The Convention, adopted by IMO in 1969, was the first successful attempt to introduce a universal tonnage measurement system.
Previously, various systems were used to calculate the tonnage of merchant ships. Although all went back to the method devised by George Moorsom of the British Board of Trade in 1854, there were considerable differences between them and it was recognized that there was a great need for one single international system.
The Convention provides for gross and net tonnages, both of which are calculated independently.
The rules apply to all ships built on or after 18 July 1982 - the date of entry into force - while ships built before that date were allowed to retain their existing tonnage for 12 years after entry into force, or until 18 July 1994.
This phase-in period was intended to ensure that ships were given reasonable economic safeguards, since port and other dues are charged according to ship tonnage. At the same time, and as far as possible, the Convention was drafted to ensure that gross and net tonnages calculated under the new system did not differ too greatly from those calculated under previous methods.
Gross tonnage and net tonnage
The Convention meant a transition from the traditionally used terms gross register tons (grt) and net register tons (nrt) to gross tonnage(GT) and net tonnage (NT).
Gross tonnage forms the basis for manning regulations, safety rules and registration fees. Both gross and net tonnages are used to calculate port dues.
The gross tonnage is a function of the moulded volume of all enclosed spaces of the ship. The net tonnage is produced by a formula which is a function of the moulded volume of all cargo spaces of the ship. The net tonnage shall not be taken as less than 30 per cent of the gross tonnage.