Displacement is the total weight of a ship, measured as the weight of water it displaces — by Archimedes’ principle, a floating vessel displaces a weight of water exactly equal to its own total weight.
Light Displacement vs Loaded Displacement
- Light displacement (lightweight) — the weight of the ship itself, with machinery and equipment, but without cargo, fuel, water, stores, or crew
- Loaded displacement — the ship’s full weight when loaded to its maximum permitted draft, including cargo, fuel, water, and everything else aboard
Displacement vs Deadweight Tonnage (DWT)
These are commonly confused:
- Displacement = the ship’s total actual weight (ship + everything aboard)
- Deadweight tonnage (DWT) = the maximum weight the ship can carry (cargo, fuel, water, stores, crew) — essentially loaded displacement minus light displacement
DWT is the figure typically used commercially to describe a ship’s cargo-carrying capacity, while displacement is more relevant to stability and structural calculations.
Why Displacement Matters
Displacement calculations are fundamental to stability assessment — since G (centre of gravity) and B (centre of buoyancy) calculations both depend on knowing the ship’s actual total weight and its distribution.
Exam Relevance
Displacement calculations and the distinction from deadweight tonnage are foundational Ship Stability topics tested in deck officer competency exams.