Tonnage Measurement - Historical Background

Tonnage measurement rules generally are spelled out in great detail in national laws. More detailed criteria for application of the laws are spelled out in regulations. The laws and regulations are interpreted by administrations established for the purpose. Since tonnages are used to determine the applicability of provisions of treaties, laws, and regulations and as bases for assessing charges, fees, and duties, any change in rules that would result in substantially different tonnage assignments for many vessels would disrupt the shipping industry. Historically, administrative decisions and rule changes have favored the ship owner, probably, because other segments of the shipping industry can protect their individual interests merely by changing rates for charges or by adopting parameters other than tonnages.

Transition from Deadweight to Volumetric Tonnage

The use of one half the breadth for the draft in the formula for approximating deadweight in the system preceding the Moorsom system led owners to acquire vessels that were poorly designed to obtain official tonnage assignments that were making the register tonnage a simple function of the volume of a vessel, the British opted for a coefficient (1/100) that would, on the average, slightly reduce the existing tonnages instead of opting for a coefficient that would more precisely approximate the deadweight.

The reasoning apparently was that if a system yielding higher, more precise deadweight tonnages were adopted, ship owners would find them burdened with higher bases for being assessed charges with no assurance that charging authorities would correspondingly reduce their rates. Other governments in amending their laws followed the example set by England. While the philosophy of avoiding radical changes in the tonnages assigned merchant vessels continues, governments can be persuaded to make their rules more logical.

In seeking a universal tonnage measurement system, the International Conference on Tonnage Measurement held in London in 1969 decided to do away with the system of exemptions and deductions from gross tonnage. The conference adopted a formula that would yield gross tonnages closely approximating those of vessels measured under present national rules without exemptions for shelter 'tween decks, deck spaces opened by tonnage openings, passenger spaces, and water-ballast spaces. On the other hand, the conference decided to maintain the net tonnage advantage enjoyed by shelter deck types and to extend that advantage to other types of vessels having low draft to depth ratios. That decision has already caused some charging authorities to shift their charge bases from net tonnage to gross tonnage.

Resistance to Illogical Changes

With varying degrees of success, governments have, at times, resisted illogical rule changes forced upon or willingly adopted by administrations of other governments. National rules usually provide that a space outside the double bottom adapted only to carry water ballast shall be included in the gross tonnage and deducted to arrive at net tonnage. They also provide that a space above deck shall be included in the gross tonnage if it is closed in and is available for the carriage of cargo or stores or for the berthing or accommodation of passengers or crew.

The United States adopted the Moorsom system by an Act of Congress dated May 6, 1864. That act specifically required passenger spaces to be included in the register tonnage. By an act dated February 28, 1865, however, the U.S. provided for the exemption of passenger spaces on or above the first deck which is not a deck to the hull. Only Liberia and Panama have followed that example. Pursuant to a law enacted February 6, 1909, until 1915 the U.S. included in the gross tonnage then deducted to arrive at net tonnage space adapted only for carrying water ballast out¬side the double bottom.

In 1915, however, although the law had not been changed, the Commissioner of Navigation published regulations exempting water-ballast space from gross tonnage. Again, of the other countries now having large merchant fleets, only Liberia and Panama adopted the practice. In order to facilitate movement in and out of foreign ports, vessels with passenger space and water-ballast Space exemptions must, at times, carry special appendices to their registers or British style tonnage certificates showing those spaces included in the gross tonnage.

The history of the open shelter deck exemption and IMCO Resolution A.48(III) dated October 18, 1963, titled Recommendations on the Treatment of Shelter-Deck and other 'Open' Spaces illustrates how governments can be persuaded in the interests of their ship owners, so to amend their rules that they become exactly contrary to a basic provision of their laws.

Basic tonnage measurement laws require that the volume of a closed-in structure available for the carriage of cargo or stores or for the berthing or accommodation of passengers or crew shall be measured and added to the gross tonnage. Under a British House of Lords decision of 1875 relating to the SS Bear, however, it was held in effect that a cargo space could be deemed to be under cover but open to the weather, that is not enclosed and not included in the gross tonnage.

Thereupon, to protect the interests of charging authorities, the British enacted a deck cargo law in 1876 providing for measuring the volume of any cargo found in an open exempted space and including that volume in the taxable tonnage. More important for tonnage measurement considerations, the British Board of Trade adopted regulations establishing dimensions of tonnage openings and the degree to which the openings could be protected by coamings and temporary closures without making the deck space closed.

Similar regulations were quickly adopted by other governments but not by the U.S. The U.S. Treasury steadfastly held that deck structures having bulkhead tonnage openings with or without tempo¬rary closures and so-called open shelter deck spaces were, in fact, closed spaces suitable for carrying cargo within the meaning of U.S. law. Except in cases where bilateral treaties provided for mutual acceptance of national tonnages, Collectors of Customs were ordered to add the volumes of open deck structures and open shelter decks to the register tonnages when computing tonnage duties.

In 1915, however, after responsibility for the administration of the tonnage measurement rules had been transferred to the Department of Commerce, the Commissioner of Navigation published regulations for exemption of open spaces similar to but more liberal than the British regulations. The principal difference was that the U.S. regulations did not, and to this day do not, require an otherwise open, exemptible space to be included in the gross tonnage if it is accessible through a hinged or watertight door. Such easy access without loss of exemption makes it possible to exempt crew space as open under the U.S. rules but not under the British and Oslo rules.

According to the 1915 Annual Report of the Commissioner of Navigation, the rationale for adoption of those criteria was that it removed a handicap for U.S. vessels and facilitated registration under U.S. laws of shelter deck vessels acquired by Americans from European owners at the outset of World War I.

Until shortly after World War II, classification societies permitted a shelter-deck vessel to be of substantially lighter scantlings than those required for a vessel of similar dimensions having its freeboard measured from the weather deck. After World War, II, classification societies in Europe required shelter-deck vessels to have increased strength.

That led to the development of the open-closed shelter-deck vessel. Such a vessel has sufficient strength to qualify for assignment of a minimum geometric freeboard measured from the weather deck when its tonnage openings are sealed and from the second deck when the openings meet the criteria of the tonnage measurement rules. When in the closed condition, a convertible vessel is permitted to carry more weight than in the open condition but it does so at the cost of higher gross and net register tonnages than it would have in the open condition.

Open-closed shelter-deck vessels did not develop in the United States. In fact a number of vessels built as open shelter-deck vessels after World War II had their tonnage openings closed because MarAd adopted a policy to discourage vessels that could not meet a one-compartment subdivision standard. Without the ability to open their upper tween-deck bulkheads, U.S. vessels were assigned tonnages as much as 50 percent higher than their European counterparts in the open condition.

U.S. shipowners operated for years under that handicap until after the international maritime community through IMCO Resolution A.48(III) of October 18, 1963, recommended an interim solution to the problem of the reduced safety of open shelter-deck vessels pending development and adoption of a simplified, universal system of tonnage measurement to replace the several national systems.

The Soviet Union was the first country to adopt the recommendation, and the U.S., by a law enacted September 29, 1965, was the second. Most countries with merchant marines adopted the recommendation in one form or another as quickly as they could. In its most widely accepted form it is known as the Tonnage Mark Scheme.

The Tonnage Mark Scheme provides for assignment and recognition of open shelter-deck tonnages for a full scantling vessel without tonnage openings while operating at a freeboard approximating that it would be assigned if it were fitted with tonnage openings.

In practice under the scheme, a vessel may either be issued a certificate reciting two sets of gross and net tonnages, one set corresponding to the tonnages of an open shelter-deck vessel and the other set corresponding to the tonnages of a closed full scantling vessel, or the vessel may be issued a certificate reciting only the lower tonnages.

In the case of the dual tonnage assignment, the freeboard is calculated and the load line assigned with the upper deck as the freeboard deck and a tonnage mark, comprising a horizontal line 380 mm long surmounted by a 300 mm equilateral triangle resting on its apex, is placed on each side a prescribed distance below the second deck. When the horizontal line is submerged, the higher tonnages apply for all purposes for which register tonnages are used.

When the tonnage mark is not submerged, the lower set of tonnages may be used as bases for assessing duties and other charges. Since a dual tonnage vessel measured under the Tonnage Mark Scheme needs only to submerge its tonnage mark to be of the higher tonnages, governments use the higher tonnages to determine the applicability of treaties, laws, and regulations relating to safety.

In the case of the single low tonnage assignment, the second deck is used as the freeboard deck and the tonnage mark is fixed at the upper level of the load line grid. Since, in such a case, the tonnage mark cannot be submerged without submerging the load line, the assigned low tonnages are used to determine the applicability of treaties, laws and regulations relating to safety, as well as, to assess charges.

Closed spaces for dry cargo and stores in detached superstructures and deck houses are exempted under the resolution whether or not the tweendeck spaces are exempted. Although the concept that a closed space available for dry cargo and stores may be exempted from gross tonnage cuts directly across the basic tonnage measurement tenet that such a space should be included in both gross and net tonnage, the resolution was enthusiastically adopted.

The U.S. Government supported the resolution because it enabled full scantling U.S. vessels to be assigned tonnages approximating those of open shelter-deck vessel of other countries without the reduction in safety attending tonnage openings. Other governments supported the resolution because it made it possible for their vessels to be made safer without an increase in tonnage.