Chapter 06 Tonnage of Ships

7/28/2019 Chapter 06 Tonnage of Ships

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Chapter 6

Tonnage of Ships

Tonnage is used for many purposes in shippingfor assessment of port and harbour 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

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The word does not imply the ships weight or the weight of the cargo but concerns the internal

volume of the ship and is therefore an indication of the ships 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 expenseof 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 CanalGross tonnage 9280 9080 9180 9240

Net tonnage 6350 6240 7290 7440For 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

measurementthe 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 overthe older systems, which had produced some serious deficiencies. The new International Tonnage hasimmediate 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


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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, Lloyds Register of

Shipping is also authorised to perform these functions for a ship which is classed with Lloyds 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 ships gross and net tonnage, the latter also being referred

to as Register tonnage since it is the figure recorded on a vessels 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 ships 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 theship. (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 upperdeck, 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 notspaces 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:

Masters and crews accommodation.

Navigation spaces.Chain lockers.

Steering and anchoring/mooring machinery spaces.


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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 waterballast capacity and double bottom space).

Deducted spaces must be certified as being in compliance with all regulations (e.g., crewaccommodation 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 19 th century steam engines werereplacing 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 view point, 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 ofengines 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 propellingmachinery, much larger than would be required for the main propulsion system. This has led to someanomalies where unnecessarily large propelling machinery space is provided for small engines so thatthe gross tonnage can be greatly reduced. The large machinery spaces could be a disadvantage to thevessels 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 thesinking 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

ships 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 ships 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 as shown in Figure 6.1 below:

Figure 6.1


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The spaces on deck between these structures were exempted from tonnage measurement but couldbe used for cargo. Owners did carry cargo in these wells and protected it by awnings and othershelters 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. See Figure 6.2:

Figure 6.2

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

and-restore the vessels integrity without being penalised by having the shelter deck space included in

the vessels tonnage. This method was not popular with port and other authorities which derive their

revenue based on the ships tonnage.

For modified tonnage, the space between the second deck from above and the upper deck is notincluded in tonnage measurement. Where these tonnages apply, a tonnage mark is placed on the ships

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 Figure 6.3:

Figure 6.3 Tonnage mark for modified tonnage

Alternativetonnage. A vesselmay 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 ~ub1ished by Lloyds 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 revenuedepending on the submersion of a mark. The submersion was variable and unpredictable yet the vessels

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 Canaltonnage. The main purpose of the special measurement system is to establish criteria which

determine the owners 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,


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they are added to the SCNT for that transit.

Examples of SCNT are:

Suez Canal Net Tonnage (SCNT)

First 5000 Next 5000 Next 10000

Type of vessel Laden Ballast Laden Ballast Laden Ballast

Crude oiltankers 7.00 5.95 3.90 3.32 3.50 2.98Products

tankers 7.00 5.95 3.90 3.32 3.55 2.98Bulkers 7.00 5.95 3.90 3.32 2.80 2.38

For container vessels, surcharges are applied depending on the number of tiers containers carried

above the weather deck, e.g., 7.5 per cent, 10 per cent and 11.5 per cent for three, four and five tiers

respectively.

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 wassuccessful 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 owners liability to pay fees

for Panama Canal transits.

From 1 October 1989, the transit tolls were:

For loaded vessels, US$2.01 per Panama Canal Net Ton (PCNT).For vessels in ballast, US$1.60 per PCNT.

In 1989 and 1990 there were considerable changes and surcharges and also extra charges for the

infrastructure, such as tugs and line handlers.

Internationaltonnage. Clearly, there are a number of differences between all the existing systems. This

Means that a ships 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 differentRegistries.

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:


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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 ships 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 ships 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 nettonnage 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 regulationsmade under SOLAS 1974/1978. Therefore provision is made for special consideration of certain ships:

These are:


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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 ships 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.

Chapter 06 Tonnage of Ships

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