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The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures in long tons. Today, tonnes are more commonly used.
Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage") to its maximum load. Numerous specific terms are used to describe varying levels of load and trim, detailed below.
Ship displacement should not be confused with measurements of volume or capacity typically used for commercial vessels and measured by tonnage: net tonnage and
The process of determining a vessel's displacement begins with measuring its draft.George, 2005. p.5. This is accomplished by means of its "draft marks" (or "load lines"). A merchant vessel has three matching sets: one mark each on the
There are terms for the displacement of a vessel under specified conditions:
Image:Archimedes principle.svg, A floating ship's displacement ''F''''p'' and buoyancy ''F''''a'' must be equal.
Image:Archimedes bath.jpg, Greek philosopher
The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures in long tons. Today, tonnes are more commonly used.
Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage") to its maximum load. Numerous specific terms are used to describe varying levels of load and trim, detailed below.
Ship displacement should not be confused with measurements of volume or capacity typically used for commercial vessels and measured by tonnage: net tonnage and gross tonnage
Gross tonnage (GT, G.T. or gt) is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage. Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weigh ...
.
Calculation
The process of determining a vessel's displacement begins with measuring its draft.George, 2005. p.5. 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
Port and starboard are nautical terms for watercraft and aircraft, referring respectively to the left and right sides of the vessel, when aboard and facing the bow (front).
Vessels with bilateral symmetry have left and right halves which are ...
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.George, 2005. p. 465.
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.George, 2005. p. 262.
Displacement is usually measured in units of tonnes or long tons.
Definitions
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", 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 approved by the load line assigning authority which is either the flag state (USCG etc) or a classification society (and designated by its "waterline").Department of the Navy, 1942. Warships have full load condition established through the Naval design process, and are exempt from commercial requirements laid out by flag state laws.Light displacement
*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.Military Sealift Command.Normal displacement
*Normal displacement is the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."United States Naval Institute, 1897. p 809.Standard displacement
*Standard displacement, also known as "Washington displacement", is a specific term defined by the Washington Naval Treaty of 1922.''Conference on the Limitation of Armament, 1922''. Ch II, Part 4. "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
Archimedes
Archimedes of Syracuse (;; ) was a Greek mathematician, physicist, engineer, astronomer, and inventor from the ancient city of Syracuse in Sicily. Although few details of his life are known, he is regarded as one of the leading scientists ...
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
* Naval architecture * Hull (watercraft) *Hydrodynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...
* Tonnage
References
Bibliography
* * * . * * * * * * {{DEFAULTSORT:Displacement (Ship) Water transport Shipbuilding Nautical terminology Ship measurements