Displacement (nautical)


May 28, 2022

The displacement of a vessel corresponds to the mass of water displaced by it when afloat, and is generally expressed in tonnes. This value is obtained by multiplying the submerged volume of the vessel by the density of the water in which it sails. In practice, the displacement represents the mass of the vessel itself at a given moment. As a measure of mass, displacement should not be confused with tonnage, which - despite the designation implying otherwise - is not a measure of weight or mass, but of volume. Displacement should not be confused with size either, since this - despite also being a measure of mass - represents only the ship's carrying capacity and not its total mass. When simply mentioning the displacement of a ship, it is understood as referring to its maximum displacement.


In fluid mechanics, the term "displacement" refers to the mass of a fluid displaced when an object is immersed in it, taking its place. The greater the density of the liquid, the less volume will be displaced by an object of the same mass. Similarly, in the nautical context, displacement corresponds to the mass of water displaced by a ship while floating there. Another way of thinking about displacement is to consider it as the amount of water that would spill out of a completely full container, if the ship were placed there. A floating ship always displaces an amount of water whose mass is the same as the mass of the ship itself. The density (mass per unit volume) of water can vary. For example, the average density of seawater at the ocean surface is 1025 kg/m³. The average density of fresh water is only 1,000 kg/m³. Assuming a 100-ton vessel moving from salt water in the sea to fresh water in a river, it would continue to displace exactly the same 100 tons of water, but this displacement would correspond to a greater volume of fresh water than water. salted. Thus, the vessel would have a slightly higher immersion in the river than it would have at sea. There are several displacement measurements, which correspond to the various loading conditions of the ship.


The traditional method of determining the actual displacement of a ship is through the use of draft marks. A merchant ship normally has six sets of draft marks, placed fore, amidships and aft, both starboard and port. These drafts can make it possible to determine the displacement of a ship with an accuracy of 0.5%. First, the average of the various drafts is calculated, obtaining the average draft. Then, the value of the average draft is introduced in the table of hydrostatic curves of the ship, obtaining the corresponding displacement. Since the 1950s, computers have been used for hydrostatic calculations, including those needed to determine displacement. The first were slide rule-like mechanical computers that could convert load levels such as size, draft, and trim. Since the 1970s, computer programs have been developed for use in portable computers.

Displacement under special conditions

There are several displacement measures corresponding to the various states of ship's load.

Light displacement

Light displacement or minimum displacement consists of the total mass of the ship, excluding cargo, fuel, ballast and crew, but with sufficient boiler water to maintain pressure.

Offset loaded

Loaded displacement, full-load displacement or maximum displacement correspond to practically the same concept. When the term "displacement" is simply mentioned, it is normally understood as referring to the loaded displacement. The displacement at full load is defined as the displacement of a vessel when