As a rule, the net tonnage is understated by the of ficial method of measurement. Sonic uniformity in this situation has been obtained by, the International Tonnage Commission.
Professor E. R. Johnson in his report on the "Measurement of Vessels for the Panama Canal" makes the following comparison of measurements in tonnage averaged from eight steamships : Tho no fixed ratio between gross and net tonnage can be given, it is usually safe to figure that in the case of most steamers two-thirds of the gross tonnage is net, while in the case of a sailing vessel seven-eighths is about the correct ratio.
G. Displacement tonnage.—Displacement tonnage is another method of indicating the size of a vessel. This is the weight of the vessel expressed in long tons of 2,240 pounds avoirdupois. The term "displace ment" refers to the fact that the weight of the vessel equals the weight of the water which it displaces when afloat. As a rule this measurement applies to the ship with its crew and supplies on board, but without fuel, passengers, or cargo. This is sometimes called the displacement "light" as contrasted with displace ment "loaded." This method of indicating the size is used for men of-war and passenger steamers. Displacement is not used as the basis for canal or harbor charge because of the great difficulty in determining the "light" and the "loaded" figure and the possibility of changing the actual displacement when entering a harbor by "blowing out" ballast. It would violate the principle of giving main consideration to earning capacity in levying canal charges.
Professor Johnson says: Tolls upon the weight or displacement of ships would be unfair as between different types of vessels, because fast pas senger steamers have maximum weight in machinery, fittings, and fuel as compared with the weight of paying load, while slow cargo steamers have a maximum capacity for freight as compared with the weight of and space occupied by machin ery, fittings, and fuel. In the case of the passenger steamer, the paying load is relatively light as compared with the non paying weight or tare, while the freight steamer has an earn ing load heavy in relation to tare. Otherwise stated, the fast ship of "fine" lines has a large displacement and small deadweight capacity, while the ship with "full" lines has large carrying space in relation to light displacement.
7. Deadweight capacity.—The difference between the displacement "light" and the displacement "loaded" is the "deadweight capacity" of a vessel. This carrying capacity of a vessel depends upon its size, the weight of its machinery, the material of which it is built, and its design. The squarer the cross sec tion of a ship the greater is the carrying capacity or the less its speed.
The tendency of owners is to load heavily to in crease a ship's earning capacity. English vessels carry a mark on the outside of the hull, known as the Plimsoll line, which must not be submerged in load ing. The British Board of Trade has the legal power to fix this line but often delegates its authority.
American vessels have no such compulsory line, but usually a scale of figures on the bow of a vessel in dicates depth, while the ship's papers indicate the maximum load she should carry. The absence of a Plimsoll line in this country has, however, an un favorable effect upon insurance rates. The opinion among underwriters is that as a rule American ships carry ten per cent more deadweight than the British law would allow. Insurance rates are fixed accord ingly.
8. Economic significance of from an engineering standpoint the size of vessels may be al most unlimited, the table below shows that a limit has apparently been reached in the Leviathan. Any fur ther development will be limited by the difficulties encountered in finding adequate port facilities, by the increasing size of the insurance premium and by the difficulty in obtaining full cargos or passenger lists.
Large vessels are relatively cheaper to operate. The first cost, too, of a large ship is lower per ton than that of a small ship. When the cost of building a 2,000-ton steamer is about $200 a deadweight ton, a 5,000-ton steamer would cost $180 per deadweight ton, an 8,000-ton steamer $175, and a 10,000-ton steamer $170 per deadweight ton. The size of the crew does not increase proportionately with the size of the vessel and the waste space decreases. In de signing a vessel for freight service the most economical size at the present time ranges between 5,000 and 8,000 tons.