Stability of ships

Stability of Ships

Ships float because they displace a lot of water - this is discussed on another Page of this Web Site. If you have not already read this page it might be helpful to do so before you read the rest of this page. To read it please click here

Most ships are longer than they are wide and approximately symmetrical about the centre line.

We can consider the weight of the ship to be a single force acting through a single point, its centre of gravity, and the buoyancy of the water to be another single force acting through another single point, the centre of buoyancy. The centre of buoyancy is the point which would have been the centre of gravity of the water which the ship has displaced. For submarines, whether on the surface or submerged, the centre of buoyancy is usually above the centre of gravity, but for most surface ships the centre of gravity is above the centre of buoyancy.

Single-hulled ships are normally designed so they float vertically and with the centre of gravity and centre of buoyancy on the centreline. If the ship now tips even slightly to one side but the centre of buoyancy does not change the weight and buoyancy will now act in a way which makes the ship want to tip over further - the ship is unstable and will probably capsize and sink.
Unstable ship

However in practice as the ship tips the centre of buoyance will always change and, depending on the design of the hull, one of four things will happen

The ship might reach another position in which the centres of gravity and buoyancy are in a vertical line, but the ship itself is not vertical - this is called listing. A ship will usually list if it has taken on water because as the ship tips the water will move to one side and the centre of gravity of the ship will move. Most ships are built with special watertight compartments to minimise this effect.
It may tip over until the top of one side of the hull is under water - this is called capsizing. At this point it may start to fill with water. A capsized ship is in danger of sinking.
The ship might turn right over. This is called turning turtle, because the upturned hull of a small boat looks like the shell of a turtle. If air is trapped inside the hull of a ship that has turned turtle it may stay afloat for a very long time - several films, including The Poseidon Adventure, have been made on this theme. A small wooden canoe or rowing boat or sailing dingy may turn turtle quite easily, but a big ship is much more likely to capsize and sink than turn turtle.
As the ship tips the centre of buoyancy may move in a way which tends to make the ship return to an upright position (right itself) - this is the way most ships are designed to act. As it rights itself it may overshoot and tip the other way, and it will then continue to tip from one side to the other. This is called rolling - rolling is an example of Simple Harmonic Motion (SHM)

Most ships roll to some extent, and roll more when the sea is rough. Some people may be seasick if the ship is rolling a lot, and passenger ships are often fitted with stabilisers to reduce rolling.

A more complete explanation of SHM is given in another page of my Web Site - to link to it please click here

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The best way to investigate stability is to do some experiments.

You will need a bowl of water, a disposable plastic cup and a milk bottle, and any other containers, tins, jamjars, test tubes, bottles etc. you can find. I am not giving you any other instructions, but you can e-mail me with your questions, ideas and results if you like.

You may find that some containers do not float upright unless you put a small amount of water into them - this is called ballast. Ballast changes the position of the centre of gravity and also, by making the ship lower in the water, the centre of buoyancy. Most ships carry ballast to make them more stable. Fixed ballast is usually in the form of blocks of iron or lead; moveable ballast is usually in the form of tanks of water which can be filled or emptied.

Lifeboats need to go to sea in very difficult conditions and are designed to right themselves if they capsize or turn turtle: to see some pictures of self-righting lifeboats please click here

Catamarans and other multi-hulled ships and outrigger canoes etc are very stable because their design is such that any sideways tipping will always produce a very large righting effect.

Ballast and ecology

Many modern merchant ships are designed to carry just one type of cargo: petroleum (crude oil), grain, bananas or new cars for example. So they have to make the return journey without any cargo, and need to carry lots of water as ballast. This water is taken in at one port and discharged at another, and this is creating very real problems of ecology, because many small animals such as jellyfish and crabs can be carried in this water and so moved to areas where they are not native. Without any natural predators they may then reproduce uncontrollably and this can have a devastating effect on the native wildlife.