{{Short description|Ship's weight}} [[File:Draft scale at the ship bow (PIC00110).jpg|right|thumb|upright=1.3|Draft marks, by showing how low a ship is sitting in the water, make it possible to determine displacement.]]

The '''displacement''' or '''displacement tonnage''' of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures in long tons.<ref>{{Cite web|url=https://www.ggarchives.com/OceanTravel/ShipTonnage/1932-06-28-ShipTonnageExplained.html|title=Ship Tonnage Explained - Displacement, Deadweight, Etc. |website=GG Archives |access-date=2019-01-14}}</ref> Today, tonnes are more commonly used.{{cn|date=December 2024}}

Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage"<ref>{{cite web |last=Dinsmore |first=Ben |date=16 May 2011 |title=A Guide to Understanding Ship Weight and Tonnage Measurements |url=http://www.themaritimesite.com/a-guide-to-understanding-ship-weight-and-tonnage-measurements/ |url-status=usurped |archive-url=https://web.archive.org/web/20110529003954/http://www.themaritimesite.com/a-guide-to-understanding-ship-weight-and-tonnage-measurements/ |archive-date=29 May 2011 |access-date=14 February 2018 |website=The Maritime Site}}</ref>) 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 of '''volume''' or '''capacity''' typically used for commercial vessels and measured by tonnage: net tonnage and gross tonnage.

==Calculation== thumb|Shipboard stability computer programs can be used to calculate a vessel's displacement. The process of determining a vessel's displacement begins with measuring its draft.<ref name="g5">George, 2005. p. 5.</ref> This is accomplished by means of its "draft marks". A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern.<ref name="g5"/> These marks allow a ship's displacement to be determined to an accuracy of 0.5%.<ref name="g5"/>

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.<ref name="g465">George, 2005. p. 465.</ref> To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1,025&nbsp;kg/m<sup>3</sup>) is more dense than fresh water (1,000&nbsp;kg/m<sup>3</sup>);<ref>Turpin and McEwen, 1980.</ref> so a ship will ride higher in salt water than in fresh. The density of water also varies with temperature.

Devices akin to slide rules have been available since the 1950s to aid in these calculations. Presently, it is done with computers.<ref name="g262">George, 2005. p. 262.</ref>

Displacement is usually measured in units of tonnes or long tons.<ref>{{cite book|title=Chapman Great Sailing Ships of the World|author=Otmar Schäuffelen|publisher=Hearst Books|date=2005|page=xix}}</ref>

==Definitions== [[File:The Arrival of the First Flotilla of Destroyers From America To the Royal Navy, Devonport, September 1940 A724.jpg|thumb|right|In this 1940 photo, USS ''Aaron Ward'', left, and USS ''Abel 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:

==={{Anchor|Loaded displacement}}Loaded displacement=== *'''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".<ref name="msc"/> *'''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 a classification society (and designated by its "load line").<ref name="don42">Department of the Navy, 1942.</ref> Warships have full load condition established through the Naval design process, and are exempt from commercial requirements laid out by flag state laws.<ref name="don42"/>

===Light displacement=== *'''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.<ref name="msc">Military Sealift Command.</ref>

===Normal displacement=== *'''Normal displacement''' is the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."<ref name="usni">United States Naval Institute, 1897. p 809.</ref>

===Standard displacement=== *'''Standard displacement''', also known as "Washington displacement", is a specific term defined by the Washington Naval Treaty of 1922.<ref name="usa1922">''Conference on the Limitation of Armament, 1922''. Ch II, Part 4.</ref> "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."<ref name="usa1922"/>

==Gallery== <gallery mode="packed"> Image:Archimedes principle.svg|A floating ship's displacement ''F''<sub>''p''</sub> and buoyancy ''F''<sub>''b''</sub> must be equal. Image:Archimedes bath.jpg|Greek philosopher Archimedes having his famous bath, the birth of the theory of displacement Image:CurvasCarenaDerecha.PNG|A ship's hydrostatic curves. Lines 4 and 5 are used to convert from mean draft in meters to displacement in tonnes (table in Spanish). </gallery>

==See also== {{Portal|Transport}} * Naval architecture * Hull (watercraft) * Hydrodynamics * Tonnage

==References== {{Reflist|30em}}

===Bibliography=== * {{cite book | last = Dear | first = I.C.B. | last2 = Kemp |first2 = Peter | title = Oxford Companion to Ships and the Sea | edition = Paperback | year = 2006 | publisher = Oxford University Press | location = Oxford | isbn = 0-19-920568-X }} *{{cite book |author=George, William E. |title=Stability & Trim for the Ship's Officer |publisher=Cornell Maritime Press |location=Centreville, Md |year=2005 |isbn=0-87033-564-2 |url-access=registration |url=https://archive.org/details/stabilitytrimfor0000unse }} * {{cite book |last=Hayler |first=William B. |title=American Merchant Seaman's Manual |year=2003 |publisher= Cornell Maritime Press |location=Cambridge, Md| isbn= 0-87033-549-9 }}. * {{cite book | last = Turpin | first = Edward A. | author2=McEwen, William A. | title = Merchant Marine Officers' Handbook | edition = 4th | year = 1980 | publisher = Cornell Maritime Press | location = Centreville, MD | isbn = 0-87033-056-X }} *{{cite web | url=https://www.history.navy.mil/content/history/nhhc/research/library/online-reading-room/title-list-alphabetically/n/nomenclature-naval-vessels.html#displacement | title=Nomenclature of Naval Vessels | author=Navy Department |year=1942 |work=history.navy.mil |publisher=United States Navy }} *{{cite web |url=http://www.msc.navy.mil/inventory/glossary.htm |title=Definitions, Tonnages and Equivalents |access-date=2008-03-24 |author=Military Sealift Command |author-link=Military Sealift Command |work=MSC Ship Inventory |publisher=United States Navy|archive-url=https://web.archive.org/web/20070205160650/http://www.msc.navy.mil/inventory/glossary.htm |archive-date=2007-02-05 }} *{{cite web |url=http://www.uscg.mil/mlcpac/mlcp/Eng%20Support/mlcpv/trim_and_stability.htm |title= Trim and Stability Information for Drydocking Calculations |access-date=2008-03-24 |author=MLCPAC Naval Engineering Division |date=2005-11-01 |publisher=United States Coast Guard}} *{{cite book |author=United States of America|title=Papers Relating to the Foreign Relations of the United States: 1922|volume=1 |year=1922 |pages=247–266 |chapter=Conference on the Limitation of Armament, 1922|chapter-url=http://www.ibiblio.org/pha/pre-war/1922/nav_lim.html }} *{{cite book |author=United States Naval Institute |author-link=United States Naval Institute |title=Proceedings of the United States Naval Institute |url=https://books.google.com/books?id=qgwKAAAAIAAJ&q=%22Normal+Displacement%22+%2B%22two-thirds+supply%22&pg=PA809 |access-date=2008-03-24 |year=1897 |publisher=United States Naval Institute }}

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{{DEFAULTSORT:Displacement (Ship)}} Category:Water transport Category:Shipbuilding Category:Nautical terminology Category:Ship measurements