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As weight is added to a ship, it submerges. Designated displacement is the ship's weight when fully loaded and submerged to her load lines.
As weight is added to a ship, it submerges. Designated displacement is the ship's weight when fully loaded and submerged to her load lines.

A ship's displacement is its mass at any given time, generally expressed in metric tons or long tons. This article is about the tonne or metric ton For other tons see Ton. 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. [1] 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. Net tonnage (often abbreviated as NT, NT or nt) is a calculated representation of a the internal volume of a ship's cargo holds Gross tonnage (often abbreviated as GT, GT or gt) is a unitless index related to a ship's overall internal volume Deadweight tonnage (also known as deadweight and variously abbreviated as DWT, D

The word displacement refers to the mass of the water that the ship displaces (moves out of the way) while floating. [2] A floating ship always displaces an amount of water of the same mass as the ship. [2]

The density (mass per unit of volume) of water can vary. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different For example, the average density of seawater at the surface of the ocean is 1025 kg/m³ (64 lb/cu ft), fresh water on the other hand has a density of about 1000 kg/m³ (62. 5 lb/cu ft). [1] 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 more fresh water to amount to 100 tons.

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.

Contents

Calculation

Shipboard stability programs are often used to calculate a ship's current displacement.
Shipboard stability programs are often used to calculate a ship's current displacement.

The traditional method for determining a ship's actual displacement is by use of draft marks. [3] A merchant vessel has six sets of draft marks: forward, midships, and astern on both the port and starboard sides. [3] These drafts can allow the determination of a ship's displacement to an accuracy of 0. 5%. [3] First, the individual drafts are averaged to find a mean draft. [4] Then the mean draft is entered into the ship's hydrostatic tables, giving a displacement. [5]

Computers have been used to assist in stability calculations, such as determining displacement, since the 1950s. [6] The first were mechanical computers, similar to slide rules which could convert cargo levels to values such as deadweight tonnage, draft, and trim. The slide rule, also known as a slipstick, is a mechanical Analog computer. Deadweight tonnage (also known as deadweight and variously abbreviated as DWT, D [6] Since the 1970s, personal computer-based programs have been developing to meet these needs. A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated [6] 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. [7] The program also alerts the user to potentially dangerous situations and offers recommendations for improvement. [7]

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 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 her greatest allowable draft as established by the classification societies. [8] For warships, an arbitrary full load condition is established. [8]

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. [9]

Standard displacement

Standard displacement is a term defined in the Washington Naval Treaty. The Washington Naval Treaty, also known as the Five-Power Treaty, limited the naval armaments of its five signatories the United States of America, the [10] 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 feed water on board. [10]

Light displacement

Light displacement is defined as the mass of the ship excluding cargo, fuel, ballast, stores, passengers, crew, but with water in boilers to steaming level. [9]

Normal displacement

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. "[11]

Gallery

A floating ship's displacement Fp and buoyancy Fa must be equal.

Greek philosopher Archimedes having his famous bath

A ship's hydrostatic curves. Lines 4 and 5 are used to convert from mean draft in meters to displacement in tonnes.

Trivia

Notes

  1. ^ a b c Turpin and McEwen, 1980.
  2. ^ a b George, 2005, p. 68.
  3. ^ a b c George, 2005. p. 5.
  4. ^ George, 2005. p. 14 - 15.
  5. ^ George, 2005. p. 465.
  6. ^ a b c George, 2005. p. 262.
  7. ^ a b MLCPAC, 2005.
  8. ^ a b Department of the Navy, 1942.
  9. ^ a b Military Sealift Command.
  10. ^ a b United States of America, 1922. Ch II, Part 4.
  11. ^ United States Naval Institute, 1897. p 809.

References

See also

Tonnage is a measure of the size or Cargo capacity of a Ship. Units of mass There are three similar units of Mass called the ton: Long ton (simply ton in countries such as the United In Fluid mechanics, displacement occurs when an object is immersed in a Fluid, pushing it out of the way and taking its Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles A hull is the body of a Ship or Boat. It is a central concept in floating vessels as it provides the Buoyancy that keeps the vessel from sinking Fluid dynamics is the sub-discipline of Fluid mechanics dealing with fluid flow: Fluids ( Liquids and Gases in motion
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