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Displacement (ship)

The more heavily loaded a ship is, the lower it sits in the water. "Designated displacement" is a measurement of the weight of water a ship displaces when fully loaded and submerged to her load lines.[citation needed]

The displacement or displacement tonnage of a ship is the ship's weight. The name reflects the fact that it is measured indirectly, by first calculating the volume of water displaced by the ship, and then calculating the weight of that water. By Archimedes' principle, this is also the weight of the ship.

Displacement should not be confused with other measurements of volume or capacity typically used for commercial vessels such as net tonnage, gross tonnage, or deadweight tonnage.


Shipboard stability programs can be used to calculate a vessel's displacement

The process of determining a vessel's displacement begins with measuring its draft[1] 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.[1] These marks allow a ship's displacement to be determined to an accuracy of 0.5%.[1]

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.[2]

To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³);[3] 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. It is done today with computers.[4]

Displacement is usually measured in units of tonnes or long tons.[5]


Two similar destroyers berthed alongside each other. The one on the outboard is more heavily loaded and displaces more water.

There are terms for the displacement of a vessel under specified conditions:

Loaded displacement

Light displacement

Normal displacement

Standard displacement


See also


  1. ^ a b c George, 2005. p.5.
  2. ^ George, 2005. p. 465.
  3. ^ Turpin and McEwen, 1980.
  4. ^ George, 2005. p. 262.
  5. ^ Otmar Schäuffelen (2005). Chapman Great Sailing Ships of the World. Hearst Books. p. xix. 
  6. ^ a b Military Sealift Command.
  7. ^ a b Department of the Navy, 1942.
  8. ^ United States Naval Institute, 1897. p 809.
  9. ^ a b Conference on the Limitation of Armament, 1922. Ch II, Part 4.