Displacement

Displacement is the weight of a ship, and it takes its name from a principle of physics: a floating body pushes aside, or displaces, a volume of water whose weight exactly equals the weight of the body. So the weight of the water a ship displaces is the same as the weight of the ship itself, together with everything it is carrying at that moment. Displacement is therefore a true weight, measured in tonnes, and it is the fundamental quantity from which many of the ship's other characteristics are derived.

Displacement changes with how the ship is loaded, so it is usually quoted for particular conditions. The lightship displacement is the weight of the empty ship as built (the hull, machinery and permanent equipment) with no cargo, fuel, stores or crew. The loaded displacement is the weight when the ship is floating at its maximum permitted draught, fully laden to its marks. The difference between the two is the deadweight, the total weight of cargo, fuel, water, stores and everything else the ship can carry. This relationship (loaded displacement minus lightship equals deadweight) is central to understanding a ship's capacity.

Displacement should not be confused with the tonnage measures used for regulation and dues. Gross and net tonnage are measures of internal volume, not weight, and are dimensionless figures used for manning, safety requirements and port charges. Deadweight is a weight, but it is carrying capacity, not the total weight of the ship. Displacement alone gives the actual weight of the whole vessel. It is important in naval architecture and in operation: it is used in stability calculations, in working out draughts, and in the draught survey method by which the weight of bulk cargo loaded is determined by measuring the change in the ship's displacement before and after loading. Displacement is thus a cornerstone concept in how ships are designed, loaded and operated.