Traditional casks Historically in
England, tunnage was the medieval import duty on
tuns of wine. A tun was a large size of
casks used for
wine), used in the wine trade. The number of tuns that a ship could carry was used as a measure of the size of the ship. The wine trade to England originated in France, which is where the tuns were made. A French standard tun cask size was established about 1450. The 15th century Bordeaux wine tun was between . When measuring the tonnage of a ship, the approximately cylindrical cask would have air space around it when stowed in a ship. Therefore the volume of hold space required for several tuns was greater than the total of the capacity of those tuns. 252 imperial gallons is just over . British practice by the 19th century was for a ton of cargo volume to be . This is derived from an estimate of the hull space needed to fit the roughly cylindrical tun and is broadly consistent with the much earlier French definition given below. The Bordeaux tun was used as a measurement elsewhere in Europe. By the 16th century, multi-decked ships which were loaded through hatchways (as opposed to earlier undecked or single decked ships) found it more convenient to use a smaller size of standard barrel. This was the , measuring a quarter of the size of the tun (in English, this barrel was termed a hogshead). For instance,
Basque ships engaged in 16th century whaling in
Labrador used this size of barrel (with the name ). The French was legally defined, in 1681, as the cubic space into which four fitted, with the casks aligned two directly above the two below (so not optimising the layout). This redefined ton worked out as 42 cubic (. The difference between this measure and, for instance, the Spanish was calculated by recognised adjustment factors.
Purpose of measuring tonnage Tonnage measurement was important for an increasing number of reasons through history. In England in the
Middle Ages, ships were often impressed by the crown for military use. To do this in an efficient and speedy manner, a measurement of size was needed. The payment to the owner of the requisitioned ship was based on the tonnage. Port dues and various licences were based on tonnage, and it was a useful measure for a ship builder who needed to build a vessel that met the new owner's requirements. In the
Tudor period a bounty per ton was paid for the construction of larger ships, so the threshold for that payment had to be determined, as well as the amount paid for those qualifying.
Method of measuring In the middle ages, the normal way of discovering the tonnage of a ship was to load her with wine and see how many tuns could be fitted in. There is an instance of the owner of a new ship, in 1459, being challenged that his safe-conduct was for a 400 ton vessel, whilst he had already loaded more than 600 tons. The excuse was accepted that he had no idea of her tonnage until she was loaded. In another case, in 1456, a dispute over the actual tonnage of a ship had to be resolved by having coopers part load her with (presumably empty) barrels to estimate what she could carry. The Tudor bounty paid for the construction of larger ship was apparently paid without any measurement system to confirm the actual size. The presumption is that it was taken from the amount of cargo unloaded after the first voyage (which would be recorded in the customs records of the relevant port).
Gross register tonnage Gross register tonnage (GRT) represents the total internal
volume of a vessel, where one register ton is equal to a volume of ; a volume that, if filled with fresh water, would weigh around 2.83
tonnes. The definition and calculation of the internal volume is complex; for instance, a ship's hold may be assessed for
bulk grain (accounting for all the air space in the hold) or for
bales (omitting the spaces into which bulk, but not baled cargo, would spill). Gross register tonnage was replaced by
gross tonnage in 1982 under the Tonnage Measurement convention of 1969, with all ships measured in GRT either scrapped or re-measured in GT by 1994.
Net register tonnage Net register tonnage (NRT) is the volume of cargo the vessel can carry—that is, the gross register tonnage less the volume of spaces that do not hold
cargo (e.g.,
engine compartment,
helm station, and
crew spaces, again with differences depending on which
port or
country does the calculations). It represents the volume of the ship available for transporting
freight or
passengers. It was replaced by
net tonnage in 1994, under the Tonnage Measurement convention of 1969. == See also ==