A ship's displacement or displacement tonnage, a term usually applied only to naval vessels, is the weight of the water that a ship displaces when it is floating. The term is defined ordinarily such that the ship's fuel tanks are full and all stores are aboard. Displacement is the actual weight of the ship, since a floating body displaces its own weight in water (Archimedes' principle). Another way of thinking about displacement is the weight of the water that would spill out of a completely filled container were the ship placed into it.
A number of synonymous terms exist for this maximum weight, such as loaded displacement, full load displacement and designated displacement. As a measurement of weight, displacement should not be confused with similarly named measurements of volume or capacity such as net tonnage, gross tonnage, or deadweight tonnage.
The density (weight per unit of volume) of water can vary. For example, the average density of seawater at the surface of the ocean is 1025 kg/m³ (10.25 lb/ga, 8.55 lb/US gallon); fresh water on the other hand has a density of about 1000 kg/m³ (10.00 lb/ga, 8.35 lb/US gallon). Consider a 100-ton ship passing from a saltwater sea into a freshwater river. It always displaces exactly 100 tons of water, but it has to displace a greater volume of fresh water to amount to 100 tons. Therefore it would sit slightly lower in the water in the freshwater river than it would in the saltwater sea.
It can be useful to know a ship's displacement when it is unloaded or loaded partially. Terms for these measurements include light displacement, standard displacement, and normal displacement. These terms are defined below.
The traditional method for determining a ship's actual displacement is by use of draft marks. A merchant vessel has six sets of draft marks: forward, midships, and astern on both the port and starboard sides. These drafts can allow the determination of a ship's displacement to an accuracy of 0.5%. First, the individual drafts are averaged to find a mean draft. Then the mean draft is entered into the ship's hydrostatic tables, giving a displacement.
Computers have been used to assist hydrostatic calculations, such as determining displacement, since the 1950s. The first were mechanical computers, similar to slide rules which could convert cargo levels to values such as deadweight tonnage, draft, and trim. Since the introduction of electronic digital computers, computer programs have been developed to meet these needs.
Displacement under special conditions
A number of measurements of displacement are defined when the ship is in a special state, such as when it is completely full or completely empty. These special types of displacement are discussed below.
Full or deep load or loaded displacement
Full load displacement and loaded displacement have almost identical definitions.
Full load displacement is defined as the displacement of a vessel when floating at its greatest allowable draft as established by classification societies. For warships, an arbitrary full load condition is established. Deep load condition means full ammunition and stores, with most available fuel capacity used.
Loaded displacement is defined as the weight of the ship including cargo, passengers, fuel, water, stores, dunnage and such other items necessary for use on a voyage, which brings the ship down to its load draft.
The standard displacement, also known as Washington displacement, is a term defined by the Washington Naval Treaty of 1922. It is defined as the displacement of the ship complete, fully manned, engined, and equipped ready for sea, including all armament and ammunition, equipment, outfit, provisions and fresh water for crew, miscellaneous stores, and implements of every description that are intended to be carried in war, but without fuel or reserve boiler feed water on board. The omission of fuel and water was to avoid penalizing the British, who had great global commitments and required greater fuel loads, and especially the United States, which had global commitments almost as great but with fewer bases to provide fueling than the Royal Navy.
Light displacement (LDT) is defined as the weight of the ship excluding cargo, fuel, water, ballast, stores, passengers, crew, but with water in boilers to steaming level.
This rare term has been used to mean the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."
Greek philosopher Archimedes having his famous bath
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