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 rather than commercial ones, and is measured when the ship's fuel tanks are full and all stores are aboard.〔Dear and Kemp, 2006, p.588〕〔George, 2005, p. 68.〕
Displacement 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〔George, 2005. p.5.〕 (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%.〔
The method is to average the individual drafts to find a mean draft.〔George, 2005. p.14–15.〕 It is then entered into the ship's hydrostatic tables, giving a displacement.〔George, 2005. p. 465.〕
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 denser than fresh water (1000 kg/m³);〔Turpin and McEwen, 1980.〕 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.〔George, 2005. p. 262.〕
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