Displacement (fluid)

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In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place, so that it can be weighed.

An object that sinks also 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.

Displacement is used as a measure of the weight/size of naval ships, as cargo carrying capacity is not a factor. The displacement of a vessel is defined as the weight of water it displaces when afloat. The size of a naval ship typically is reported as the number of long tons (a larger British ton) as opposed to regular, or short tons (a smaller American ton.) All ships use the long ton measurement because it was agreed upon as part of international treaty settlements at the end of the First World War (see below.) However, this measurement can be highly variable depending upon the loaded condition of the ship and the state of the water. Displacement is normally measured when the ship is in the water immersed to her load line. But because there is more than one load line (Summer, Winter, North Atlantic, etc), and also because the water condition will vary (temperature, density, salinity, etc.) ship displacement is an approximate measure. Merchant ships use the measurement of volume capacity, which is a fixed measurement suitable for cargo, passengers, and how rates are charged.

It should be noted as an exception, that while a ship pushing its way through the water has a displacement hull supported entirely by displacement, a hydroplaning hull gliding over the surface of the water is partially supported by hydrodynamic forces caused by the motion of the hull across the water, a hydrofoil, which lifts the hull with the use of underwater wings. This force will counteract the weight of the vessel