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How do you determine the size of a ship?
Well, it is solely a matter of how you go about it.

Tonnage regimes in shipping:
a critical overview

“Grete Mærsk” in Århus, Denmark. Recently delivered from Odense shipyard, she is now among the largest container ships in the world with an official capacity of 7,500 TEU. The actual capacity is however significantly larger. There are estimates of up to 12,000 TEU. Photo: Bent Mikkelsen

By Yakabe Fumi, Technical Official, Inspection & Measurement Division,
Maritime Bureau, Ministry of Land Infrastructure and Transport,
yakabe-f635q@mlit.go.jp

Tonnage is a phenomenon that is unique to ships and shipping. Essentially, tonnage is a concept that has to do with the size of a ship; and size can be depicted in terms of weight or space or both. Furthermore, if one were to examine the table appearing below in a attempt to figure out which of the six different types of ships is the largest, it would be almost impossible to reach a definitive conclusion. The notion of tonnage expressed as GT or NT was devised to address this basic issue.

Big and small. The ro-ro “Stena Carrier II” and the bunker vessel “Ängö”. Photo: Pär-Henrik Sjöström

Concept of tonnage – scale of ships
As a tool to indicate the size of a ship, there are different tonnages and other scales which are not based on tonnage but which also express size.
Gross Registered Tonnage (GRT) and Net Registered Tonnage (NRT): These were basically measured according to the Moorsom system. GRT shows the total enclosed capacity of a ship and consists of:

• Under-deck tonnage
• Tweendeck spaces
• Superstructures
• Deckhouses and other erections
• Exempted spaces (navigational spaces such as wheelhouses and chartrooms, galleys, stairways, light and air spaces)

One ton is equivalent to 100 cubic feet of internal space.
NRT illustrates spaces available for cargo and passengers, that is earning capacity of the ship, and is calculated by deducting certain spaces from GRT.
These terms indicate tonnages which are officially registered in each administration according to their own rules, but are not measured by any unified form.

Gross Tonnage (GT) and Net Tonnage (NT): These are tonnages which are determined under the 1969 IMO Tonnage Convention. The moulded volume of all enclosed spaces are counted into GT, and by using a standard formula; the volume expressed in cubic metres is transformed into dimensionless GT. Such non-dimensional value was chosen to express sizes of ships, as a result of consideration for the adverse effect to the industry’s economics. Since GT includes all enclosed spaces and is measured in cubic metres, it was a concern that the largely different figures between GRT and GT would produce confusion and misunderstanding in the industry.

NT is a measure of the useful capacity of a ship and is also dimensionless, based on the moulded volume of the cargo spaces.

Deadweight (DWT) and Twenty-feet Equivalent Unit (TEU): DWT is the weight of a ship’s content. It is calculated from the difference between loaded displacement and lightweight of the ship, consisting of the total weight of cargo, fuel, water ballast, fresh water, stores, crew, passengers and luggage.

The container capacity of a ship is expressed in TEU. One TEU is equivalent to one twenty-feet shipping container.

Compensated Gross Tonnage (CGT): This is the measurement of commercial shipbuilding output and productivity, which are not to be determined in GT or DWT, because some ships need a much higher work content per GT than others. CGT expresses the amount of necessary workload for the particular ship, and additionally CGT can be used as a parameter of shipyards, since cost per CGT can show efficiency of the shipyard (Bertram & Weiss, 1997, p 21).

Legal regime
The basis of those scales and methods of measurement are essential to be internationally recognized.
1969 International Convention on Tonnage Measurement of Ships: As mentioned above, this is a universal system of tonnage measurement, established by the Inter-Governmental Maritime Consultative Organization adopted in 1969, and entered into force in 1982 with an interim period until 1994. 141 States in the world have contracted in and 98.18 per cent of the world tonnage complies with the Convention. This Convention applies to ships of 24 metres or longer in length engaged on international voyages, and provides for rules and the form of the International Tonnage Certificate (1969), hereby ITC69, and technical regulations for determining GT and NT of ships.

“Freedom of the Seas”, the largest cruise ship of today, measured by GRT (158,000). Photo: Aker Finnyards

Panama and Suez Canal: Before the Convention was created, the Authorities at the Suez Canal and the Panama Canal established their own systems of tonnage measurement. Those systems originated from the Moorsom system, and in Suez Canal the unique system has been used until today. IMO recommended acceptance of use of the Convention rule at both Panama and Suez Canal in 1981, and the first change occurred at Panama Canal in 1994, while the Convention became mandatory to all existing ships. The Panama Canal Universal Measurement System (PC/UMS) uses the same basis of the Convention. In 2002, the Panama Canal changed its flat rate for all ships to flexible rates depending on ship size and type, and separate locomotive usage rates were added to the toll structure. It aims at more equitable charges to each ship. Accordingly, the Panama Canal Commission is proposing to consider the volume of the capacity of containers carried on or above the main deck regarding computation of canal tolls (Nelson, 2004).

Use of tonnages
As a measurement of the size of the ship, tonnage is used by those who may have interests in a ship to charge and to regulate.

Port dues: According to UNCTAD, a port charges ships with tariffs of various functions. Apart from facilities and service tariffs, there are general tariffs such as port dues and wharfage, which is assessed by the value that users attach to them. In order to provide continuously sufficient quality of service to most valued customers while giving open access to all users, these main tariffs are critical to port authorities. Although the charging unit for wharfage is freight, port dues are based on GT, NT, Length of ship or Length x Breadth x Draft and are also differentiated among types of ships. In many ports in the world, GT is used as the unit determining port dues.

Regulations – Major IMO Documents: In order to define the range of applications of individual regulations on safety and pollution prevention at sea, in some relevant cases, regulations include provisions specifying which ships are applicable to that particular set of regulations. Such application could be limited according to the size of ship, type of ship, type of engaged voyage, construction date, equipment or cargo etc. The size of a ship is often represented by GT or length, in many international, regional and national regulations.

Tonnage Tax: Recently many States have adopted a tonnage tax regime for shipping companies. It allows companies to choose the method of measuring taxable profits from shipping activities by reference to the total tonnage of their ships instead of by reference to variable business results. According to Smith & Middleditch (2004, pp 4–8), accounting and financial benefits are pointed out in the UK tonnage tax regime, even though the tonnage tax system may not bring about large savings of tax. For instance, it abridges accounting work as a consequence because tax payable does not depend on deferral reliefs such as capital allowances.

Another beneficial point is proof of certainty of the company. As investors and business partners will be able to confirm the level of tax payable at any time, it should provide more flexible financing. Tonnage tax has attracted shipowners and in fact, an increase in the number of ships under each national flag has been observed. In many cases, the system has NT criterion to determine the rate of tax. Here tonnage is used as a representative scale to measure the ability of the shipping activities of a company.

Tonnage vs. maritime safety and pollution prevention
From the shipowners’ point of view, tonnages are sensitive factors since they bring about the matter of costs as mentioned above. Therefore, efforts to reduce the tonnages of a ship have never come to an end, while maximizing DWT is always a target for naval architects. This contradiction leads to discussions about the seaworthy design of ships, as there are some warning articles of industrial journals.

“Jahre Viking” was the world’s largest tanker (564,763 MDWT) until 2004, when she was converted into an FSO and renamed “Knock Nevis”. Photo: Shipping Publications

Longitudinal strength: The resistance against longitudinal stresses at sea is calculated from the bending moment and sheer force caused by loaded cargo and sagging and hogging. A smaller depth, which would allow carriage of more deck cargo is one way of reducing GT, however it diminishes the moment of inertia of the transverse section of the hull, which resists against bending moments, and consequently lessens the longitudinal strength against bending moments. Furthermore, especially in the case of container ships, it should be taken into account that the on-deck cargo adds more longitudinal stress on the structure of the ship.

Stability: Transverse static stability is defined from the centre of gravity of a ship and the centre of buoyancy of the submerged part of the hull. In fact, the hull form and the distribution of weights determine static stability (Derrett, 1999). When intending to reduce GT, the depth and breadth of the ship may be minimized at the design stage. Consequently, it may happen that an inadequately shaped hull cannot achieve the desired stability. Furthermore, a significant stability problem has been observed in terms of weight distribution in container ships, that is, heavier cargo is loaded above deck than below, as a result of the reduction of the depth.

Container vessels
Gray (2002) wrote representative articles about the design of container ships regarding size and seaworthiness, while the issue has brought about vigorous discussion. The sizes of container ships have been growing rapidly in those days according to improvement of ports and port facilities.

Propelling power is another factor that determines the size of container ships, as container ships require a service speed of about 25 knots. Apart from these restrictions, factors determining the size of ships, such as cargo volume on a specific route, number of ships in the fleet to be employed for that route and the time span for one round trip, indicate that the trend of more demand of large container ships will continue (Lee, 2003, p 132).

There are some significant features in the design of container ships related to the technical development, such as ultimately minimised freeboard, high stacks of containers, cargo on deck and open-top. These have generated various concerns over seaworthiness. The reason for that evolution is often described as reduction of GT, moreover, accessibility to the cargo, visibility and less building cost are notable contributors.

Tankers
There are some recommendations that come out of considerations for measures to prevent oil pollution. In 1967 IMO recommends that machinery space exclusively used for prevention of oil pollution and slop tanks should be deducted from GT. Tonnage of segregated water ballast tanks that comply with MARPOL regulations should be declared in ITC69 according to IMO resolution A.388(X) of 1977, to be used for reduction of the GT for calculating tonnage based fees. For example, the Council of European Union created a regulation and recommendation to deduct the segregated ballast tank tonnage from GT on which dues are based, in order to encourage the use of oil tankers fitted with segregated ballast capacity (EUROPA, 2003).

Tonnage measurement
Tonnage measurement consists of calculation using plans and calculation data from the shipyard and survey at the shipyard and on board, and takes place when ships are newly built and when structural alterations are made on ships in service.

Main dimensions, GT, NT, spaces included in tonnage and excluded spaces are to be specified as per ITC69. Accuracy and fairness are the most important factors to be borne in mind, while carrying out tonnage measurement.
Displacement volumes are generally computed according to Simpson’s first rule or any other integration method, which is accepted as sound naval architectural practice.

Sections above the waterline are often of standard geometric shape and therefore calculated with simple geometric formulas to yield volumes.

New technology in computing tonnages
The major shipyards and related ship-designing companies have created special computer softwares to calculate tonnages, which have been approved by many authorities, who determine and issue tonnage certificates, for example, the Napa Ltd., a separated company from a historic shipyard in Finland, provides computer-based systems for ship safety and performance, which is now used by 12 authorities in the world. Those designated softwares are accurate and flexible so they can be applied to a wide range of ship types. Considerable advantages of use of such softwares have been raised. Those softwares are often programmed to be able to visualize the result of computation, and therefore wrong input data and miscalculation can be easily detected in earlier stages.

Another benefit is unification and simplification of the calculating process in determining both tonnages and load line, since they are often combined with the program of stability and load calculation. Additionally, calculation of longitudinal bending moments and sheer forces can be included in the program.
On the other hand, there are some concerns regarding the use of these softwares, which should be taken into account. It is possible that different programs may yield different results depending on their methods of computation.

In this case, it is still doubtful to call tonnages as reliable unified units to express the size of a ship. The development of systems and programs in both hardware and software may also cause problems, as a result of recent rapid changes in information technology. Compatibility always comes into question when old systems are replaced by new systems. Since a tonnage certificate is basically valid for the ship’s lifetime, data on tonnage calculation is necessarily stored by the authority and should be available at any time. Use of computer-aided systems should be dealt with carefully.

Conclusion
Recently, some doubt has been raised in the maritime industry, as to whether the present tonnage system is appropriate for “safer and cleaner ocean.” This question seems to be derived from the fact, that tonnages do not literally represent the size or earning capacity of a ship and is in the nature of challenge to save costs. Container ships, for example, can carry more cargo on deck, which is not included in the tonnage. The challenge of saving costs causes rigorously minimized enclosed spaces other than cargo spaces to reduce GT, and there is a risk that this effort also affects seaworthiness. It may result in reduction of safe and healthy life of crew and higher risk of cargo and environmental pollution at sea.

To solve this dilemma in the industry, a new system which harmonizes all such aspects and avoids conflicts between safety and economics of ships may be desirable. However, before going further, it will be necessary to review the usage of tonnages with those concerns and make appropriate use of tonnages or other scales to fit to each purpose.

Fairplay Solutions (2002, p 24) recommends an alternative measure to determine port dues expressed by the formula Length x Beam x Draft divided by Depth. In this formula, ship’s occupation in port is clearly taken into account and it does not reflect the volume by higher freeboard.

For smaller ships which have GT close to the margin of application of each regulation, there should be consideration given to achieving the purpose of the regulation. As mentioned earlier, various other conditions exist on which the application can be determined; moreover, the stage of implementation of such regulations needs to take into consideration sufficient and effective application. The method of tonnage measurement has an important role. Therefore clear and unified practical interpretation of tonnage measurement rules is essential, in addition to accuracy and fairness.

The tonnage tax regime does not seem to carry such negative effects directly. The point of discussion here is that tonnage tax should be based on a truthful scale when determining payable tax. Since tonnage has been recognized as a unified measure, it become suitable, even though it displays only a ship’s total volume and is not exact about the revenue raised from the ship. This fact should be respected when discussing the present tonnage regimes.

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