tonnage

tonnage, originally the charge for the hire of a ship at so much a ton of its burthen. It was also a tax, first levied in 1303 by Edward I of England, on all imports brought by ship into England. A second tax, known as tunnage, of three shillings on each tun of wine imported, was levied in 1347 by Edward III.

It was from the first of these meanings, the cost of the hire of a ship, that the word tonnage came into use as an alternative to burthen. Although tonnage was still theoretically based on the number of tuns of wine that a ship could carry in its holds, it became necessary, both for taxation purposes and for calculating the harbour dues payable by a ship, to devise a rough and ready formula by which the tonnage could be quickly calculated. It was found, in the general design of ships of those early days, that the vessel's length in feet, multiplied by its maximum beam in feet, multiplied by the depth of its hold below the main deck in feet, with the product divided by 100, gave a reasonably accurate measurement of its tonnage, and this was the formula used for measuring warships as well as merchant vessels.

In 1694, when a law was introduced in Britain requiring the marking of a waterline on merchant ships, both when in ballast and fully laden, this tonnage formula was officially adopted, though marginally amended to make the product of length, beam, and depth divisible by 94 instead of 100. This remained the standard of ship measurement until 1773, when more accurate limits of measurement were established by a formula known as the Builders Old Measurement (BOM). This remained in force until the advent of iron for shipbuilding and steam propulsion revolutionized the design and shape of ships.

The BOM served its purpose well for the typical bluff-bowed, full-bodied ship of the timber and sail era but had no relevance to the longer, finer hulls of the iron ship in which the ratio of length to beam increased from the average three to one to four, five, and even six to one. In place of the old BOM a new calculation, known as Moorsom's Rule, devised by the Admiralty at the request of the Board of Trade in the mid-19th century, was introduced. The total capacity of a ship's hull below the upper deck was calculated in cubic feet and, by dividing it by 100, the resultant figure became known as a ship's gross tonnage.

But this figure did not, of course, bear very much resemblance to its cargo-carrying capacity, since it was calculated on the total hull space below the ship's upper deck and made no allowance for space taken up by crew's quarters, ship's stores, fuel, engines, etc. So a second calculation was made of the capacity in cubic feet of these spaces and, still taking 100 cubic as equivalent to one ton, was deducted from the figure of its gross tonnage to give a net tonnage.

Both these tonnages are known as register tonnages as they are entered on the ship's certificate of registration. It is on the figure of a ship's net register tonnage that such charges as port and harbour dues, dues for navigational aids, towage charges, and salvage assessments are normally levied.

Another tonnage measurement of a merchant vessel is deadweight tonnage, normally shortened to dwt. This is a measurement of the weight of the cargo it carries based on the long ton of 2,240 pounds (1,017 kg). The figure is arrived at by calculating the amount of water displaced by a ship when it is unloaded, but with its fuel tanks full and stores on board, and the amount of water similarly displaced when it is fully loaded with its cargo holds full. The difference expressed in tons (35 cubic ft of seawater = one ton) gives the ship's deadweight tonnage.

Naval vessels are usually measured in terms of displacement tonnage. This is the weight of the water a ship displaces when it is floating with its fuel tanks or bunkers full, and with all stores on board. This, at the rate of 35 cubic feet per ton, is the actual weight of the ship, since a floating body displaces its own weight in water. During the last half of the 19th century, and for the first half of the 20th century, yachts were measured by the Thames Measurement rule.

The introduction of the metric system into most countries that previously used the imperial measurement has led to ships' displacement being given in metric tonnes. However, as the weight of the metric tonne (1,000 kg) and the avoirdupois, or imperial, long ton (2,240 lb) are so close—there is only 1.16% difference between the two—large-scale reregistration of vessels has not been necessary.

In 1969 the International Maritime Organization adopted the International Convention on Tonnage Measurement of Ships which entered into force in 1982. This was the first successful attempt to introduce a universal tonnage measurement system, the new rules applying to all ships built on or after 18 July 1982. Those ships built before that date were allowed to retain their existing tonnage for twelve years before they needed to be remeasured. The Convention meant a transition from the traditional gross register tons (GRT) and net register tons (NRT) to gross tonnes (GT) and net tonnes (NT). Gross tonnage forms the basis for manning regulations, safety rules, and registration fees, while both are used to calculate port dues. The gross tonnage is a function of the moulded volume of all enclosed spaces of the ship, while the net tonnage is produced by a formula which is a function of the moulded volume of all cargo spaces of the ship.