Displacement (ship)

The process of determining a vessel's displacement begins with measuring its draft. This is accomplished by means of its "draft marks". A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern. To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1,025 kg/m3) is more dense than fresh water (1,000 kg/m3); so a ship will ride higher in salt water than in fresh. The density of water also varies with temperature. Devices akin to slide rules have been available since the 1950s to aid in these calculations. Presently, it is done with computers. Displacement is usually measured in units of tonnes or long tons. ==Definitions==

, left, and USS Abel P. Upshur are destroyers of comparable size, but because the latter is more heavily loaded, it sits lower, displacing more water. There are terms for the displacement of a vessel under specified conditions: Loaded displacement • Loaded displacement is the weight of the ship including cargo, passengers, fuel, water, stores, dunnage and such other items necessary for use on a voyage. These bring the ship down to its "load draft". Warships have full load condition established through the Naval design process, and are exempt from commercial requirements laid out by flag state laws. Normal displacement • Normal displacement is the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board." Standard displacement • Standard displacement, also known as "Washington displacement", is a specific term defined by the Washington Naval Treaty of 1922. "It is the displacement of the ship complete, fully manned, engined, and equipped ready for sea, including all armament and ammunition, equipment, outfit, provisions and fresh water for crew, miscellaneous stores, and implements of every description that are intended to be carried in war, but without fuel or reserve boiler feed water on board." ==Gallery==

Image:Archimedes principle.svg|A floating ship's displacement Fp and buoyancy Fb must be equal. Image:Archimedes bath.jpg|Greek philosopher Archimedes having his famous bath, the birth of the theory of displacement Image:CurvasCarenaDerecha.PNG|A ship's hydrostatic curves. Lines 4 and 5 are used to convert from mean draft in meters to displacement in tonnes (table in Spanish). ==See also==