Net tonnage is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula. Defined in
The International Convention on Tonnage Measurement of Ships that was adopted by the
International Maritime OrganizationThe International Maritime Organization , formerly known as the Inter-Governmental Maritime Consultative Organization , was established in Geneva in 1948, and came into force ten years later, meeting for the first time in 1959...
in 1969, the net tonnage replaced the earlier
net register tonnageNet register tonnage is a ship's cargo volume capacity expressed in "register tons", one of which equals to a volume of . It is calculated by reducing non-revenue-earning spaces i.e. spaces not available for carrying cargo, for example engine rooms, fuel tanks and crew quarters, from the ship's...
(NRT) which denoted the volume of the ship's revenue-earning spaces in "register tons", units of volume equal to 100 cubic feet (2.83 m³). Net tonnage is used to calculate the port duties and should not be taken as less than 30 per cent of the ship's
gross tonnageGross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage...
.
Net tonnage is not a measure of the weight of the ship or its cargo, and should not be confused with terms such as
deadweight tonnageDeadweight tonnage is a measure of how much weight a ship is carrying or can safely carry. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew...
or
displacementA ship's displacement is its weight at any given time, generally expressed in metric tons or long tons. The term is often used to mean the ship's weight when it is loaded to its maximum capacity. A number of synonymous terms exist for this maximum weight, such as loaded displacement, full load...
. Also, unlike the net register tonnage, the net tonnage is unitless and thus can not be defined as "tons" or "net tons".
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
net register tonsNet register tonnage is a ship's cargo volume capacity expressed in "register tons", one of which equals to a volume of . It is calculated by reducing non-revenue-earning spaces i.e. spaces not available for carrying cargo, for example engine rooms, fuel tanks and crew quarters, from the ship's...
(nrt) and gross register tons (grt) to net tons (NT) and gross tons (GT) . 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. All previous methods traced back to
George MoorsomThe Moorsom System is a method created in Great Britain of calculating the tonnage or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees...
of
Great BritainGreat Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
's
Board of TradeThe Board of Trade is a committee of the Privy Council of the United Kingdom, originating as a committee of inquiry in the 17th century and evolving gradually into a government department with a diverse range of functions...
who devised one such method in 1854.
The 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 tonnage to use of NT and NGT. 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" where
gross tonnageGross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register 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 tonnageGross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage...
.
Choice of draft value
The net tonnage calculation is based on a number of factors, one of which is the moulded draft
d. The choice of the value to use for
d can be complicated. For ships subject to the
International Convention on Load LinesInternational Convention on Load Lines is an International Convention on Load Lines , signed in London on 5 April 1966, amended by the 1988 Protocol and further revised in 2003...
, the Summer Load Line draft is used, with the exception of cases where that is a timber load line. For passenger ships, the draft used is the deepest subdivision load line assigned in accordance with the
International Convention for the Safety of Life at SeaThe International Convention for the Safety of Life at Sea is an international maritime safety treaty. The SOLAS Convention in its successive forms is generally regarded as the most important of all international treaties concerning the safety of merchant ships.- History :The first version of the...
. Otherwise, if a ship has been assigned a load line by its national government, the draft for that summer load line is used. If the ship has no load line, instead, a maximum draft assigned by its national government, that value is used, if it has been assigned a maximum. Finally, for a ship to which none of the above applies, the value of
d is taken as 75 per cent of the moulded depth amidships.
12 or fewer passengers
The Net tonnage calculation is defined in Regulation 4 of Annex 1 of
The International Convention on Tonnage Measurement of Ships, 1969. It is based on two main variables:
- V_{c}, the total volume of the ship's cargo spaces in cubic meters (m³), and
- d, the ship's moulded draft amidships in meters,
The first step in calculating NT is to find the value known as
K_{2}, a multiplier based on
V_{c}. It is obtained by using the following formula:
And then these three values are used to calculate NT using this formula:
Where the factor 4d
^{2} will not exceed 1, the term V
_{c} ×Multiplication is the mathematical operation of scaling one number by another. It is one of the four basic operations in elementary arithmetic ....
K
_{2} ×Multiplication is the mathematical operation of scaling one number by another. It is one of the four basic operations in elementary arithmetic ....
4d
^{2} will not be less than 0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.
13 or more passengers
When calculating NT for ships certified to carry 13 or more passengers, an additional term is used in the NT formula. It is based on three additional variables:
- GT, the ship's gross tonnage
Gross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage...
,
- N_{1}, number of passengers in cabins with not more than 8 berths, and
- N_{2}, number of other passengers,
First, a multiplier K
_{3}, based on the ship's
gross tonnageGross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage...
is found,
Then the net tonnage is calculated:
Where the factor 4d
^{2} will not exceed 1, the term
will not be less than 0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.
The difference between the cases of 12 of fewer passengers and 13 or more passengers is due to a restriction given in the net tonnage definition that states "...when N
_{1} + N
_{2} is less than 13, N
_{1} and N
_{2} shall be taken as zero."
Usage
In the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, net tonnage is used to determine eligibility for registering boats with the federal government. Vessels with net tonnage of five or more are eligible for federal registration and not required to display state registration numbers. Most vessels longer than 25 feet are five tons or more.