Displacement_(fluid)

In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced (the volume of the immersed object will be equal to the volume of the displaced fluid).

An object that sinks displaces an amount of fluid equal to the object\'s volume. Thus buoyancy is expressed by Archimedes\' Principle which states that when this happens, the weight of the object is reduced by its volume times the density of the fluid. If the weight of the object is less than this quantity, it will float, if more it will sink. The amount of fluid displaced is directly related (via Archimedes\' Principle) to its weight.

In the case of an object which sinks the amount of fluid displaced will be equal in volume to the immersed object, whereas in the case of an object which floats the amount of fluid displaced will be equal in weight to the displacing object.

Ship Displacement

Displacement is used as a measure of the weight of ships (see: Hull (watercraft), load line and Stability conditions (watercraft). Displacement is a measure of the weight of water that a vessel displaces, including some, all, or none of its contents depending on the definition. Vessels such as naval ships and icebreakers are often measured by their displacements. It is usually expressed in long tons or in metric tonnes. It is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.)

Displacement is a measure of weight and should not be confused with tonnage, which is a measures of volume, and is usually used to express the cargo carrying capacity of various types of merchant ships.

Standard displacement

Weight or 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 feed water on board." (Definition of "standard displacement" given in the Washington and London Naval Treaties).

Normal displacement

The same as standard displacement, but with two-thirds supply of stores and fuel (the United States Navy definition). Can also include all fuel, oils/lubricants and water. This is regarded as an average operational displacement.

Full load displacement

Weight/displacement of the fully loaded ship, including all stores, supplies, munitions, personnel and(unlike standard tonnage) fuel and reserve feed water.

See also

• Naval architecture
• Hull (watercraft)
• hydrodynamics

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