Shipping Tonnage Terms

For purposes of tonnage, the length is measured along the centre line of the upper surface of the tonnage deck from a point forward where the inner surface of the frames or lining crosses the deck at the bow to a similar point aft. The tonnage breadth is taken horizontally from face to face of the inside of the frames, or of the inside of the lining on the frames provided the lining is not more than 3 in. thick. The depth for tonnage in vessels with double bottoms is measured from the upper surface of the inner bottom plating at the centre line to the under side of the tonnage deck, from which is deducted one-third of the curvature or "round" of the deck beam, while a further deduction is made if a lining is fitted on the inner bottom.

For purposes of registration, the length is taken from the fore part of the bow, on the line of the forecastle deck, to the after side of the head of the sternpost (or the centre of the rudder stock where no sternpost exists). The breadth is the maximum breadth measured to the outside of the plating of a steel ship, or to the outside of the planking of a wooden ship. The depth is measured from the top of the tonnage deck beam at the centre line amid ships to the top of the inner bottom plating. In the measurement of tonnage, the area of cross section in the space under the tonnage deck is determined at a number of places—depending on the size of the ship, and from these areas—which are obtained by actual measurement at the ship—the volume of the under-deck tonnage is ascertained. The result in cubic feet divided by 100 gives the under-deck tonnage. The spaces above the tonnage deck are now measured, such spaces being either those which extend through a complete 'tween deck or those which are situated above the upper deck. All spaces which are deemed to be properly enclosed are required to be measured and included in the gross tonnage. Such spaces as hatchways have to be measured and their volume ascer tained, and any excess over a certain amount is added to the tonnage. There are, however, certain spaces (such as shelter decks, forecastles, bridges or poops) which may be closed in certain defined ways, and which so long as they have the accepted methods of closing are not measured for tonnage nor included in the gross tonnage.

"Gross" and

"Net."—From the gross tonnage—which is the sum of the under-deck tonnage and of the tonnage above the ton nage deck—the register or net tonnage is obtained by making certain deductions on account of the space in which the machinery or engines and boilers are situated, on account of crew spaces, and on account of certain small spaces necessary for the general work ing of the ship. The most important of these is the allowance for the machinery space; the other two items, although of importance, amounting to only some io% of the total deductions permitted. The allowance for machinery spaces appears to be designed to include not only the volume actually occupied by the machinery itself, but also an allowance on account of the fuel necessary to propel the ship. The present allowance is based on the use of coal as a fuel and on the use of steam machinery. It is quite evident that the fuel space allowance must be the subject of compromise, since for the same ship the length of voyage may be very variable. It is usual to provide in all ships a certain amount of permanent bunker space, which may, for the average sea-going ship, be roughly described as that necessary for a month's voyage. The spaces occupied by the main machinery (including the necessary light and air spaces and shaft tunnels) are measured. The total

deduction permitted is either 14 times the propelling space, or 32% of the gross tonnage where the actual measurement of the propelling space lies between 13% and 20% (both inclusive) of the gross tonnage. It will therefore appear that the difference between the volume of the machinery space and the volume allowed to be deducted represents the allowance made for the space deemed to be occupied by fuel. In order to take full advantage of this deduc tion, all low-powered steamships have the volume of machinery space so arranged that it is never less than 13% of the gross tonnage. The Merchant Shipping Act of 1907, however, limits the deduction which may be permitted for propelling power to a maximum of 55% of the gross tonnage. It must be recognized that with the improvement in marine engines and the adaption of the oil motor for propelling purposes, the tendency is for the machinery spaces to be less in volume than the 13% necessary for the ordinary marine steam machinery, and in this respect it will probably be necessary in the future to reconsider the allowance which may properly be made. The register or net tonnage is now determinable (being the gross tonnage less the deductions per mitted as described above), and is the tonnage which appears on the register of the vessel and on which dues are assessed. The British system of measurement has been generally adopted by all the maritime nations, the principal differences in those which do not identically comply being on account of the method of esti mating the propelling power deduction. It may be said that gen erally the tonnage as measured by any maritime country will not differ more than about 2% from that obtained by the British measurement.

Suez and Panama "Tons..

This satisfactory state of affairs does not, however, obtain with the Suez and Panama canal authorities, whose regulations differ from the British in giving less allowance for deductions, and in requiring more of the superstruc tures to be taken into the gross tonnage. Approximately it may be stated that the gross tonnage for the Suez Canal is 5% higher than the British, and that for the Panama Canal io% higher, while the net tonnage for the Suez Canal is nearly 3o% higher, and for the Panama Canal about 25% higher, than the British meas urement.

It is difficult to express briefly the relation between tonnage and size of ship. As an example, an ordinary cargo steamer which can carry a nominal deadweight of about 8,000 tons avoirdupois of cargo, fuel and stores will have a displacement of about 11,50o tons, a gross tonnage of about 5,200, and a registered or net ton nage of about 3,200. The net tonnage is relatively highest in this type of steamer, where it may be as much as 65% of the gross tonnage. This figure is reduced to 5o% for coasting cargo vessels of small size. In fast steamers, where a large proportion of the space is taken up by machinery, the net tonnage may be as low as 5o% of the gross tonnage, and for such extreme types as the cross-channel boat the ratio may be as low as 40%.

BIBLIOGRAPHY.—Board of Trade Instructions as to' the Tonnage Measurement of Ships (H. M. Stationery Office) ; E. R. Johnson, Measurement of Vessels for the Panama Canal, Washington Govern ment Printing Office (1913) includes references to tonnage measure ment systems adopted by various nationalities; A. Van Driel, Tonnage Measurement, The Hague Government Printing Office (1925).

(W. S. A.)