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, 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 of ships (see: Hull (watercraft), load line and Stability conditions (watercraft). The displacement of a vessel is equal to the weight of water it displaces when afloat.)

Vessels such as naval ships and icebreakers are often measured by their displacements. The unit of measure can be long tons or metric tons depending on the country of origin. The ship can be measured in light condition, fully loaded, or normal (usually fully loaded, but with about two-thirds of fuel and unconsumables). For official purposes, the Washington Naval Treaty introduced the standard displacement, which was the displacement fully loaded but with no fuel or reserve feed water.