Displacement (ship)

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.

[citation needed] Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage"[2]) to its maximum load.

Numerous specific terms are used to describe varying levels of load and trim, detailed below.

The process of determining a vessel's displacement begins with measuring its draft.

A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern.

Draft marks , by showing how low a ship is sitting in the water, make it possible to determine displacement.
Shipboard stability computer programs can be used to calculate a vessel's displacement.
In this 1940 photo, USS Aaron Ward , left, and USS Abel P. Upshur are destroyers of comparable size, but because the latter is more heavily loaded, it sits lower, displacing more water.