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. Although all went back to the method devised by George Moorsom of the British Board of Trade in 1854, there were considerable differences between them and it was recognized that there was a great need for one single international system. The Convention provides for gross and net tonnages, both of which are calculated independently.
The rules apply to all ships built on or after 18 July 1982 – the date of entry into force – while ships built before that date were allowed to retain their existing tonnage for 12 years after entry into force, or until 18 July 1994.
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, and as far as possible, the Convention was drafted to ensure that gross and net tonnages calculated under the new system did not differ too greatly from those calculated under previous methods.
Gross tons and net tons
The Convention meant a transition from the traditionally used terms gross register tons (grt) and net register tons (nrt) to gross tons (GT) and net tons (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 molded volume of all enclosed spaces of the ship. The net tonnage is produced by a formula that is a function of the molded volume of all cargo spaces of the ship. The net tonnage shall not be taken as less than 30 percent of the gross tonnage.
Determination of Tonnages.
The determination of gross and net tonnages shall be carried out by the Administration which may, however, entrust such determination either to persons or organizations recognized by it. ln, in every case the Administration concerned shall accept full responsibility for the determination of gross and net tonnages.
(1)A ship flying the flag of a State the Government of which is a Contracting Government shall be subject, when in the ports of other Contracting Government, to inspection by officers duly authorized by such Governments. Such inspection shall be limited to the purpose of verifying:
(a) that the ship is provided with a valid International Tonnage Certificate (1969); and (b) that the main characteristics of the ship correspond to the data given in the certificate.
(2) In no case shall the exercise of such inspection cause any delay to the ship.
(3) Should the inspection reveal that the main characteristics of the ship differ from those entered on the international Tonnage Certificate (1969) so as to lead to an increase in the gross tonnage or the net tonnage, the Government of the State whose flag the ship is flying shall be informed without delay.
Definitions of Terms
(1) Upper Deck
The upper deck is the uppermost complete deck exposed to weather and sea, which has permanent means of weather-tight closing of all openings in the weather part thereof, and below which all openings in the sides of the are fitted with permanent means of watertight closing. In a ship having a stepped upper deck the lowest line of the exposed deck and the continuation of that line parallel to the upper part of the deck is taken as the upper deck.
(2) Molded depth
(a) The molded depth is the vertical distance measured from the top of the keel to the underside of the upper deck at the side. In wood and composite ships, the distance is measured from the lower edge of the keel rabbet.
Where the form at the lower part of the midship section is of hollow character, or where thick garboards are fitted, the distance is measured
from the point where the line of the flat of the bottom continued inwards cuts the side of the keel.
(b) In a ship having rounded gunwales, the molded depth shall be measured to the point of intersection of the molded lines of the deck and side shell plating, the lines extending as though the gunwales were of angular design.
(c) Where the upper deck is stepped and the raised part of the deck extends over the point at which the molded depth is to be determined, the
molded depth shall be measured to a line of reference extending from the lower part of the deck along a line parallel with the raised part. (IACS UI TM 1)
The breadth is the maximum breadth of the ship, measured amidships to the molded frame in a ship with a metal shell and to the outer
the surface of the hull in a ship with a shell of any other material.
(4) Enclosed Spaces
Enclosed spaces are all those spaces that are bounded by the ship’s hull, by fixed or portable partitions or bulkheads, by decks or coverings other than permanent or movable awnings. No break in a deck, nor any opening in the ship’s hull, in a deck or in a covering of space, or in the partitions or bulkheads of space, nor the absence of a partition or bulkhead, shall preclude a space from being included in the enclosed space.
(5) Excluded Spaces
Notwithstanding the provisions of paragraph (4) of this Regulation, the spaces referred to in subparagraph (a) to (e) inclusive of this paragraph shall be called excluded spaces and shall not be included in the volume of enclosed spaces, except that any such space which fulfills at least one of the following three conditions 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:
(i) A space within an erection opposite an end opening extending from deck to deck except for a curtain plate of a depth not exceeding by more than 25 millimeters (one inch) the depth of the adjoining deck beams, such opening having a breadth equal to or greater than 90 percent of the breadth of the deck at the line of the opening of the space. This provision shall be applied so as to exclude from the enclosed spaces only the space between the actual end opening and a line drawn parallel to the line or face of the opening at a distance from the opening equal to one-half of the width of the deck at the line of the opening
(ii) Should the width of the space because of any arrangement except by convergence of the outside plating, become less than 90 percent of the breadth of the deck, only the space between the line of the opening and a parallel line drawn through the point where the athwartships width of the space becomes equal to, or less than, 90 percent of the breadth of the deck shall be excluded from the volume of enclosed spaces
(iii) Where an interval which is completely open except for bulwarks or open rails separates any two spaces, the exclusion of one or both of which is permitted under sub-paragraphs (a) (i) and/or (a) (ii), such exclusion shall not apply if the separation between the two spaces is less than the least half breadth of the deck in way of the separation.
A space under an overhead deck covering open to the sea and weather, having no other connection on the exposed sides with the body of the ship than the stanchions necessary for its support. In such a space, open rails or a bulwark and curtain plate may be fitted or stanchions fitted at the ship’s side, provided that the distance between the top of the rails or the bulwark and the curtain plate is not less than 0.75 meters (2.5 feet) or one-third of the height of the space, whichever is the greater.
A space in a side-to-side erection directly in way of opposite side openings is not less in height than 0.75 meters (2.5 feet) or one-third of the height of the erection, whichever is greater. lf the opening in such an erection is provided on one side only, the space to be excluded from the volume of enclosed spaces shall be limited inboard from the opening to a maximum of one-half of the breadth of the deck in way of the opening.
A space in an erection immediately below an uncovered opening in the deck overhead provided that such an opening is exposed to the weather and the space excluded from enclosed spaces is limited to the area of the opening.
A recess in the boundary bulkhead of an erection that is exposed to the weather and the opening of which extends from the deck to deck without means of closing provided that the interior width is not greater than the width at the entrance and its extension into the erection is not greater than twice the width of its entrance.
A passenger is every person other than:
(a) the master and the members of the crew or other persons employed or engaged in any capacity on board a ship on the business of that ship; and
(b) a child under one year of age.
(7) Cargo Spaces
Cargo spaces to be included in the computation of net tonnage are enclosed spaces appropriated for the transport of cargo that is to be discharged from the ship, provided that such spaces have been included in the computation of gross tonnage. Such cargo spaces shall be certified by permanent marking with the letters CC (cargo compartment) to be so positioned that they are readily visible and not to be less than 100 millimeters (4 inches) in height.
Watertight means that in any sea conditions water will not penetrate into the ship