displacement ton

n.
A unit for measuring the displacement of a ship afloat, equivalent to one long ton or about one cubic meter of salt water.

The ton used to express the mass of ships, traditionally the long ton of 2 240 lb (1 016.0~ kg), but typically equated with a volume of 35 ft³ (0.991 09~ m³), the approximate volume of 1 long ton of salt water.

Since, by Archimedes' principle, the mass of a floating object equals the mass of water displaced, the mass of a ship can be deduced, as its displacement, from the three-dimensional plans of its hull, cut at the level of flotation. Clearly that level depends on the load added to the fixtures (from hull to engines to gun-turrets). Its full displacement tonnage corresponds to its being fully laden, including with fuel, the level of immersion then according with the Plimsoll line for the pertinent sea. Its displacement when empty of fuel and freight, and of any temporary ballast, is called the lightweight displacement; the difference between that and the full displacement is termed the deadweight tonnage, which is used to express the capacity of freight ships, but is of little relevance for warships. However, a distinct measurement ton is widely used for assessing the commercial size of a ship for dues and similar purposes. See also shipping ton.