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Gross tonnage (GT, G.T. or gt) 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 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. It is the sum of the weights of cargo, fuel, fresh water, ballast water, pro ...
or displacement. Gross tonnage, along with net tonnage, was defined by the ''International Convention on Tonnage Measurement of Ships, 1969'', adopted by the
International Maritime Organization The International Maritime Organization (IMO, French: ''Organisation maritime internationale'') is a specialised agency of the United Nations responsible for regulating shipping. The IMO was established following agreement at a UN conference ...
(IMO) in 1969, and came into force on 18 July 1982. These two measurements replaced gross register tonnage (GRT) and net register tonnage (NRT). Gross tonnage is calculated based on "the moulded volume of all enclosed spaces of the ship" and is used to determine things such as a ship's manning regulations, safety rules, registration fees, and port dues, whereas the older gross register tonnage is a measure of the volume of only certain enclosed spaces.


History

The International Convention on Tonnage Measurement of Ships, 1969 was adopted by IMO in 1969.International Maritime Organization, 1982. The Convention mandated a transition from the former measurements of gross register tonnage (grt) and net register tonnage (nrt) to gross tonnage (GT) and net tonnage (NT). It was the first successful attempt to introduce a universal tonnage measurement system. Various methods were previously used to calculate merchant ship tonnage, but they differed significantly and one single international system was needed. Previous methods traced back to George Moorsom of
Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It ...
's
Board of Trade The Board of Trade is a British government body concerned with commerce and industry, currently within the Department for International Trade. Its full title is The Lords of the Committee of the Privy Council appointed for the consideration of ...
who devised one such method in 1854. The tonnage determination rules apply to all ships built on or after 18 July 1982. Ships built before that date were given 12 years to migrate from their existing gross register tonnage (GRT) to use of GT and NT. The phase-in period was provided to allow ships time to adjust economically, since tonnage is the basis for satisfying manning regulations and safety rules. Tonnage is also the basis for calculating registration fees and port dues. One of the convention's goals was to ensure that the new calculated tonnages "did not differ too greatly" from the traditional gross and net register tonnages. Both GT and NT are obtained by measuring ship's volume and then applying a mathematical formula. Gross tonnage is based on "the moulded volume of all enclosed spaces of the ship" whereas net tonnage is based on "the moulded volume of all cargo spaces of the ship". In addition, a ship's net tonnage is constrained to be no less than 30% of her gross tonnage.


Calculation

The gross tonnage calculation is defined in Regulation 3 of Annex 1 of ''The International Convention on Tonnage Measurement of Ships, 1969''. It is based on two variables, and is ultimately an increasing one-to-one function of ship volume: * ''V'', the ship's total volume in cubic metres (m3), and * ''K'', a multiplier based on the ship volume. The value of the multiplier ''K'' varies in accordance with a ship's total volume (in cubic metres) and is applied as an amplification factor in determining the gross tonnage value. For smaller ships, ''K'' is smaller, for larger ships, ''K'' is larger. ''K'' is calculated with a formula which uses the common or base-10 logarithm: : K = 0.2 + 0.02 \times \log_(V)\, Once ''V'' and ''K'' are known, gross tonnage is calculated using the formula, whereby GT is a function of V: : GT = V \times K\, which by substitution is: :GT = V\times(0.2 + 0.02\times\log_(V)) Note that the units of gross tonnage, which involve both cubic metres and log-metres, have no physical significance, but were rather chosen for historical convenience. The ''inverse'', on the other hand, is not as simple. A root-finding algorithm may be used for obtaining an approximation to a ship's volume given its gross tonnage. The exact formula is: : V = \frac where ln is the
natural logarithm The natural logarithm of a number is its logarithm to the base of the mathematical constant , which is an irrational and transcendental number approximately equal to . The natural logarithm of is generally written as , , or sometimes, if ...
and W is the Lambert W function.


See also

* Compensated gross tonnage * List of world's largest ships by gross tonnage *
Ton Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses. Mainly it describes units of weight. Confusion can arise because ''ton'' can mean * the long ton, which is 2,240 pounds ...
*
Tonnage 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 r ...


Notes


References

* * * * {{DEFAULTSORT:Gross Tonnage Shipbuilding Nautical terminology Ship measurements 1969 introductions de:Schiffsmaße#Brutto- und Nettoraumzahl (BRZ, NRZ)