Tonnage is used for many purposes in shipping – for assessment of port and harbor dues, pilotage charges, canal tolls, insurance premiums, manning levels, maritime statistics, limitations of liability, and as a criteria for application of regulations made under International Conventions, in particular, SOLAS 74/78.
The word does not imply the ship’s weight or the weight of the cargo but concerns the internal volume of the ship and is therefore an indication of the ship’s cargo carrying capacity. Ships are measured internally and issued with a certificate which states their “gross” and “net” tonnage. Crudely, “gross tonnage” is a measure of the volume of a vessel, and “net” tonnage represents the volume actually available for cargo.
Ships’ internal structures are complicated with a number of internal spaces used for various purposes, not all of them for cargo carrying. Also, large amounts of money (fees, premiums, dues, and the expense of complying with regulations, the stringency of which generally increases with gross tonnage) depend on the certified tonnage. Therefore shipowners, on the one hand, and various authorities, on the other hand, have different criteria which they would prefer applied in the measurement of ships’ internal volume.
The result is that various systems were used to determine the tonnage of cargo and passenger ships. For example, for transit through the Suez and Panama Canals, the rules of those authorities apply to give the SCNT (Suez Canal Nett Tonnage) and the PCNT (Panama Canal Nett Tonnage). Different tonnage measurement systems can give different tonnages for the same vessel. For example, for a general cargo vessel of 15,000 deadweight tonnes, the figures could be as below:
System / British / Liberian / Panama Canal / Suez Canal
Gross tonnage 9280 9080 9180 9240
Net tonnage 6350 6240 7290 7440
For a bulk carrier of about 34,000 deadweight tonnes, the tonnages could be:
Gross tonnage 23500 21000 23500 25000
Net tonnage 14800 13500 18000 20500
However, international cooperation at the level of 1MG has resulted in a new basis for tonnage measurement – the International Convention on the Tonnage Measurement of Ships, adopted in 1969 and in force from 18 July 1982. Not all countries have accepted the 1969 Tonnage Convention, e.g., Panama and. Suez Canals still use their own rules.
The purpose of the Convention is to achieve a new unified system of measurement of tonnage of all ships of all nations and presumably for all purposes. There are a number of expected advantages over the older systems, which had produced some serious deficiencies. The new “International Tonnage” has immediate application to certain classes of vessels. These are:
(a) “new” ships of 24 metres or more in length (built after 1982);
(b) existing ships of 24 metres or more in length, whose tonnage is substantially altered (e.g., by increase in size which is known as “jumboisation”);
(c) existing ships of 24 metres or more in length, whose owners apply to have them measured under the new system.
However, in specified instances, e.g., in the case of existing ships and “new” ships which are less than 24 metres in length, the older systems are still permitted to be ..used until 17 July 1994. The International Tonnage Measurement system will apply to all ships after that date.
Therefore, previous systems will first be considered before a consideration of the Tonnage Convention 1969.
A ship is “measured” and issued with a Tonnage Certificate. Since 1965, Lloyd’s Register of Shipping is also authorized to perform these functions for a ship which is classed with Lloyd’s Register. However, since 18 July 1982, certain other Classification Societies can be “Certifying Authorities” in addition to the appropriate government department.
The certificate of tonnage specifies a ship’s “gross” and “net” tonnage, the latter also being referred to as “Register” tonnage since it is the figure recorded on a vessel’s certificate of registry. If deck cargo is carried in spaces not included in tonnage measurement, the volume occupied by this cargo is added to the ship’s certified tonnage for tonnage purposes.
Gross tonnage. Under the pre-1982 British tonnage system, for purposes of measurement, gross tonnage is the sum of the following volumes:
“underdeck tonnage” + tweendeck space between upper and second deck + volume of permanently closed in spaces on/above upper deck + “excess of hatchways” (volume of hatchways > l/20/o of gross tonnage) + lighting and ventilation spaces for propelling machinery.
Underdeck tonnage. Space below the “tonnage deck”, above the double bottom tanks, open floors or ceilings and between inboard faces of frames or sparring and including protuberances such as shaft bossings, bulbous bows, and so on. (The “tonnage deck” is the second deck from above except in the case of single-decked vessels in which case it is the upper deck.)
Lighting/ventilation spaces are included in the gross tonnage if:
(a) The owner has applied for these spaces to be included in the propelling machinery space of the ship. (This propelling machinery space is deducted from gross tonnage to arrive at net tonnage.)
(b) The spaces are permanently marked by a notice stating their purpose; and
(c) A surveyor certifies that the spaces are seaworthy, properly constructed, reasonable in extent and cannot be used for other purposes.
Excluded spaces. These are not included in the “gross tonnage”: e.g., dry cargo space above the upper deck, machinery spaces above the upper deck, wheelhouse, navigation spaces, galley, dedicated water ballast tanks above the upper deck and so on.
Such excluded spaces (except dry cargo space) must be certified by a surveyor to be reasonable in extent, properly constructed and permanently marked as to their purpose. Double bottom tanks are not spaces included in the gross tonnage.
Registered tonnage. This is Gross tonnage less “Deductions” less “Allowance for Propelling Machinery Space”.
Deductions can only be allowed if they were originally included in the gross tonnage:
Master’s and crew’s accommodation.
Steering and anchoring/mooring machinery spaces.
Safety equipment and battery storage spaces.
Workshops and storerooms for maintenance equipment.
Pumprooms and pumping machinery spaces.
Dedicated water ballast tanks (up to 19 per cent of the gross tonnage when added to exempted water ballast capacity and double bottom space).
Deducted spaces must be certified as being in compliance with all regulations (e.g., crew accommodation standards, construction rules, etc.) and reasonable in extent. They must also be permanently marked by a notice stating their purpose.
Allowance for propelling machinery space. In the middle of the 19th century steam engines were replacing sails as the main means of propulsion.
Ships were no longer at the mercy of the weather; in particular, the wind, and engines could generally be used to get a ship out of danger. Therefore, from a safety viewpoint, authorities encouraged owners to fit engines to their ships. Early engines were steam reciprocating and occupied a large space in relation to the power they produced. Modern engines are reduced in size for the same power. However, an effect of the large early engines and also of the authorities’ attempts to encourage the fitting of engines was that the machinery space was allowed to be deducted from gross tonnage according to a sliding scale formula which advantaged ships with larger propelling machinery spaces.
The consequence is that owners have ships designed with large spaces reserved for propelling machinery, much larger than would be required for the main propulsion system. This has led to some anomalies where unnecessarily large propelling machinery space is provided for small engines so that the gross tonnage can be greatly reduced. The large machinery spaces could be a disadvantage to the vessel’s reserve buoyancy and could cause the vessel to sink after damage. In Hong Kong, for example, a ferry sank in 1977 after a collision. The subsequent marine inquiry established that the cause of the sinking was the large open spaces, although the engine was small. The allowance is determined on a formula.
Modified tonnage. Sometimes a shipowner may find that the cargoes his ship carries rarely submerge the ship’s normal load lines. This would be the case with light, bulky cargoes or with livestock or other low density cargo, which can have a low deadweight. He can apply to the authorities for the load lines to be marked lower down the side of the ship. Calculations for the distance from the load lines to the deck (i.e., the “freeboard”) are made with reference to the second deck from above (in the usual general cargo ship type to which this applies) and not to the uppermost deck. This is called a “greater than minimum freeboard” under the Load Line Convention. (One advantage of larger freeboards may be that Classification Societies allow lesser “scantlings” or standard dimensions for the structural components of the vessel.)
The ship’s gross tonnage is measured, this time excluding the space between the second and upper decks. Naturally, the ship will also have a lower registered tonnage. This tonnage is known as “modified tonnage”.
Modified tonnage was a concept introduced in 1967, mainly by 1MG, because of the problem posed by the type of vessel known as a “shelter decker”. The scheme resulted in a special “Tonnage mark” placed on each side of the ship.
The traditionally designed general cargo vessels of the 1950s and 1960s (still found in some old-fashioned trading areas today) was the “three-island type” with raised forecastle, accommodation block and raised “poop” .
The spaces on deck between these structures were exempted from tonnage measurement but could be used for cargo. Owners did carry cargo in these “wells” and protected it by awnings and other shelters which gradually became more permanent. Eventually the spaces were totally enclosed with a permanent deck above. These spaces became known as the “shelter deck spaces
However, to maintain the fiction that the space above the original upper deck was still “open”, the shelter deck spaces were interconnected by openings and a small “tonnage hatch” at the after end of the ship led to the shelter deck space.
The tonnage hatch could not be permanently or substantially covered. This led to a concern for the safety of these vessels. For example, if there was fire or flooding, the danger could spread from one end of the ship to the other without hindrance. The tonnage mark was introduced to encourage shipowners to close all these “tonnage openings” andrestore the vessel’s integrity without being penalised by having the shelter deck space included in the vessel’s tonnage. This method was not popular with port and other authorities which derive their revenue based on the ship’s tonnage.
For modified tonnage, the space between the second deck from above and the upper deck is not included in tonnage measurement. Where these tonnages apply, a “tonnage mark” is placed on the ship’s side, in line with the deepest load line. The tonnage mark functions as a badge rather than as a level to which the vessel may be loaded. See Picture:
Tonnage mark for modified tonnage
Alternative tonnage. A vessel may sometimes load a full cargo, which takes it down to the normal load line marks. At other times it may load a full cargo of low-density, light commodities. The shipowner can request modified tonnage in addition to the normal (e.g., British) gross and registered tonnage. A special certificate is issued, showing both tonnages. When the details of the vessel are included in publications, e.g., in the “Register” published by Lloyd’s Register of Shipping, both tonnages will also be shown.
A modified tonnage may apply to a vessel as an alternative to its normal tonnage. The same criteria for modified tonnage would apply. Once again, a tonnage mark would be used but in this case it is placed at a distance below the line of the second deck from above. This distance depends on the length and depth of the vessel. It is common for the tonnage mark to be closer to the keel than the deepest load line.
In this situation the tonnage mark is not like a “badge”. If the tonnage mark is not submerged, the modified (lower) tonnage applies for various purposes, such as harbour and pilotage dues. If it is submerged, because of the loading of the vessel, the normal gross and net tonnages will determine the various charges on the vessel. Port authorities react unfavourably even to this system, their revenue depending on the submersion of a mark. The submersion was variable and unpredictable yet the vessel’s earning capacity was not affected if the cargo was light and bulky so that the normal load lines were not submerged nor was the tonnage mark.
Suez Canal tonnage. The main purpose of the special measurement system is to establish criteria which determine the owner’s liability to pay Suez Canal tolls.
These were, raised to take effect on 1 January 1991. The tolls are expressed in “Special Drawing Rights” (SDRs) per Suez Canal Nett Ton (SCNT) and vary whether the vessel is laden or in ballast. There are a number of similarities and some significant differences with the British system of tonnage measurement. An example of the latter is the treatment of double bottom tanks. These spaces are not included in the British system but if these spaces are used for cargo or bunkers during a Canal transit, they are added to the SCNT for that transit.
The Suez Canal Authority felt that the tolls were driving vessels to use the Cape routes and, in 1987, introduced a scheme to encourage long-haul vessels to use the Canal rather than the Cape by offering them a toll rebate equivalent to the difference in cost between the two routes. The scheme was successful in attracting back some of the vessels which could use the Canal. At the end of 1989 other schemes were introduced to attract even larger vessels, e.g., VLCCs to take the Canal route partly laden and load fully at Sidi Kerir, on the Mediterranean. A reduced toll package was offered based on a round trip.
Panama Canal tonnage. This is tonnage measured according to rules published by the United States Government from 1912 under authority delegated by the Panama Canal Company. The main purpose of the special measurement system is to establish criteria, which determine the owner’s liability to pay fees for Panama Canal transits.
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 shipowners.
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’’.
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.
-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.