Net tonnage

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Net tonnage is calculated by measuring a ship's internal volume and applying mathematical formulae.
Net tonnage is calculated by measuring a ship's internal volume and applying mathematical formulae.

Net tonnage (often abbreviated as NT, N.T. or nt) is a calculated representation of a the internal volume of a ship's cargo holds. It is expressed in "tons", a unit of volume defined as 100 cubic feet (~2.83 m³).[1] Net tonnage is not a measure of the ship's weight or displacement and should not be confused with terms such as deadweight tonnage, net register tonnage, or displacement.

Net tonnage, along with gross 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 tons (grt) and net register tons (nrt). Net tonnage is calculated based on "of the moulded volume of all cargo spaces of the ship" and is used to determine things such as a ship's port dues.

Contents

[edit] History

The International Convention on Tonnage Measurement of Ships, 1969 was adopted by IMO in 1969.[2] The Convention mandated a transition from the former measurements of net register tons (nrt) and gross register tons (grt) net tons (NT) and gross tons (GT) .[2] It was was the first successful attempt to introduce a universal tonnage measurement system.[2]

Various methods were previously used to calculate merchant ship tonnage, but they differed significantly and one single international system was needed.[2] All previous methods traced back to George Moorsom of Great Britain's Board of Trade who devised one such method in 1854.[2]

The rules apply to all ships built on or after July 18, 1982.[2] Ships built before that date were given 12 years to migrate from their existing tonnage to use of NT and NGT.[2] 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.[2] 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.[2]

Both NT and GT are obtained by measuring ship's volume and then applying a mathematical formula.[2] Net tonnage is based on "the moulded volume of all cargo spaces of the ship where gross tonnage is based on "the moulded volume of all enclosed spaces of the ship"."[2] In addition, a ship's net tonnage is constrained to be no less than 30% of her gross tonnage.[2]

[edit] Calculation

[edit] 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 Lines, 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 Sea. 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.

[edit] 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:

  • Vc, 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 K2, a multiplier based on Vc. It is obtained by using the following formula:

K2 = 0.2 + 0.02 × log10(Vc)

And then these three values are used to calculate NT using this formula:

NT = Vc × K2 × 4d2

Where the factor 4d2 will not exceed 1, the term Vc × K2 × 4d2 will not be less than 0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.

[edit] 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,
  • N1, number of passengers in cabins with not more than 8 berths, and
  • N2, number of other passengers,

First, a multiplier K3, based on the ship's gross tonnage is found,

     1.25 × (GT + 10,000)
K3 = --------------------
          10,000

Then the net tonnage is calculated:

NT = Vc × K2 × 4d2 + K3 × (N1 + N2)

Where the factor 4d2 will not exceed 1, the term Vc × K2 × 4d2 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 N1 + N2 is less than 13, N1 and N2 shall be taken as zero."

[edit] Notes

  1. ^ 100 cubic feet is 2.8316846592 cubic meters exactly.
  2. ^ a b c d e f g h i j k l m International Maritime Organization, 1982.

[edit] References

[edit] See also

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