telegraph

This was what I was referring to

He added: “A ship is designed to float in a certain depth of water. An ocean cruise ship is not designed to float in 20ft of water. It needs much more than that to remain upright."
Surely as long as there is water under the keel, it will stay upright.?
 
provided it is within its design constraints, but a very big hole down one side isn't.
you can flood the other side to restore the balance, but surely not as fast as that hole in the concordia.
As the ship heels, the rectangular underwater section changes to a triangle, and the centre of buoyancy moves towards the deeper side more than the centre of mass above it, and so pushing it back upright.

If that triangle is not buoyant due to flooding in some sections, that effect will be diminished , and if flooding is extensive, lost altogether, the centre of mass will move more than the centre of buoyancy. It will then capsize.
Water under the keel will not stop it.
 
jamesmar said:
Apropos the Costa Concordia accident,Someone mentioned that a large cruise type ship is less stable in shallow water .

I can't see how the depth of water makes any difference,the upthrust must surely be the same whether there is 1 metre or 100 metres under the keel.

Have I picked this up wrongly?
Click to expand...

"Upthrust" isn't the same as stability. Stability is about resisting any force tending to tilt the ship from vertical (e.g wind, waves). Not that it should be influenced by depth, though.

On the other hand it is well known that waves will increase in height when approaching shallows, which will be more of a challenge to stability of any vessel.

Mike.
 
jamesmar said:
This was what I was referring to

He added: “A ship is designed to float in a certain depth of water. An ocean cruise ship is not designed to float in 20ft of water. It needs much more than that to remain upright."
Surely as long as there is water under the keel, it will stay upright.?
Click to expand...

What was being said, not terribly clearly, is thay once the ship is aground, which she certainly will be in 20ft of water, she will no longer be stable, because of the transfer of weight from the water to the ground.
 
Minn said:
What was being said, not terribly clearly, is thay once the ship is aground, which she certainly will be in 20ft of water, she will no longer be stable, because of the transfer of weight from the water to the ground.
Click to expand...
Level ground, and a flat bottom to the hull would surely give maximum stability!
 
Most ships bottoms are flat and probably all passenger ships have cross flooding ducts to ensure as much of the water is spread evenly about the compartment as can be. The ship will always sink but the theory is that it will sink upright and allow the lifeboats to be used.
The fact that the vast majority got to safety, and the boats were able to be lowered is a good reflection on the training of the crew and the design of the ship, regardless as to why it happened.
Rgds
Bob
 
Looking at the cross-section of a typical cruise ship, I can't help thinking that the angle of vanishing stability must be around 45 degrees or so. Past that, and there is little chance of launching lifeboats as she rolls over. I fear that it is only a matter of time before one encounters seas huge enough to roll the ship- the resultant disaster doesn't bear thinking about.
 
From the survivor accounts it sounds as if free surface effect was the problem. Perhaps the bottom tanks and engine rooms were not separated properly.


And lifeboats. Not beyond the wit of man (or even naval architects) to work out a system of lowering them at great angles and filling them with Pax ?
 
tobermoryphil said:
Looking at the cross-section of a typical cruise ship, I can't help thinking that the angle of vanishing stability must be around 45 degrees or so. Past that, and there is little chance of launching lifeboats as she rolls over. I fear that it is only a matter of time before one encounters seas huge enough to roll the ship- the resultant disaster doesn't bear thinking about.
Click to expand...

The roll on this one seems quite extreme (I think the port prop cavitates at one point:eek:) and yet she bobbed back up:

 
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