Tonnage Calculation
About Tonnage Calculation
Navalista performs tonnage
calculation in the shortest possible time
according to the terms of
reference.
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 or displacement.
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Gross tonnage, along with net tonnage, was defined by the
International
Convention on Tonnage Measurement
of Ships, 1969, adopted by the International Maritime Organization 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.
A small digression into history
The International Convention on Tonnage Measurement of Ships, 1969 was adopted by
IMO in 1969. 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.
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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's Board of Trade who devised one such method in 1854.
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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.[2] 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.
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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.