Buoyancy

Buoyancy is the ability to float. If the average density of an object is less than that of a fluid it floats in the fluid, at such a level that the weight of the object is equal to the weight of the displaced fluid, i.e. of an amount of fluid equal to that which is absent below the surface level of the fluid due to the presence of the object.

Objects of a material of higher density than the fluid, e.g. a metal object in water, can still float if it has a suitable shape that allows keeping air below the surface level of the fluid. In that case, for the average density mentioned above, the air is counted also, which may reduce this density to less than that of the fluid.

This is the principle of vessels such as boats and ships. discovered by Archimedes.

See also Hull (ship), Naval architecture.