Displacement
Displacement is the weight of a yacht, expressed in tonnes and equal to the mass of water her hull displaces by Archimedes' principle. It is a mass measurement, not a hull form - distinct from a displacement hull. Builders quote it in three states: light ship, half load and full load (loaded displacement).
What is yacht displacement?
Displacement is the weight of a yacht, expressed in tonnes. By Archimedes' principle, a floating yacht displaces a volume of water whose weight equals her own - so measuring displaced water volume and multiplying by seawater density (approximately 1.025 t/m³) yields the yacht's mass directly.
Displacement is quoted in three loading states:
- Light ship displacement - the bare yacht: hull, machinery, fixed outfit and lubricants only. Used for structural and stability calculations.
- Half-load displacement - light ship plus 50 per cent of consumables. The reference state for performance predictions.
- Full-load (loaded) displacement - light ship plus 100 per cent of consumables, crew, guests, tenders and toys, at maximum draft assigned by the load line authority.
The same yacht therefore has three displacement figures, often differing by 20-30 per cent. Class societies - Lloyd's Register, DNV, Bureau Veritas, RINA, ABS under the IACS umbrella - define these states in their rules.
Why it matters for yacht owners
Displacement drives almost every operating figure on a motor yacht. Fuel burn at displacement speed scales roughly with mass and with the cube of speed. Draft, freeboard and stability margin all shift as fuel and water are consumed, which is why charter captains plan load on a per-passage basis.
Crucially, displacement is not gross tonnage. Gross tonnage measures internal volume for regulatory purposes; displacement measures mass. Confusing the two when comparing yachts is one of the most common buyer errors.
Key facts
- Displacement is measured in tonnes (metric tonnes, 1,000 kg).
- Three reference states: light ship, half load, full load (loaded displacement).
- Seawater density used in the calculation is approximately 1.025 t/m³.
- Archimedes' principle is the physical basis: weight of yacht equals weight of water displaced.
- Indicative full-load displacement: 50m steel displacement motor yacht - 500-800 tonnes; 70m - 1,200-1,800 tonnes; 100m+ - 3,000-5,000+ tonnes.
- Displacement is mass, gross tonnage is volume - the two are not interchangeable.
- Maximum permissible displacement is set by the load line under the IMO International Convention on Load Lines, 1966.
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View moreFAQ
Is displacement the same as a displacement hull?
No. Displacement is a weight measurement - the mass of a yacht in tonnes. A displacement hull is a hull form. A yacht always has a displacement; she may or may not have a displacement hull.
What is the difference between displacement and gross tonnage?
Displacement measures mass (the yacht's weight in tonnes); gross tonnage measures internal volume (a non-linear function of enclosed spaces). The two are not interchangeable.
How is yacht displacement measured?
Calculated by the naval architect from the submerged hull volume at a given waterline, multiplied by water density. Verified at delivery by an inclining experiment witnessed by the class society.
What do light ship, half load and full load mean?
Light ship is the bare yacht. Half load is light ship plus 50% consumables. Full load is the yacht at her maximum permitted draft with all consumables, crew, guests, tenders and toys aboard.
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