Displacement — what a ship actually weighs

Displacement is a ship's actual weight — and because you cannot put a 500,000-tonne tanker on a scale, it is measured indirectly, by Archimedes' principle. A floating ship pushes aside a volume of water that weighs exactly what the ship weighs. Measure that displaced volume from the ship's draft and hull shape, multiply by the density of the water, and you have her weight.

Displacement changes constantly with load, so naval architects use fixed reference points. Light displacement is the empty ship — hull, engines, fittings, nothing else. Full-load (or loaded) displacement is the ship down to her maximum legal draft. The gap between the two is the deadweight: her carrying capacity.

Density matters more than people expect. Seawater at 1,025 kg per cubic metre is denser than fresh water at 1,000, so the same ship rides higher in the sea than in a river — which is exactly why the Plimsoll line has separate marks for fresh, tropical and winter North Atlantic water. It is also why displacement, not deadweight, is the honest figure for comparing warships, which carry weapons rather than cargo.