Big ship stability.

[Appleyard]

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?

 

[Monique]

Basic matey. You are correct.

Whoever spouted the thought needs some basic knowledge of basic physics.

 

[chewi]

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?

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.

 

[sarabande]

Regret that the basic physics are right. Read up on "ship squat".


http://www.ship-squat.com/shallow_water_signs.htm


Now I am not saying that CC was influenced by squat....

 

[Reverend Ludd]

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.

I think that maybe true but the depth of water would not really make any difference as I understand it.

 

[Monique]

Regret that the basic physics are right. Read up on "ship squat".


http://www.ship-squat.com/shallow_water_signs.htm


Now I am not saying that CC was influenced by squat....

I agree with your site.... if the ship is moving. Otherwise, buoyancy = weight.

 

[chewi]

Are you saying they continued at significant if not full speed? If not then surely their stability would be noticeably reduced?

 

[Appleyard]

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.?

 

[chewi]

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.

 

[mjcoon]

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?

"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.

 

[Kukri]

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.?

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.

 

[[32511]]

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.

Level ground, and a flat bottom to the hull would surely give maximum stability!

 

[Kukri]

Yes, that is perfectly true as far as it goes, but ships bottoms are not quite flat and the seabed is not always flat either. Imagine a flat bottomed ship sitting on a rock pinnacle - will she tip over when the water level falls...

 

[RichardS]

will she tip over when the water level falls...

.....not in the Med though.

Richard

 

[Kukri]

Yes; that does tend to complicate salvage operations on grounded ships in the Med!

But a ship travelling at any speed when she grounds will tend to sue up out of her draft; the effect is the same.

 

[chiefeng]

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

 

[[32511]]

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.

 

[Kukri]

For the record, the Carnival Group's design office is in London and employs a couple of hundred naval architects, who are very well respected. The design looks to me to have been proven, not the reverse.

 

[sarabande]

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 ?

 

[Boomshanka]

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.

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