Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents). It is usually applied to naval vessels, and measured when the ship's fuel tanks are full and all stores are aboard.
Displacement measures weight of a volume of water displaced, not the vessel's absolute weight in pounds put on a scale. It should not be confused with other measurements of volume or capacity typically used for commercial vessels such as net tonnage, gross tonnage, or deadweight tonnage.
A variety of terms denoting degree of displacement as a function of load are defined below.
The traditional method of determining a vessel's displacement uses draft marks (also known as "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%.
Displacement, however, is not absolute for a given vessel and load: it varies by the density of water being displaced. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³); thus a ship with a given mass (i.e. dead weight in kg) will displace less seawater than fresh. In practical terms, it will ride higher in salt water, lower in fresh.
Computers have been used to assist hydrostatic calculations such as determining displacement since the 1950s. These were originally manual, similar to slide rules which could convert cargo levels to values such as deadweight tonnage, draft, and trim. Digital computers employ programs to make the calculations.
A variety of terms are used to describe varying degrees of displacement under specific loads:
- 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 to skew it in favor of Great Britain and the United States. 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.
- The omission of fuel and water was to avoid penalizing the British, who had great global commitments and required greater fuel loads, and especially the United States, which had global commitments almost as great but with fewer bases to provide fueling than the Royal Navy.
- Dear and Kemp, 2006, p.588
- George, 2005, p. 68.
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