Displacement (ship)

Last updated
Load lines, by showing how low a ship is sitting in the water, make it possible to determine displacement. Draft scale at the ship bow (PIC00110).jpg
Load lines, 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. [1] Today, tonnes are more commonly used.

Contents

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 of volume or capacity typically used for commercial vessels and measured by tonnage: net tonnage and gross tonnage.

Calculation

Shipboard stability computer programs can be used to calculate a vessel's displacement. USCoastGuardShipsStabilityProgram.jpg
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. [3] 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. [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 to slide rules have been available since the 1950s to aid in these calculations. Presently, it is done with computers. [6]

Displacement is usually measured in units of tonnes or long tons. [7]

Definitions

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. The Arrival of the First Flotilla of Destroyers From America To the Royal Navy, Devonport, September 1940 A724.jpg
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:

Loaded displacement

Light displacement

Normal displacement

Standard displacement

See also

Related Research Articles

<span class="mw-page-title-main">Hull (watercraft)</span> Watertight buoyant body of a ship or boat

A hull is the watertight body of a ship, boat, or flying boat. The hull may open at the top, or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, derrick, or mast. The line where the hull meets the water surface is called the waterline.

Ship Large watercraft

A ship is a large vessel that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally distinguished from boats, based on size, shape, load capacity and purpose. Ships have supported exploration, trade, warfare, migration, colonization, and science. Ship transport is responsible for the largest portion of world commerce.

<span class="mw-page-title-main">United States Merchant Marine</span> U.S. civilian mariners

The United States Merchant Marine is an organization composed of United States civilian mariners and U.S. civilian and federally owned merchant vessels. Both the civilian mariners and the merchant vessels are managed by a combination of the government and private sectors, and engage in commerce or transportation of goods and services in and out of the navigable waters of the United States. The Merchant Marine primarily transports domestic and international cargo and passengers during peacetime, and operate and maintain deep-sea merchant ships, tugboats, towboats, ferries, dredges, excursion vessels, charter boats and other waterborne craft on the oceans, the Great Lakes, rivers, canals, harbors, and other waterways. In times of war, the Merchant Marine can be an auxiliary to the United States Navy, and can be called upon to deliver military personnel and materiel for the military.

The long ton, also known as the imperial ton or displacement ton, is the name for the unit called the "ton" in the avoirdupois system of weights or Imperial system of measurements. It was standardised in the 13th century. It is used in the United Kingdom and several other Commonwealth of Nations countries alongside the mass-based tonne defined in 1799, as well as in the United States for bulk commodities.

<span class="mw-page-title-main">Deadweight tonnage</span> Measure of how much weight a ship can carry

Deadweight tonnage or tons deadweight (DWT) is a measure of how much weight a ship can carry. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew.

<span class="mw-page-title-main">Tanker (ship)</span> Ship designed to transport liquids or gases in bulk

A tanker is a ship designed to transport or store liquids or gases in bulk. Major types of tankship include the oil tanker, the chemical tanker, and gas carrier. Tankers also carry commodities such as vegetable oils, molasses and wine. In the United States Navy and Military Sealift Command, a tanker used to refuel other ships is called an oiler but many other navies use the terms tanker and replenishment tanker. Tankers were first developed in the late 19th century as iron and steel hulls and pumping systems were developed. As of 2005, there were just over 4,000 tankers and supertankers 10,000 LT DWT or greater operating worldwide.

<span class="mw-page-title-main">Waterline length</span> Size of a ship

A vessel's length at the waterline is the length of a ship or boat at the level where it sits in the water. The LWL will be shorter than the length of the boat overall as most boats have bows and stern protrusions that make the LOA greater than the LWL. As a ship becomes more loaded, it will sit lower in the water and its ambient waterline length may change; but the registered L.W.L it is measured from a default load condition.

USNS <i>Guadalupe</i> Oiler of the United States Navy

USNS Guadalupe (T-AO-200) is a Henry J. Kaiser-class underway replenishment oiler operated by the Military Sealift Command to support ships of the United States Navy.

<span class="mw-page-title-main">Sea captain</span> Commander of a ship or other sea-going vessel

A sea captain, ship's captain, captain, master, or shipmaster, is a high-grade licensed mariner who holds ultimate command and responsibility of a merchant vessel. The captain is responsible for the safe and efficient operation of the ship, including its seaworthiness, safety and security, cargo operations, navigation, crew management, and legal compliance, and for the persons and cargo on board.

The stability conditions of watercraft are the various standard loading configurations to which a ship, boat, or offshore platform may be subjected. They are recognized by classification societies such as Det Norske Veritas, Lloyd's Register and American Bureau of Shipping (ABS). Classification societies follow rules and guidelines laid down by International Convention for the Safety of Life at Sea (SOLAS) conventions, the International Maritime Organization and laws of the country under which the vessel is flagged, such as the Code of Federal Regulations.

USNS <i>Triumph</i>

USNS Triumph (T-AGOS-4) is a Stalwart-class ocean surveillance ship formerly of the United States Navy. She was struck from the Naval Vessel Register in 1995. On 1 October 2012 the ship was disposed of by Navy title transfer to the Maritime Administration. As of May 2015, Triumph was held as a reserve asset for spare parts for sister ships General Rudder and State of Michigan.

<i>Prevail</i> (IX-537)

Prevail (IX-537) is a modified Stalwart-class auxiliary general ocean surveillance ship (AGOS) of the United States Navy previously operated by the U.S. Military Sealift Command as T-AGOS 8. Prevail was reclassified as Unclassified Miscellaneous (IX) in October 2003 and is unofficially referred to as TSV-1. In this context, TSV stands for Training Support Vessel, and should not be confused with the U.S. Army's USAV Spearhead Theater Support Vessel initiative.

<span class="mw-page-title-main">Draft (hull)</span> Vertical distance between the waterline and the bottom of the hull (keel)

The draft or draught of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel). The draught of the vessel is the maximum depth of any part of the vessel, including appendages such as rudders, propellers and drop keels if deployed. Draft determines the minimum depth of water a ship or boat can safely navigate. The related term air draft is the maximum height of any part of the vessel above the water.

USNS <i>Tippecanoe</i> Oiler of the United States Navy

USNS Tippecanoe (T-AO-199) is a Henry J. Kaiser-class underway replenishment oiler operated by the Military Sealift Command (MSC) to support ships of the United States Navy. She serves in the United States Pacific Fleet. Tippecanoe, the thirteenth ship of the Henry J. Kaiser class, was laid down at Avondale Shipyard, Inc., at New Orleans, Louisiana, on 19 November 1990 and launched on 16 May 1992. She entered non-commissioned U.S. Navy service under the control of the MSC with a primarily civilian crew on 8 February 1993.

<i>Hatsuharu</i>-class destroyer Ship class

The Hatsuharu-class destroyers were a class of Imperial Japanese Navy destroyers in the service before and during World War II. The final two vessels in the class, completed after modifications to the design, are sometimes considered a separate "Ariake class".

<span class="mw-page-title-main">Net tonnage</span> Ship cargo space volume

Net tonnage is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula. Defined in The International Convention on Tonnage Measurement of Ships that was adopted by the International Maritime Organization in 1969, the net tonnage replaced the earlier net register tonnage (NRT) which denoted the volume of the ship's revenue-earning spaces in "register tons", units of volume equal to 100 cubic feet (2.83 m3). Net tonnage is used to calculate the port duties and should not be taken as less than 30 per cent of the ship's gross tonnage.

USNS <i>Kanawha</i> Oiler of the United States Navy

USNS Kanawha (T-AO-196) is a Henry J. Kaiser-class fleet replenishment oiler of the United States Navy in non-commissioned service in the Military Sealift Command.

USNS <i>Patuxent</i> Oiler of the United States Navy

USNS Patuxent (T-AO-201) is a Henry J. Kaiser-class underway replenishment oiler operated by the Military Sealift Command to support ships of the United States Navy.

Ship measurements consist of a multitude of terms and definitions specifically related to ships and measuring or defining their characteristics.

<i>Kaivan</i>-class patrol craft

The Kaivan is a class of large patrol craft operated by the Islamic Republic of Iran Navy. They ships in the class are modified versions of the American Cape-class vessels, built by the United States Coast Guard Yard in the 1950s.

References

  1. "Ship Tonnage Explained - Displacement, Deadweight, Etc". GG Archives. Retrieved 2019-01-14.
  2. 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. Retrieved 14 February 2018.{{cite web}}: CS1 maint: unfit URL (link)
  3. 1 2 3 George, 2005. p.5.
  4. George, 2005. p. 465.
  5. Turpin and McEwen, 1980.
  6. George, 2005. p. 262.
  7. Otmar Schäuffelen (2005). Chapman Great Sailing Ships of the World. Hearst Books. p. xix.
  8. 1 2 Military Sealift Command.
  9. 1 2 Department of the Navy, 1942.
  10. United States Naval Institute, 1897. p 809.
  11. 1 2 Conference on the Limitation of Armament, 1922. Ch II, Part 4.

Bibliography