What is Gross Tonnage?

apl_austria001.jpg
Intermodal Containers Increase Capacity. World Shipping Council

Definition:

Gross Tonnage is a measure of volume inside a vessel. This includes all areas from keel to funnel and bow to stern.

The means to calculate gross tonnage is laid out in the International Convention on Tonnage Measurement of Ships.

Measurement of the total interior volume of a vessel in cubic meters is multiplied in a formula by a second factor which is determined by the volume of the vessel.

The formula is:

K=0.2+0.02*log10(V)

Gross Tonnage (GT) = K*V

Where V = Interior volume of a vessel in cubic meters

Gross Tonnage is used to determine the number of crew, safety rules, registration fees, and port dues. It is the standard most often used to define a vessel.

Since most ships were involved with the transport of goods, otherwise known as cartage, the ships were rated and valued on the maximum amount of cargo that could be stuffed into every nook inside a ship. On long sailing voyages private traders may buy bundles of lumber, spices, and cloth, along with decorative goods after selling their loads of cookware, tools, and machines. Every space was surely stuffed full to maximize profit on both legs of the voyage.

Some people see the term tonnage and think it's a measure of weight which is reasonable since a ton is a measure of weight. But gross tonnage is definitely a measure of volume which lets someone know about the potential amount of cargo that can be stuffed into every bit  of free cargo space.

In the past this included the crew accommodations which are now left out of the measurement. On sailing ships in particular the crew and cargo intermingled in order to balance the ship and attain a low center of gravity.

One place that was exempt was the bilge area where ballast was held since no cargo could be stored here without damage since wooden ships of the time required a wet bilge.

Ballast stones were used on sailing ships that were leaving with a light load and returning with a heavy cargo. This might be the case when transporting a finished metal like copper to a port where raw copper ore was loaded for the trip back to England for refining.

Piles of these foreign stones the size of bowling balls can be found underwater near historic ports allover the world. Today we use water as ballast since pumps and tanks are better but it come with its own problems.