TONNAGE, in regard to ships, a measure both of cubical capacity and of dead-weight carrying capability. The term, used by itself, may have reference severally to "builders' old measurement" tonnage (B. 0. M.), "register" tonnage, "displacement" tonnage, "freight" ton nage, etc., each of which expressions is more or less current in shipping circles. From very early times in the history of shipping a scale of one sort or other must have been employed to determine the relative capacity or carrying power of different vessels, and in point of fact the term "tonnage" in this connection can be traced back for at least 500 years. In Great Britain the first Act of Parliament dealing with the subject was passed in 1422, a second was introduced in 1694, and a third in 1720, but the application of these was limited to particular classes of ships, or those employed in particular trades.
"Displacement" tonnage is by general consent regarded as the fairest measure for the tonnage of naval ships since they are designed to carry certain maximum weights and to float at certain load lines which are fixed with reference to the character of service. It has for many years been the official tonnage for the warships of France and other European nations, and since 1872—prior to which date the B. 0. M. rule was the only one
employed—the tonnage of British naval ships has been based on the displacement principle. The United States navy has also adopted displacement tonnage; in fact, it is all but universally employed.
"Freight" tonnage, a system of meas urement commonly employed in connec tion with stowage by merchants and shipowners, although it has no legal capacity, is simply a measure of cubical capacity. A freight ton, or "unit of measurement cargo," simply means 40 cubic feet of space available for cargo, and is therefore two-fifths of a register ton.
In connection with yachts tonnage is measured by special rules for the purpose of regulating time allowances in racing. These rules are numerous and varied almost as the yacht clubs and associa tions employing them, but the rule which has hitherto been most generally adopted in Great Britain is known as the "Thames Rule," and is simply a slight modification from the B. 0. M. tonnage.