Tonnage

Gross tonnage (GT) is a function of the volume of all of a ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing.

Gross tonnage is therefore a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety, and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though it derives from the volumetric capacity in cubic metres.

PC/UMS is based on a mathematical formula to calculate a vessel's total volume; one PC/UMS net ton is equivalent to 100 cubic feet (2.83 m3) of capacity.

[6]: 13  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.

For instance, Basque ships engaged in 16th century whaling in Labrador used this size of barrel (with the name barrica).

The French tonneau de mer was legally defined, in 1681, as the cubic space into which four barrique bordelaise fitted, with the casks aligned two directly above the two below (so not optimising the layout).

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.

[5] 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.

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.

[7][1] 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).