Ship Tonnage, Weights and Measures
SS Empress of Austrlia (1937) of the Canadian Pacific Line, 21,833 Tons Gross. Length 590 Feet, Breadth 75 Feet. GGA Image ID # 2000d1e5ec
Everyone who has looked at steamship specifications is often bewildered by the many tonnages used for the same vessel. An ocean liner may have different gross tonnage, depending on which country's rules were used in determining the weight. This article from 1932 provides a good explanation of just what the tonnages for ships are.
There are five different kinds of tonnage. Ordinarily, three tonnage terms are commonly used in discussing shipping: deadweight tonnage, gross tonnage, and net tonnage. The other two kinds are displacement tonnage and cargo tonnage.
History of the Measurement and Tonnage Laws through 1920.
Under present United States laws, regulations, and interpretations, the tonnage basis for charges on an American ship does not exceed that on a foreign vessel and, in some cases, is materially less.
Weight of world's seaborne trade.—No organism can be understood unless its functions are understood. The function of world shipping is to carry the seaborne trade of all countries. Therefore, it is imperative to discuss the cargo, its nature, and its volume.
This is important, especially to those about to construct or otherwise acquire ships, as a clear understanding of our measurement rules will lead to a more thorough knowledge of the relation between tonnage and carrying capacity.
Generally, net tonnage may be described as that portion of the ship's internal capacity devoted to commercial uses—the carriage of passengers and freight—and is the tonnage left after certain deductions have been made from the gross tonnage.
The displacement ton is a unit applied to vessels and not cargo. Still, to ascertain the dead-weight tonnage a ship can carry, it is first necessary to determine the vessel's displacement tonnage.
A vessel's dead-weight tonnage is the difference between the weight or displacement of the ship when "light" and when loaded to its maximum authorized draft.
The rules for measuring vessels to determine their gross and net tonnage are not the same, and the interpretation and application of the rules are not uniform.
The rules or methods followed in measuring vessels to determine their gross tonnage originated with Mr. George Moorsom of England in the British Tonnage Act of 1854.