The displacement or displacement tonnage of a ship is its

weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar qua ...

. As the term indicates, it is measured indirectly, using

Archimedes' principle Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes' ...

, 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,

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...

s 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.

Calculation

The process of determining a vessel's displacement begins with measuring its

draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vesse ...

.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 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 rule The slide rule is a mechanical analog computer which is used primarily for multiplication and division, and for functions such as exponents, roots, logarithms, and trigonometry. It is not typically designed for addition or subtraction, which ...

s 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

s or long tons.

Definitions

The Arrival of the First Flotilla of Destroyers From America To the Royal Navy, Devonport, September 1940 A724

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 Dunnage is inexpensive or waste material used to load and secure cargo during transportation; more loosely, it refers to miscellaneous baggage, brought along during travel. The term can also refer to low-priority cargo used to fill out transport ca ...

and such other items necessary for use on a voyage. These bring the ship down to its "load draft", colloquially known as the "

waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...

". *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 A ship classification society or ship classification organisation is a non-governmental organization that establishes and maintains technical standards for the construction and operation of ships and offshore structures. Classification societies ...

(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 The Washington Naval Treaty, also known as the Five-Power Treaty, was a treaty signed during 1922 among the major Allies of World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Nav ...

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

Image:Archimedes principle.svg, A floating ship's displacement ''F''_''p'' and buoyancy ''F''_''a'' 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).

References

Bibliography

* * * . * * * * * * {{DEFAULTSORT:Displacement (Ship) Water transport Shipbuilding Nautical terminology Ship measurements