Draft marks, by showing how low a ship is sitting in the water, make it possible to determine displacement.
Thedisplacement ordisplacement tonnage of a ship is itsweight. As the term indicates, it is measured indirectly, usingArchimedes' principle, by first calculating the volume of waterdisplaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures inlong tons.[1] Today,tonnes are more commonly used.[citation needed]
Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage"[2]) to its maximum load. Numerous specific terms are used to describe varying levels of load and trim, detailed below.
Ship displacement should not be confused with measurements ofvolume orcapacity typically used for commercial vessels and measured bytonnage:net tonnage andgross tonnage.
Shipboard stability computer programs can be used to calculate a vessel's displacement.
The process of determining a vessel's displacement begins with measuring itsdraft.[3] This is accomplished by means of its "draft marks". A merchant vessel has three matching sets: one mark each on theport and starboard sides forward, midships, and astern.[3] These marks allow a ship's displacement to be determined to an accuracy of 0.5%.[3]
The draft observed at each set of marks is averaged to find a mean draft. The ship's hydrostatic tables show the corresponding volume displaced.[4]To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1,025 kg/m3) is more dense than fresh water (1,000 kg/m3);[5] so a ship will ride higher in salt water than in fresh. The density of water also varies with temperature.
Devices akin toslide rules have been available since the 1950s to aid in these calculations. Presently, it is done with computers.[6]
In this 1940 photo,USSAaron Ward, left, andUSSAbel P. Upshur are destroyers of comparable size, but because the latter is more heavily loaded, it sits lower, displacing more water.
There are terms for the displacement of a vessel under specified conditions:
Loaded displacement is the weight of the ship including cargo, passengers, fuel, water, stores,dunnage and such other items necessary for use on a voyage. These bring the ship down to its "load draft".[8]
Full load displacement and loaded displacement have almost identical definitions. Full load is defined as the displacement of a vessel when floating at its greatest allowable draft as approved by the load line assigning authority which is either the flag state (USCG etc) or aclassification society (and designated by its "load line").[9] Warships have full load condition established through the Naval design process, and are exempt from commercial requirements laid out by flag state laws.[9]
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.[8]
Standard displacement, also known as "Washington displacement", is a specific term defined by theWashington Naval Treaty of 1922.[11] "It is 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."[11]
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