The displacement or displacement tonnage of a ship is the ship's weight. The name reflects the fact that it is measured indirectly, by first calculating the volume of water displaced by the ship, and then calculating the weight of that water. By Archimedes' principle, this is also the weight of the ship.
The process of determining a vessel's displacement begins with measuring its draft This is accomplished by means of its "draft marks" (or "load lines"). A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern. These marks allow a ship's displacement to be determined to an accuracy of 0.5%.
The draft observed at each set of marks is averaged to find a mean draft. The ship's hydrostatic tables show the corresponding volume displaced.
To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³); so a ship will ride higher in salt water than in fresh. The density of water also varies with temperature.
There are terms for the displacement of a vessel under specified conditions:
- 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", colloquially known as the "waterline".
- Full load displacement and loaded displacement have almost identical definitions. Full load is defined as the displacement of a vessel when floating at its greatest allowable draft as established by classification societies (and designated by its "waterline"). Warships have arbitrary full load condition established.
- Deep load condition means full ammunition and stores, with most available fuel capacity used.
- Light displacement (LDT) is defined as the weight of the ship excluding cargo, fuel, water, ballast, stores, passengers, crew, but with water in boilers to steaming level.
- Normal displacement is the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."
- Standard displacement, also known as "Washington displacement", is a specific term defined by the Washington Naval Treaty of 1922. It is defined as 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.
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