The word displacement refers to the mass of the water that the ship displaces (moves out of the way) while floating. A floating ship always displaces an amount of water of the same mass as the ship.
The density (mass 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 (8.55 lb/ga), fresh water on the other hand has a density of about 1000 kg/m³ (8.35 lb/ga). 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 partially loaded. Terms for these measurements include light displacement, standard displacement, and normal displacement. These terms are defined fully 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 in stability 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 1970s, personal computer–based programs have been developing to meet these needs. The image to the right is a screen-shot of the U.S. Department of Homeland Security's FCCSWin program which aids in cargo loading and calculates a ship's current hydrostatic properties. The program also alerts the user to potentially dangerous situations and offers recommendations for improvement.
Full load displacement is defined as the displacement of a vessel when floating at her greatest allowable draft as established by the classification societies. For warships, an arbitrary full load condition is established.
Loaded displacement is defined as the mass 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 her load draft.