SHIPPING: TONNAGE TERMS. The term "Tonnage" has a variety of meanings. According to the British Merchant Shipping acts and the Suez and Panama Canal regulations it is a measurement of the capacity or volume of a ship expressed in units of ioo cubic feet—one unit of such volume being termed a ton measurement. The purpose of measuring a ship is two-f old— primarily to form a basis for the payment of the various charges which are levied by Port and Harbour authorities, by Lighthouse Boards and for pilotage services; secondly for use in the registra tion or identification of the ship itself. (See SHIPPING: Registra tion, Classification and State Regulation.) Tonnage legislation dates from very early times, not only in the Mediterranean but also in the Far East, where in China some form of measurement has existed since before the Christian era.
The earliest known record in Great Britain occurs in A.D. 1422, which provides that "keels (barges) that carry coals at Newcastle shall be measured and marked." In 1694 the actual weight carried was recorded and was ascertained by placing known iron or lead weights on board, 2ocwt. avoirdupois being taken as a ton. In France in 1681 the volume was measured approximately and ex pressed in tons of 42 cubic feet, on the assumption that 4 wine tuns—which average that particular volume—weighed I ton avoir dupois. There is thus from early times a considerable confusion of thought as to what is the real meaning of the word "ton," it being apparently connected both with the ton avoirdupois and with the tun of wine.
tonnage, which is the sum of the under-deck tonnage and of all enclosed spaces above the tonnage deck—there being, however, certain portions above the tonnage deck which may be exempted from measurement. (c) Register or net tonnage, which is derived from the gross tonnage by deducting the volumes of certain "non earning" spaces—spaces which cannot, broadly speaking, be utilized for the carriage of cargo or passengers.
(2) Displacement Tonnage. This system is invariably used for warships, and represents the weight expressed in tons avoirdupois of the vessel when fully loaded. It is called displacement because it is generally ascertained by calculating the weight of the water displaced by the form of the vessel when immersed to the loadline.
(3) Deadweight Tonnage. Deadweight tonnage is the weight in tons avoirdupois of cargo, passengers, fuel and stores which can be carried by a vessel when fully loaded. It is the difference between the weight of a vessel (including machinery)—or what is termed the light ship—and the weight of the vessel and the contents when fully loaded.
(4) Freight Tonnage. Freight tonnage is a measure of the total cubic capacity of a vessel which is available for the carriage of cargo, and is usually expressed in tons of 4o cu.ft. measurement, it being assumed that 4o cu.ft. weigh 1 ton avoirdupois. Under the British system of tonnage measurement, the tonnage deck is the upper deck in vessels which have one or two decks, and in all other vessels it is the second continuous deck from below. The upper deck is the uppermost complete deck the openings of which are closed in such a way that any space below the deck is regarded as a closed-in space by the Board of Trade. The tonnage dimen sions, i.e., length, breadth and depth, differ from the registered dimensions (see SHIPPING : Registration, Classification and State Regulation)—although these are both ascertained at the same time by the surveyor who is measuring the tonnage.