International tonnage

International tonnage. Clearly, there are a number of differences between all the existing systems. This Means that a ship’s tonnage can vary depending on the applicable rules. The variations are reflected in financial consequences for ship-owners.


However, it is more serious that the different rules are used by shipowners to their financial advantage in ways which could lead to anomalies and possible unsafe practices.

For example, identical ships may have different tonnages because they operate under different Registries.

The attempt in 1963 to solve some of the problems by the introduction of the “tonnage mark” has generally failed and therefore IMCO (as it was) convened a Conference to establish an international system.

Advantages of 1969 Tonnage Measurement

1. It is relatively easy to apply, the Gross Tonnage (GT) being based on the total volume of all enclosed spaces and a formula:

GT = K1 x V

where V = Total volume of all enclosed spaces of the ship in cubic metres.

K1 = a coefficient in the tonnage regulations.

“net” tonnage is determined by a formula based on the volume of cargo spaces and/or the number of passengers carried.

NT = K2 x Vc (4d / 3D)2+ K3 (N1 + N2/10)

where K2 = a coefficient in the tonnage regulations.

Vc= total volume of cargo spaces in cubic metres.

K3 = 1.25 (GT + 10000)/10000.

D = moulded depth amidships (M.).

d = moulded draught amidships.

N1 = passengers in cabins with eight or less berths.

N2 = other passengers.

2. It is therefore quicker to apply: a 50 per cent saving in time has been experienced.

3. It provides a more realistic value of ship’s size (GT) and earning capacity (NT). The absence of a variety of exempted and deducted spaces will reduce, if not completely neutralise, manipulation of the rules by ship designers and ship-owners.

4. Tonnage marks and dual tonnages will be eliminated, but ship safety will not be prejudiced by ‘‘tonnage openings’’.

Significant features

1. Spaces open to the sea are completely excluded from measurement.

2. Cargo spaces will be marked with the letters “cc”.

3. The tonnage of (non-earning) segregated ballast tanks in an oil tanker complying with MARPOL 73/76 will be specially noted on the ship’s “International Tonnage Certificate (1969)”.

4. Deck cargo carried in any uncovered space on deck is added to the tonnages by a formula by which only about one-third of the actual volume of the cargo is added.

5. The certificate becomes invalid if the variables for the Gross Tonnage and Net Tonnage formulae are altered. If the formula variables are altered, e.g., if the number of passengers carried is reduced, the net tonnage cannot be reduced and re-certified more than one a year. Increases in net tonnage are permitted at any time! 6. If the ship is transferred between flag countries, the certificate becomes invalid, unless the transfer is to the flag of a state, which has also adopted the Tonnage Convention. In this case the certificate remains valid for three months after the transfer or until the new state of registry issues a new International Tonnage Certificate (1969).

The Interim Scheme for tonnage measurement for certain ships

It is to be expected, as with any changes, that some ships would have different tonnages than if one of the older systems applied. For example, open shelter deck ships and ships with large exempted spaces will have an increased gross tonnage. Other ships, such as ro/ro vessels and car ferries, will have increased net tonnages, up to 300 per cent. (Some ships, such as bulk carriers, ore carriers and ships of under 500 gross tons, may have reduced net tonnages.)

These tonnages could cause an increased obligation on shipowners to comply with safety regulations made under SOLAS 1974/1978. Therefore provision is made for special consideration of certain ships.

These are:

—all new ships whose keels were laid before 1986.

—new cargo ships of less than 1,600 gross tonnage (measured under the older system) whose keels are laid before 18 July 1994.

These ships will have gross and net tonnages determined by the 1969 Convention. They may also have a gross tonnage determined by an older method.

United Kingdom vessels, which have these two gross tonnages, are issued with a special “British Tonnage Certificate” which is endorsed to the effect that the ship is measured according to the interim scheme.

The gross tonnage determined by the older method may be used only for the purpose of complying with SOIAS 1974/78.

Safety certificates issued under the SOLAS Convention will contain the ship’s “previous” gross tonnage and a special endorsement to the effect that the certified gross tonnage was ascertained according to the regulations in force before 18 July 1982.


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