A floating ship pushes water aside. Displacement is the weight of water displaced by the submerged part of the hull; by Archimedes’ principle it equals the total weight of the ship and everything aboard.

Lightweight is the weight of the empty ship; deadweight is the difference between displacement at load draft and lightweight. Naval architects use displacement curves to relate draft to weight. Displacement changes as fuel is burned, ballast is adjusted, and cargo is loaded or discharged.

Accurate knowledge of displacement is essential for stability calculations and for determining how much cargo can still be loaded. Hydrostatic tables on board convert observed drafts into displacement after corrections for trim and water density.

• Weight of water displaced by the floating ship
• Equals the total weight of ship and contents
• Used in stability and loading calculations
• Derived from draft readings and hydrostatic data