A ship's displacement is its mass at any given time, generally expressed in metric tons or long tons. The term is often used to mean the ship's mass when it is loaded to its maximum capacity. A number of synonymous terms exist for this maximum mass, such as loaded displacement, full load displacement and designated displacement. Displacement is a measurement of mass, and should not be confused with similarly named measurements of volume or capacity such as net tonnage, gross tonnage, or deadweight tonnage.
The word displacement refers to the mass of the water that the ship displaces while floating. Another way of thinking about displacement is the amount of water that would spill out of a completely filled container, were the ship to be placed into it. 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 (10.25 lb/gallon, 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 partially loaded. Terms for these measurements include light displacement, standard displacement, and normal displacement. These terms are defined fully below.
