Displacement and Lightweight

A floating ship displaces water. As a ship specification, displacement refers to the ship’s mass in its current loading condition: when the load changes, so does this mass. Displacement therefore does not indicate the size of the enclosed spaces on board.

Lightweight is a defined reference condition that excludes variable load. Under the SOLAS definition, it excludes cargo, fuel and lubricating oil, ballast, fresh and feed water in tanks, consumable stores, as well as passengers, crew and their effects.Deadweight (DWT), or carrying capacity, is neither the ship’s total displacement nor cargo alone. SOLAS defines it as the difference between the displacement at the draught corresponding to the assigned summer freeboard in water with a relative density of 1.025 and the lightweight. This difference therefore includes the combined variable masses, such as cargo, fuel, water, stores and people.

A simplified example: if lightweight is 8,000 tonnes and displacement in the specified summer load condition is 11,500 tonnes, DWT is 3,500 tonnes. This example illustrates the difference between the two masses; it is not a figure for any particular ship. Displacement changes with a different loading condition, while the definition of DWT remains tied to the specified summer condition.

Why GT is different

Gross Tonnage (GT) and Net Tonnage (NT) are measurement values under the International Convention on Tonnage Measurement of Ships, 1969. The International Maritime Organization (IMO) describes GT as a function of the moulded volume of all enclosed spaces; NT is determined using a separate formula based on the moulded volume of cargo spaces. The two values are calculated independently. NT must not be less than 30 percent of GT.

GT and NT are not masses measured in tonnes, nor are they direct measurements of space in cubic metres; they are dimensionless figures derived from the Convention’s formulas. They serve purposes different from those of displacement and DWT: GT is used, among other things, as the basis for crewing requirements, safety rules and registration fees; both tonnage values also factor into port dues. Displacement, by contrast, describes the mass in a given condition, while DWT is a defined difference between masses. The IMO summarizes the

Tonnage Convention and its measurement values

.The displacement describes the mass of the water displaced. It is a measure of mass and should not be equated with volumetric GT.Displacement and lightweight

Displacement describes the mass of the water displaced. It is a measure of mass and should not be equated with volumetric GT.