Gross tonnage is a unitless index related to a ship's overall internal volume. Gross
tonnage is different from gross register tonnage. Neither gross tonnage nor gross register tonnage
is a measure of the ship's displacement and should not be confused with terms such as
deadweight tonnage 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 in 1969, and came into force on July 18, 1982.
These two measurements replaced gross register tonnage and net register tonnage. 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 certain enclosed spaces.
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 and net register tonnage to gross
tonnage and net tonnage. 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's Board of Trade who devised
one such method in 1854. The tonnage determination rules apply to all
ships built on or after July 18, 1982. Ships built before that date were given 12 years
to migrate from their existing gross register tonnage 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:
V, the ship's total volume in cubic meters, and
K, a multiplier based on the ship volume. The value of the multiplier K varies in accordance
with a ship's total volume and is applied as a kind of reduction factor in determining
the gross tonnage value - which does not have a unit such as cubic metres or tons. For smaller
ships, K is smaller, for larger ships, K is larger. K ranges from 0.22 to 0.32 and is
calculated with a formula which uses the common or base-10 logarithm:
Once V and K are known, gross tonnage is calculated using the formula, whereby GT is a function
of V:
which by substitution is . As an example, we can calculate the gross
tonnage of a ship with 10,000 m3 total volume.
Then the gross tonnage is calculated:
The inverse, on the other hand, is not as simple. Newton's method may be used for obtaining
an approximation to a ship's volume given its gross tonnage.
See also
Tonnage Moorsom System
Gross register tonnage Compensated gross tonnage
Ton List of world's largest ships by gross tonnage
Notes
References "International Convention on Tonnage Measurement
of Ships, 1969". 1969-06-23. Retrieved 2008-03-23. International Maritime Organization. "International
Convention on Tonnage Measurement of Ships, 1969". International Maritime Organization.
Retrieved 2008-03-23. Hayler, William B.. American Merchant Seaman's
Manual. Cornell Maritime Press. ISBN 0-87033-549-9. . Turpin, Edward A.; McEwen, William A.. Merchant
Marine Officers' Handbook. Centreville, MD: Cornell Maritime Press. ISBN 978-0-87033-056-8.