History
The International Convention on Tonnage Measurement of Ships, 1969 was adopted by IMO in 1969. Ships built before that date were given 12 years to migrate from their existing tonnage to use of NT and GT. 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 units "did not differ too greatly" from the traditional GRT and NRT units.
Both NT and GT are obtained by measuring ship's volume and then applying a mathematical formula. Net tonnage is based on "the moulded volume of all cargo spaces of the ship" while gross tonnage is based on "the moulded volume of all enclosed spaces of the ship". In addition, a ship's net tonnage is constrained to be no less than 30% of her gross tonnage.
Read more about this topic: Net Tonnage
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