# Displacement (ship)

The more heavily loaded a ship is, the lower it sits in the water. "Designated displacement" is a measurement of the weight of water a ship displaces when fully loaded and submerged to her load lines.[citation needed]

The displacement or displacement tonnage of a ship is the ship's weight. The name reflects the fact that it is measured indirectly, by first calculating the volume of water displaced by the ship, and then calculating the weight of that water. By Archimedes' principle, this is also the weight of the ship.

Displacement should not be confused with other measurements of volume or capacity typically used for commercial vessels such as net tonnage, gross tonnage, or deadweight tonnage.

## Calculation

Shipboard stability programs can be used to calculate a vessel's displacement

The process of determining a vessel's displacement begins with measuring its draft[1] This is accomplished by means of its "draft marks" (or "load lines"). A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern.[1] These marks allow a ship's displacement to be determined to an accuracy of 0.5%.[1]

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.[2]

To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³);[3] 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. It is done today with computers.[4]

Displacement is usually measured in units of tonnes or long tons.[5]

## Definitions

Two similar destroyers berthed alongside each other. The one on the outboard is more heavily loaded and displaces 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",[6] colloquially known as the "waterline".
• 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 established by classification societies (and designated by its "waterline").[7] Warships have arbitrary full load condition established.[7]
• Deep load condition means full ammunition and stores, with most available fuel capacity used.[citation needed]

### 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.[6]

### Normal displacement

• Normal displacement is the ship's displacement "with all outfit, and two-thirds supply of stores, ammunition, etc., on board."[8]

### Standard displacement

• Standard displacement, also known as "Washington displacement", is a specific term defined by the Washington Naval Treaty of 1922.[9] 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 boiler feed water on board.[9]

## References

1. George, 2005. p.5.
2. George, 2005. p. 465.
3. Turpin and McEwen, 1980.
4. George, 2005. p. 262.
5. Otmar Schäuffelen (2005). Chapman Great Sailing Ships of the World. Hearst Books. p. xix.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
6. Military Sealift Command.
7. Department of the Navy, 1942.
8. United States Naval Institute, 1897. p 809.
9. Conference on the Limitation of Armament, 1922. Ch II, Part 4.

### Bibliography

• Dear, I.C.B.; Kemp, Peter (2006). Oxford Companion to Ships and the Sea (Paperback ed.). Oxford: Oxford University Press. ISBN 0-19-920568-X.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• George, William E. (2005). Stability & Trim for the Ship's Officer. Centreville, Md: Cornell Maritime Press. ISBN 0-87033-564-2.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• Hayler, William B. (2003). American Merchant Seaman's Manual. Cambridge, Md: Cornell Maritime Press. ISBN 0-87033-549-9.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>.
• Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87033-056-X.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• Navy Department (1942). "Nomenclature of Naval Vessels". history.navy.mil. United States Navy. Retrieved 2008-03-24.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• Military Sealift Command. "Definitions, Tonnages and Equivalents". MSC Ship Inventory. United States Navy. Retrieved 2008-03-24.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• MLCPAC Naval Engineering Division (2005-11-01). "Trim and Stability Information for Drydocking Calculations". United States Coast Guard. Retrieved 2008-03-24.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• United States of America (1922). "Conference on the Limitation of Armament, 1922". Papers Relating to the Foreign Relations of the United States: 1922. 1. pp. 247–266.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
• United States Naval Institute (1897). Proceedings of the United States Naval Institute. United States Naval Institute. Retrieved 2008-03-24.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>