International Convention on Tonnage Measurement of Ships
It was adopted in 1969 and entered into force in 1982.
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. There were considerable differences between them and it was recognized that there was a great need for one single international system for tonnage measurement of ships.
The Convention provides for gross and net tonnages, both of which are calculated independently.
The rules apply to all ships built on or after the date of entry into force, whereas the ships built before that date were allowed to retain their existing tonnage for 12 years after entry into force. 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, the Convention was drafted in such a way, so as 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.
The Convention does not apply to War ships and ships of less than 24m in length.
The determination of gross and net tonnages shall be carried out by the Administration which may, however, entrust this task either to persons or organizations recognized by it. In every case, the Administration concerned shall accept full responsibility for the determination of gross and net
tonnages.
Under this Convention, an International Tonnage Certificate (1969) shall be issued to every ship, the gross and net tonnages of which have been determined in accordance with the present Convention. Such certificate shall be issued by the Administration or by any person or organization duly authorized by it. In every case, the Administration shall assume full responsibility for the certificate.
The Certificate does not have an expiry date. However, under following circumstances, it shall be cancelled or shall cease to be valid:
- If alterations have taken place in the arrangement, construction, capacity, use of spaces, total number of passengers the ship is permitted to carry, assigned load line or permitted draught of the ship.
- Transfer of Flag (grace period of maximum three months after transfer)
Definition of terms used in the Annexes
- Upper Deck – The upper decks is the uppermost complete deck, exposed to weather and sea, which has permanent means of weathertight closing of all openings in the weather part thereof, and below which all openings in the side of the ship are fitted with permanent means of watertight closing.
- Moulded Depth – The moulded depth is the vertical distance measured from the top of the keel to the underside of the upper deck at side.
- Breadth – It is the maximum breadth of the ship, measured amidships.
- Enclosed spaces –
- Enclosed spaces are all those spaces which are bounded by the ship’s hull, by fixed or portable partitions or bulkheads, by decks or coverings other than permanent or movable awnings (canopy).
- No break in a deck, nor any opening in the ship’s hull, in a deck or in a covering of a space, or in the partitions or bulkheads of a space, nor the absence of a partition or bulkhead, shall prevent a space from being included in the enclosed space.
- Excluded Spaces –
- Apart the enclosed spaces, the spaces referred to in subparagraphs (a)
to (e) below shall be called excluded spaces and shall not be included in
the volume of enclosed spaces. However, if any space fulfils at least one
of the following three conditions, then that space shall be treated as an
enclosed space:- the space is fitted with shelves or other means for securing cargo or stores
- the openings are fitted with any means of closure
- the construction provides any possibility of such openings being closed
- A space shall be considered an ‘Excluded space’ in the following cases:
- A space within an erection opposite to an end opening extending from deck to deck.
- A space under an overhead deck covering, open to the sea and weather, with no other connection to the ship side other than stanchions.
- A space in a side-to-side erection directly in way of opposite side openings.
- A space in erection immediately below an uncovered opening in the deck overhead, which is exposed to weather.
- A recess in the boundary bulkhead of an erection which is exposed to the weather and the opening of which extends from deck to deck without means of closing.
- In layman’s terms, excluded spaces are those in which cargo could not be conceivably carried .Therefore; they are excluded from the computation of tonnages as per the Convention.
- Apart the enclosed spaces, the spaces referred to in subparagraphs (a)
CALCULATION OF GROSS TONNAGE AND NET TONNAGE:
The Gross Tonnage and Net Tonnage of a ship are determined in accordance with the Regulations given in the Annex 1 of the Convention.
Separate formulae are given for Gross Tonnage and Net Tonnage.
Gross tonnage is a function of the total volume of all enclosed spaces of the ship, whereas, net tonnage is a function of the total volume of cargo spaces of the ship.
Whenever there are alterations which affect the measured GT or NT, the new GT and NT must be determined in accordance with the Regulations and applied without delay. A new Tonnage certificate will also be issued.
All volumes included in the calculation of gross and net tonnages shall be measured, irrespective of the fittings and construction material or method.
Volumes of appendages shall be included in the total volume.
Volumes of spaces open to the sea may be ‘excluded’ from the total volume.
All measurement used in the calculation of volumes shall be taken to the nearest centimetre or inch.
The volumes shall be calculated by generally accepted methods for the space concerned and with accuracy acceptable to the Administration. The calculation shall be sufficiently detailed to permit easy checking.
CONTENTS OF THE INTERNATIONAL TONNAGE CERTIFICATE (1969)
a) Name of the Issuing Authority (Flag State)
b) Name of the Ship, Distinctive Numbers or Letters (Call Sign)
c) Port of Registry
d) Date on which Keel was laid or underwent major alterations
e) Main Dimensions i.e. Length, Breadth and Moulded Depth
f) Moulded draft (corresponding to Summer Load line, or deepest subdivision load line for passenger ships)
g) Gross Tonnage
h) Net Tonnage
i) Date and Place of Original Measurement / Re-measurement
j) Details of Spaces included and excluded in Calculation
k) Date and Place of Issue of the Certificate
l) Signature of the Issuing Official and Seal of the Issuing Authority
m) Remarks