Crane Glossary: Definitions of Nautical Terms: Shipping and Tonnage Measurement

2. DEADWEIGHT TONNAGE

Deadweight tonnage is the maximum weight of cargo (not cargo of exceptionally high or low density), fuel, and stores which can be safely be carried by a ship at load draught. It is frequently quoted for cargo ships as an indication of their size for purposes of trade, e.g. in connexion with the present American building programme. It can be regarded as the freight-earning capacity of a ship, subject to variations arising from the nature of the cargo and of the voyage.

Displacement minus deadweight = lightweight (or weight of the fabric).

 

3. GROSS AND NET TONNAGE

These figures are entered on the register of shipping in the country of ownership, and for this reason they are often referred to as gross register tonnage and net register tonnage. In GREAT BRITAIN "register tonnage" is held in law to be net register tonnage. Sometimes net and gross register tonnage are referred to on the Continent as "new measurement", as they were in this country for a period after the adoption of the existing rules, beginning in 1854. In BELGIUM the net register ton is often called the "Moorsom" ton.

Net tonnage is the basis for the assessment of port dues in all countries, and of light dues for ships trading to GREAT BRITAIN. Net and gross tonnage also form a basis for negotiation among brokers and underwriters. Furthermore, figures of net tonnage entered and cleared are usually quoted in statistics of port traffic.

Gross (register) tonnage and net (register) tonnage are expressed in units of MEASUREMENT and not of weight; the "ton" was originally the "tun", a measurement of capacity in the wine trade. A gross or net ton is a measurement of space calculated from the average bulk of light freight or "measurement cargo".

Measurements of computations in the country are carried out by Board of Trade Surveyors. The methods of measurement and the rules for carrying them out differ somewhat in various countries. The chief rules for the measurement of merchant ships are
(1) English rule,
(2) old German rule,
(3) Danubian rule,
(4) Suez Canal rule,
(5) Panama Canal rule.

In fact, however, the English rule is generally followed, save for certain differences which occur in some countries. These differences may apply to the general computations of gross tonnage, as in SWEDEN, or to the methods of making allowances for net tonnage, as with the old German rule for the computation of net tonnage, which is in force in SWEDEN, BELGIUM, and CHILE. Owing to these differences in computation, totals of net tonnage in Swedish and Belgian statistics, for both ship and port statistics, need to be reduced by an average of 20.6 per cent and 15 per cent respectively for comparison with the statistics of other countries. The British rules are those laid down by the Merchant Shipping Acts of 1854 and 1894, as amended slightly in 1906.

Sometimes a vessel arriving in a country in which the standard differs appreciably from the standard applied in the country of origin is examined by a surveyor and the tonnage computed according to the national standard. British ships arriving in SWEDEN for the first time are partially measured to entitle them to a Swedish certificate; Swedish ships are usually measured in SWEDEN for British tonnage as well as for Swedish.

Thus the collation of various published figures may lead to the discovery of unavoidable discrepancies. The certificates of tonnage of the following countries, with or without qualifications, have been accepted by the British Government:

*Belgium
*Denmark
Estonia
*Finland
*France
*Germany
Greece
Iceland
*Italy
*Japan
Latvia
*Netherlands
*Norway
*Portugal
*Spain
*Sweden
U.S.A.
*U.S.S.R.

Ships of other countries trading with this country must, therefore, be measured by the Board of Trade on arrival.

*The countries shown in the above list with an asterisk, together with DANZIG and GREAT BRITAIN, are those of which the gross and net register tonnage are accepted in U.S.A. (except the PANAMA CANAL).

 

4. GROSS TONNAGE

Working from the "tonnage deck" (the upper deck in all ships which have less than three complete decks, and the second deck from the bottom in all other ships) established formulae are used to determine the cubical capacity of the ship in feet. Divided by 100 this becomes the register underdeck tonnage. Upper deck capacities are added, and the result, subject to certain deductions (e.g. of structures for sheltering passengers on the top deck), gives the gross register tonnage. If the cubical capacity is computed in cubic metres, then division by 2.83 will give the tonnage.

Gross tonnage is sometimes employed as the unit in classifying ships for purposes of legislation, e.g. M.S. (Wireless Telegraphy) Act, 1919.

 

5. NET TONNAGE

When deduction from the gross tonnage is made for all spaces occupied by propelling machinery, navigating equipment, crew's quarters, double bottom, and water ballast tanks, the result is the net register tonnage, equivalent to the amount of space in the ship which can be devoted to cargo.

A tug, therefore, could have a net tonnage of o. Generally speaking the faster the ship the higher is the gross/net percentage; slow cargo ships have a low gross/net percentage.

Suez and Panama Tons. The Suez Canal Company and the United States Government apply special methods of computing the tonnage of ships which pass through the canals. Each method, by including more superstructure than is customary, arrives at a higher figure for gross and net tonnage than the figures on the register. The canal computations are higher than the British figures as follows:

SUEZ: gross 5 per cent; net 30 per cent approximately
PANAMA: gross 10 per cent; net 30 per cent approximately

These figures are applied irrespective of flag.

 

6. RELATIONS BETWEEN TONNAGE FIGURES

Introduction.
There is not general formula to express the relation between displacement, deadweight, gross, and net, for it varies according to type and build of the ship, the speed it is designed for, and many other factors. For example, a fast steamship and a slow steamship of the same displacement would have different net tonnages owing the the greater space taken up by the propelling machinery space in the former.

Cargo ships.
For a medium cargo steamship, of standard construction and about 390 feet in length, the following figures would be representative:


It is possible, however, to establish the relationship between all the tonnage figures in ships of closely similar type. For cargo steamers of standard form and of the full scantling type, with erections covering 50 per cent of the length, for example, the relationship is as follows:

Underdeck, deadweight, and displacement tonnage as a percentage of net tonnage


In a warship the gross tonnage approximates to an average of 60 per cent of the standard displacement.

In the mercantile shipping circles, however, the tonnage normal dealt with is gross, net, and deadweight. A rough formula in use is "Deadweight = 2 1/2 times net"; a better and more comprehensive formula, employed for many years, is 100 net tons = 160 gross tons = 240 deadweight tons. The relationship between tonnage measurements for various classes of ships in service may be studied in a calculation based upon 3.448 British ships in service in 1936. (These ships were covered by the Classification of Shipping Enquiry of the Chamber of Shipping of the United Kingdom, 1936.)


Gross and deadweight tonnage as a percentage of net tonnage for 3,448 British ships, June 1936.


The relation between deadweight and net tonnage changes with progress in ships design and with the demands of trade. Between 1914 and 1936, for example, the number of deadweight tons per 100 tons net for British foreign-going tramps rose from 264 to 281, and for British foreign-going tankers from 234 to 256. In the same period, however, for cargo liners the figure dropped from 247 to 233, owing to the necessity of providing refrigerated cargo space and additional space for propelling machinery arising from the higher speeds expected. Other factors affect the position, too: following an agreement relating to load lines in 1932, it was found that British tankers were permitted to sail with less freeboard than formerly.

Some specific examples of tonnage measurements for British and foreign ships are given below.


The ore carrier Amerikaland has exceptional measurement figures because the ship was designed to carry no other cargo than iron ore; owing to the high specific gravity of the cargo, the cargo space was very restricted and thus the net tonnage was very low for the deadweight. A tramp carrying only iron ore would have the greater part of its cargo space empty.

 

7. IMPORTANCE OF DISTINGUISHING TYPES OF MEASUREMENT EMPLOYED

This is particularly important at the present time in connexion with shipping losses and construction programmes. It would be true to say that a million tons of shipping lost is replaced by a million tons of American construction only if the same units are employed throughout. Thus, with standard cargo ships of 3,200 net tonnage and 8,000 tons deadweight, a million tons of shipping would mean 312 ships if net tonnage were implied, but only 192 if gross tonnage were implied, and only 125 if deadweight tonnage were implied.

 

8. MEASUREMENT CAPACITY

A further system of measurements, by volume, give the true cubic capacity of holds, or what amount of "light measurement" goods can be carried (i.e. of any goods which might ever fill the holds of a normal cargo ship).

e.g.
Cubic capacity of holds
Cubic capacity of bale space
Cubic capacity of grain
Capacity bale space (tons of 40 cubic feet)

 

SOURCES
E.W. BLOCKSIDGE: Hints on the Register Tonnage of Merchant Ships (Liverpool, 1933).
Encyclopaedia Britannica, 11th edition.
Kempe's Engineering Yearbook (London, annual).
Modern Dock Operation, D.R. JOHNSON (London, 1929).
Lloyd's Register III (Confidential).
Lloyd's Confidential Index, Foreign, 1939; British, 1939.
Admiralty Navigation Manual, vol. i (London, 1938).
L. ISSERLIS: "Tramp Shipping Cargoes and Freights", Journal Royal Statistical Society, vol. 101, pp. 53 - 134 (London, 1938).
Directory of Shipowners, Shipbuilders, and Marine Engineers (London, 1940).
Paasch: "From Keel to Truck", Dictionary of Naval Terms, 5th ed. (London, 1937).


USUAL FRENCH EQUIVALENTS
Gross: la jauge brute
Net: la jauge net
Deadweight: le port en lourd