Displacement

Displacement is the mass of water displaced by a floating vessel, equal to the total weight of the ship including hull, machinery, cargo, fuel, water, stores, and crew. By Archimedes' principle, a vessel floats when its displacement equals the weight of water it pushes aside. Displacement is measured in metric tonnes and varies with the ship's draught; at light ship draught the displacement is the light ship weight, and at full load draught it equals the light ship weight plus the full deadweight. Hydrostatic tables—part of the ship's stability booklet—tabulate displacement against mean draught, allowing the chief mate to determine the total displacement by reading the ship's draught marks. Displacement is used in stability calculations: the metacentric height GM and all righting lever (GZ) calculations are performed at a specific displacement. When the ship is loaded at port, the chief mate reads the draught marks fore, aft, and amidships, corrects for trim and density of water, and reads off the corresponding displacement from the hydrostatic tables. The difference between departure and arrival displacement—accounting for fuel, water, and cargo consumed or discharged—must be consistent with the voyage record; discrepancies indicate errors in cargo, bunker, or ballast accounting. Accurate displacement monitoring underpins all stability and loading calculations.