Thedisplacementordisplacement tonnageof a ship is itsweight.As the term indicates, it is measured indirectly, usingArchimedes' principle,by first calculating the volume of waterdisplacedby the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures inlong tons.[1]Today,tonnesare more commonly used.
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 ofvolumeorcapacitytypically used for commercial vessels and measured bytonnage:net tonnageandgross tonnage.
Calculation
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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 starboardsides 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 ruleshave been available since the 1950s to aid in these calculations. Presently, it is done with computers.[6]
Displacement is usually measured in units oftonnesorlong tons.[7]
Definitions
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There are terms for the displacement of a vessel under specified conditions:
Loaded displacement
edit- Loaded displacementis the weight of the ship including cargo, passengers, fuel, water, stores,dunnageand such other items necessary for use on a voyage. These bring the ship down to its "load draft".[8]
- Full loaddisplacement 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
edit- 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]
Normal displacement
edit- Normal displacementis the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."[10]
Standard displacement
edit- Standard displacement,also known as "Washington displacement", is a specific term defined by theWashington Naval Treatyof 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]
Gallery
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A floating ship's displacementFpand buoyancyFbmust be equal. -
Greek philosopherArchimedeshaving his famous bath, the birth of the theory ofdisplacement -
A ship'shydrostaticcurves. Lines 4 and 5 are used to convert from mean draft in meters to displacement in tonnes (table in Spanish).
See also
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References
edit- ^"Ship Tonnage Explained - Displacement, Deadweight, Etc".GG Archives.Retrieved2019-01-14.
- ^Dinsmore, Ben (16 May 2011)."A Guide to Understanding Ship Weight and Tonnage Measurements".The Maritime Site.Archived from the original on 29 May 2011.Retrieved14 February2018.
{{cite web}}:CS1 maint: unfit URL (link) - ^abcGeorge, 2005. p. 5.
- ^George, 2005. p. 465.
- ^Turpin and McEwen, 1980.
- ^George, 2005. p. 262.
- ^Otmar Schäuffelen (2005).Chapman Great Sailing Ships of the World.Hearst Books. p. xix.
- ^abMilitary Sealift Command.
- ^abDepartment of the Navy, 1942.
- ^United States Naval Institute, 1897. p 809.
- ^abConference on the Limitation of Armament, 1922.Ch II, Part 4.
Bibliography
edit- Dear, I.C.B.; Kemp, Peter (2006).Oxford Companion to Ships and the Sea(Paperback ed.). Oxford: Oxford University Press.ISBN0-19-920568-X.
- George, William E. (2005).Stability & Trim for the Ship's Officer.Centreville, Md: Cornell Maritime Press.ISBN0-87033-564-2.
- Hayler, William B. (2003).American Merchant Seaman's Manual.Cambridge, Md: Cornell Maritime Press.ISBN0-87033-549-9..
- Turpin, Edward A.; McEwen, William A. (1980).Merchant Marine Officers' Handbook(4th ed.). Centreville, MD: Cornell Maritime Press.ISBN0-87033-056-X.
- Navy Department (1942)."Nomenclature of Naval Vessels".history.navy.mil.United States Navy.
- Military Sealift Command."Definitions, Tonnages and Equivalents".MSC Ship Inventory.United States Navy. Archived fromthe originalon 2007-02-05.Retrieved2008-03-24.
- MLCPAC Naval Engineering Division (2005-11-01)."Trim and Stability Information for Drydocking Calculations".United States Coast Guard.Retrieved2008-03-24.
- United States of America (1922)."Conference on the Limitation of Armament, 1922".Papers Relating to the Foreign Relations of the United States: 1922.Vol. 1. pp. 247–266.
- United States Naval Institute(1897).Proceedings of the United States Naval Institute.United States Naval Institute.Retrieved2008-03-24.