Deadweight tonnage

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The more heavily loaded a ship is, the lower it sits in the water. Maximum DWT is the amount of weight a ship can carry without riding dangerously low in the water. Draft scale at the ship bow (PIC00110).jpg
The more heavily loaded a ship is, the lower it sits in the water. Maximum DWT is the amount of weight a ship can carry without riding dangerously low in the water.
Scale for a 6,000 tonne DWT ship. Echelle Port en lourd.jpg
Scale for a 6,000 tonne DWT ship.

Deadweight tonnage (also known as deadweight; abbreviated to DWT, D.W.T., d.w.t., or dwt) or tons deadweight (DWT) is a measure of how much weight a ship can carry. [1] [2] [3] It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew. [1]

Contents

DWT is often used to specify a ship's maximum permissible deadweight (i.e. when it is fully loaded so that its Plimsoll line is at water level), although it may also denote the actual DWT of a ship not loaded to capacity.

Definition

Deadweight tonnage is a measure of a vessel's weight carrying capacity, not including the empty weight of the ship. It is distinct from the displacement (weight of water displaced), which includes the ship's own weight, or the volumetric measures of gross tonnage or net tonnage (and the legacy measures gross register tonnage and net register tonnage).

Deadweight tonnage was historically expressed in long tons [note 1] but is now usually given internationally in tonnes (metric tons). [4] In modern international shipping conventions such as the International Convention for the Safety of Life at Sea and the International Convention for the Prevention of Pollution From Ships, deadweight is explicitly defined as the difference in tonnes between the displacement of a ship in water of a specific gravity of 1.025 (corresponding to average density of sea water) at the draft corresponding to the assigned summer freeboard and the light displacement (lightweight) of the ship. [5] [6]

See also

Notes

  1. One long ton (LT) is 2,240 pounds (1,016 kg)

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Tonnage is a measure of the cargo-carrying capacity of a ship, and is commonly used to assess fees on commercial shipping. 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. Although tonnage (volume) should not be confused with displacement, the long ton of 2240 lbs is derived from the fact that a "tun" of wine typically weighed that much.

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Gross register tonnage or gross registered tonnage, is a ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m3). Replaced by Gross Tonnage (GT), gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume. Typically this is used for dockage fees, canal transit fees, and similar purposes where it is appropriate to charge based on the size of the entire vessel.

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Waterline length Size of a ship

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Length between perpendiculars

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Gross tonnage Nonlinear measure of a ships overall internal volume

Gross tonnage is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage. Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weight such as deadweight tonnage or displacement.

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Displacement (ship) Ships weight

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Net tonnage 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.

Net register tonnage is a ship's cargo volume capacity expressed in "register tons", one of which equals to a volume of 100 cubic feet (2.83 m3). It is calculated by subtracting non-revenue-earning spaces i.e. spaces not available for carrying cargo, for example engine rooms, fuel tanks and crew quarters, from the ship's gross register tonnage. Net tonnage is thus used in situations where a vessel's earning capacity is important, rather than its mere size. Net register tonnage is not a measure of the weight of the ship or its cargo, and should not be confused with terms such as deadweight tonnage or displacement.

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Freeboard (nautical) Distance from the waterline to the upper deck level of a ship

In sailing and boating, a vessel's freeboard is the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship. In commercial vessels, the latter criterion measured relative to the ship's load line, regardless of deck arrangements, is the mandated and regulated meaning.

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

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References

  1. 1 2 Turpin, Edward A.; William A. McEwen (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, Maryland: Cornell Maritime Press. pp. 14–21. ISBN   0-87033-056-X.
  2. Hayler, William B. (2003). American Merchant Seaman's Manual (7th ed.). Centreville, Maryland: Cornell Maritime Press. p. G-10. ISBN   0-87033-549-9.
  3. Gilmer, Thomas C. (1975). Modern Ship Design (2nd ed.). Naval Institute Press. p. 25. ISBN   0-87021-388-1.
  4. McNicholas, Michael (2011-08-29). Maritime Security: An Introduction. Butterworth-Heinemann. p. 30. Retrieved 2014-08-25.
  5. SOLAS Consolidated Edition 2009. London: International Maritime Organization. 2009. p. 33. ISBN   978-92-801-1505-5.
  6. MARPOL Consolidated Edition 2011. London: International Maritime Organization. 2011. p. 44. ISBN   978-92-801-1532-1.