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 based on the amount of water its hull displaces at varying loads. It is measured indirectly using Archimedes' principle by first calculating the volume of water displaced by the ship then converting that value into weight displaced. Traditionally, various measurement rules have been in use, giving various measures in long tons. [1] Today, metric tonnes are more 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, such as net tonnage, gross tonnage, or deadweight tonnage.

Calculation

Shipboard stability programs can be used to calculate a vessel's displacement USCoastGuardShipsStabilityProgram.jpg
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 [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 (1025 kg/m3) is more dense than fresh water (1000 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. It is done today with computers. [6]

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

Definitions

USS Aaron Ward (DD-132), a Wickes-class destroyer, and USS Abel P. Upshur (DD-193), a Clemson-class destroyer, berthed alongside each other. USS Abel P. Upshur on the outboard lies lower in the water as it is more heavily loaded and displaces more water. The Arrival of the First Flotilla of Destroyers From America To the Royal Navy, Devonport, September 1940 A724.jpg
USS Aaron Ward (DD-132), a Wickes-class destroyer, and USS Abel P. Upshur (DD-193), a Clemson-class destroyer, berthed alongside each other. USS Abel P. Upshur on the outboard lies lower in the water as it is more heavily loaded and displaces 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

Hull (watercraft) Watertight buoyant body of a ship or boat

A hull is the watertight body of a ship or 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 buoyant watercraft

A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying goods 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 tradition. In the Age of Sail a "ship" was a sailing vessel defined by its sailplan of at least three square riged masts and a full bowsprit.

Tonnage is a measure of the cargo-carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine. In modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Tonnage should not be confused with displacement, which refers to the actual weight of the vessel. Tonnage is commonly used to assess fees on commercial shipping.

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 thirteenth century. It is used in the United Kingdom and several other British Commonwealth of Nations countries alongside the mass-based metric tonne defined in 1799, as well as in the United States for bulk commodities.

<i>Kagerō</i>-class destroyer ship class

The Kagerō-class destroyers were a class of nineteen 1st Class destroyers built for the Imperial Japanese Navy during the 1930s, and operated by them during the Pacific War, when all but one of these nineteen were lost.

Bulk carrier merchant ship specially designed to transport unpackaged bulk cargo

A bulk carrier,bulker is a merchant ship specially designed to transport unpackaged bulk cargo, such as grains, coal, ore, steel coils and cement, in its cargo holds. Since the first specialized bulk carrier was built in 1852, economic forces have led to continued development of these ships, resulting in increased size and sophistication. Today's bulk carriers are specially designed to maximize capacity, safety, efficiency, and durability.

Deadweight tonnage Unit of mass

Deadweight tonnage or tons deadweight (DWT) is a measure of how much weight a ship can carry, not its weight, empty or in any degree of load. DWT is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew.

Tanker (ship) 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.

Waterline length Size of a ship

A vessel's waterline length 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> (T-AO-200)

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.

Sea captain 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> (T-AGOS-4)

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.

USNS <i>Worthy</i> (T-AGOS-14)

The USAV Worthy (T-AGOS-14) is a Missile Range Instrumentation Ship operated by the United States Army. The USAV Worthy was a Stalwart-class Modified Tactical Auxiliary General Ocean Surveillance Ship of the United States Navy.

Draft (hull) The 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), with the thickness of the hull included; in the case of not being included the draft outline would be obtained. Draft determines the minimum depth of water a ship or boat can safely navigate.

<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".

<i>Etorofu</i>-class escort ship Japanese ship class

The Etorofu-class escort ships were a group of fourteen kaibōkan escort vessels built for the Imperial Japanese Navy during World War II. Eight of the fourteen ships were sunk during the war. The class was also referred to by internal Japanese documents as the "Modified A-class" coastal defense vessel.

Net tonnage Dimensionless index calculated from the total moulded volume of the ships cargo spaces by using a mathematical formula

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.

<i>Chidori</i>-class torpedo boat

The Chidori-class torpedo boat was an Imperial Japanese Navy class of torpedo boats that were built before and served during the Second World War. The design initially proved to have too much armament for its small displacement, and the capsizing of Tomozuru (友鶴) shortly after completion in heavy weather resulted in a scandal which called into question the basic design of many Japanese warships of the time. After extensive modification, the class became satisfactory sea-boats and saw service in the Battle of the Philippines and the Dutch East Indies campaign as escorts and continued in that role for the rest of the war. Three were sunk during the war and the fourth was seized by the British at Hong Kong after the end of the war, where it was scrapped later.

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

References

  1. "Ship Tonnage Explained - Displacement, Deadweight, Etc. | GG Archives". www.gjenvick.com. Retrieved 2019-01-14.
  2. "A Guide to Understanding Ship Weight and Tonnage Measurements – The Maritime Site". www.themaritimesite.com. Retrieved 14 February 2018.
  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