XIX. Register Tonnage
Three methods of measuring the capacity of a ship are more or less in general use in the maritime world.
The displacement tonnage, or weight of the volume of water displaced by the ship when fully loaded with all her crew, coal, supplies, etc., is in general use by the navies of the world for assuring accuracy and uniformity, but of course is not adapted to merchant vessels on which the cargo varies from voyage to voyage.
The deadweight tonnage, or actual weight of the cargo which a merchant ship will transport, obviously is adaptable only for vessels carrying bulk cargoes and not for general cargo ships. Each of these measurements is recorded in long tons avoirdupois.
The American registered tonnage system follows the Moorsom rules adopted in England in 1854, which are now in effect in practically every maritime country. It aims to express the entire cubical content of a merchant ship in unit tons of 100 cubic feet, this figure having been arrived at in England, on the adoption of the present system, when it was found that the ratio of the total registered tonnage of the British merchant marine to cubic feet of contents was slightly over 98.
The measurement rules of the United States are carefully and elaborately defined in the statutes themselves.
Under the statutes net tonnage is ascertained by deducting from gross tonnage that proportion of the ship's space occupied by engine's machinery, boilers, coal bunkers and certain other minor spaces, such as those which inclose the steering gear below deck, the boatman's stores, chart-houses, donkey engine and sail room.
To encourage the building of ample forecastles, crews' quarters, etc., as well as for other reasons, the rule is adopted by almost all maritime nations that tonnage taxes and other tonnage dues shall be collected not on gross but upon the net tonnage. This also includes the usual commercial charges for towage, dockage and wharfage. Official U. S. statistics of entrances and clearances are in terms of net-register tonnage, as also time charter rates when not specifically based on deadweight tonnage. The incentive to understate net register is thus strong.
In the case of tugs engaged in foreign service and which are therefore subject to tonnage duties, it becomes important to see that the net tonnage, which should ordinarily be a very small figure, is held to the lowest limit. For instance, seagoing and oceangoing tugs have been reported with a tonnage as low as eight and ten tons. On the other hand, many American tugs of no larger capacity are in the habit of carrying a net tonnage far exceeding this amount.