The displacement or displacement tonnage of a ship is its

weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...

. 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,

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...

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 v ...

.George, 2005. p. 5. This is accomplished by means of its "draft marks". A merchant vessel has three matching sets: one mark each on the

port and starboard Port and starboard are Glossary of nautical terms (M-Z), nautical terms for watercraft and spacecraft, referring respectively to the left and right sides of the vessel, when aboard and facing the Bow (watercraft), bow (front). Vessels with bil ...

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

s 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". *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 Shipping, ships and Offshore platform, offshore structure ...

(and designated by its " load line").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

Image:Archimedes principle.svg, A floating ship's displacement ''F''_''p'' and buoyancy ''F''_''b'' must be equal. Image:Archimedes bath.jpg, Greek philosopher

Archimedes Archimedes of Syracuse ( ; ) was an Ancient Greece, Ancient Greek Greek mathematics, mathematician, physicist, engineer, astronomer, and Invention, inventor from the ancient city of Syracuse, Sicily, Syracuse in History of Greek and Hellenis ...

having his famous bath, the birth of the theory of

displacement Displacement may refer to: Physical sciences Mathematics and physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

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