Ship Tonnage, Weights and Measures
Everyone who has looked at specifications for steamships is often bewildered by the many different 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. Below is an article from 1932 that provides a good explanation on just what the tonnages for ships really are.
There are five different kinds of tonnage. Ordinarily, in discussing shipping, three tonnage terms are commonly used: deadweight tonnage, gross tonnage, and net tonnage. The other two kinds are displacement tonnage and cargo tonnage.
Measurement and Tonnage Laws - 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 ship, 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, a discussion of the cargo, its nature and volume is imperative.
This is a matter of importance, especially to those who are 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.
In a general way, net tonnage may be described as that portion of the ship's internal capacity which may be 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.
Displacement Tonnage (1913)
The displacement ton is a unit applied to vessels and not to cargo, but in order to ascertain the dead-weight tonnage a vessel can carry it is first necessary to determine the vessel's displacement tonnage.
Dead-Weight Tonnage (1913)
A vessel's dead-weight tonnage is the difference between the weight or displacement of the vessel when "light" and when loaded to its maximum authorized draft.
Net Tonnage (1913)
The rules for the measurement of vessels to determine their gross and net tonnage are not the same, and the interpretation and application of the rules are not uniform.
The present 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.