Tonnage, in regard to ships, is 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.O.M.), 'register' tonnage, 'displacement' tonnage, 'freight' tonnage, &c., 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 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.
The first legal system of measurement having general application to all classes of merchant-ships was introduced in 1773, and is known as 'builders' old measurement.' Long before its legal enforcement, however, it, or a similar rule, had become the established practice of British shipbuilders, although not applied in an exactly uniform manner by different builders. Shortly stated, the rule was to multiply the length (minus three-fifths of the breadth) by the breadth, the product by half the breadth, and to divide by 94, the quotient expressing the tonnage. During the long period prior to the 19th century, when naval architecture made little progress, and types of ships and methods of construction were almost stereotyped, this system seems to have quite answered its original purpose—i.e. that of roughly expressing the 'dead-weight' carrying power of ships. But with the growth of shipping, and with the legal assessment of dues according to the tonnage as computed by this system, various methods of evasion were resorted to, the effects of which were most hurtful to true progress in ship architecture. The rule only concerned itself with length and breadth, and took no account of actual form or of depth, but assumed the depth to be equal to half the breadth. Builders had simply to increase the depth, leaving the length and breadth unchanged, or to make the form 'fuller,' in order to increase the carrying power without in any way increasing the legal and taxable tonnage. Ships were in consequence built with little beam and of such excessive depth as to render them unsafe and unseaworthy. The inadequacy and perniciousness of this system of measurement were only slowly recognised, and it remained in force until 1836, when a 'new measurement' law was substituted. For purposes of convenience, however, it continued to be used by builders in estimates as to cost and comparison of shipping, and even yet it is sometimes to be met with in builders' statements.
The 'new measurement' gave place in 1854 to 'register' tonnage, a system resembling it in principle, but much more perfect in its details, and which, with minor modifications, is still in force as the legal basis of measurement upon which dock, harbour, light, and other dues are assessed. It determines, in an accurate and easy manner, the entire internal capacity in cubic feet of a ship below the 'tonnage deck.' At suitably placed and equi-distant stations throughout the length of ship measurements are taken of the vertical transverse area of the hold, and these areas are used, in conjunction with the length of ship, in obtaining the volume of hold space. If a vessel's hull is continued unbroken to support a deck above the tonnage-deck, the volume of the space between the deck is separately estimated, and all closed-in spaces above the upper deck, such as poop, forecastle, deck-houses, &c., intended for accommodation or stowage, are also calculated. The sum of all these volumes in cubic feet, divided by 100, gives the 'gross register' tonnage. A 'register' ton, therefore, is simply 100 cubic feet of space, and has only very indirect relation to a 'dead-weight' ton of 20 cwt. avoirdupois. What is known as 'nett register' tonnage—and actually forms the tonnage inscribed in the register of British shipping—is the figure remaining after certain deductions are made from the gross. It is intended to represent the space actually available in a ship for remunerative service, such as the stowage of cargo or the accommodation of passengers. In sailing-vessels the deduction is very small indeed, representing the space occupied by the crew, and usually only from 4 to 5 per cent. of the gross. In steamers the case is very different, for not only the crew-space, but that occupied by the engines, boilers, coals, &c. is deducted. Details as to the method of computing these deductions cannot be given here, but it may be stated generally that for the majority of ocean-going steamers the nett tonnage is reduced from the gross by as much as about 35 per cent. The question of deductions has all along presented many difficulties and anomalies, and in some cases the original law has had to be amended to obviate these. On the whole, however, the principle on which the present system of tonnage measurement is based has served the purpose admirably—certainly in a way much superior to any previous system—and during the years it has been in vogue comparatively few changes have had to be made, while, on the other hand, it has been adopted by all the principal maritime nations and by various international commissions. Alternative systems of tonnage measurement have been advocated from time to time when existing inequalities have been found to press specially hard, but none have found entire favour. One is the substitution of a dead-weight basis of measurement, on the grounds that internal capacity in the present conditions of the shipping trade is not the fair measure of the possible earnings of ships under most circumstances, while dead-weight capability is, and that such a mode of measurement would obviate all disputes and difficulties as to deductions and allowances. Both this proposal and a somewhat similar one, based on a vessel's 'displacement'—i.e. the total weight of a ship in tons when immersed to her maximum draught or load-line—have the great disadvantage of necessitating the fixing of an equitable maximum load-line for the various classes of vessels, and for the vessels of the several maritime nations. The fixing of a universal load-line in the case of cargo-vessels is a problem fraught with almost insuperable difficulty, although a partial solution has been attained in connection with certain types of British merchant-ships.
'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.O.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 measurement commonly employed in connection with stowage by merchants and shipowners, although it has no legal authority, 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. It is a purely arbitrary measure, based on the assumption that 40 cubic feet are required to stow a ton weight, but for some kinds of cargo of a lighter nature 45 and even 50 cubic feet form the unit of measurement.
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 associations employing them, but the rule which has hitherto been most generally adopted in Britain is known as the 'Thames Rule,' and is simply a slight modification from the B.O.M. tonnage.