Buoyancy

Buoyancy is that quality whereby a ship, or any other floating body, is enabled to support a certain weight. In the case of a ship, it is necessary that such weight should be carried without her sinking too deeply in the water, or floating too lightly on it. The weight of a ship, not loaded with any cargo, is exactly equivalent to the weight of the volume of water she displaces (see HYDROSTATICS). Therefore, given a certain draught-line to which a ship is to be loaded, multiply the number of cubic feet of the volume of the immersed part by the weight of a cubic foot of sea-water (64 lb.), and the product will be the weight of water displaced by the ship at the given draught-line. If from this the weight of the ship herself be subtracted, the residue is the amount of extra weight, or cargo she is capable of carrying at that draught-line, and is a measure of her quality of buoyancy.