Tonnage in boats and large vessels or ships has several meanings depending on what the term is referring to. It can at times be weight and at times be volume. The origin of the word in its maritime sense was the tun, a large cask in which wine was transported. The measurement of an old sailing ship was in tunnage, or the number of tuns of wine that could be carried. Following are the most commonly used definitions:
Gross tonnage is based on a vessels volume and represents the total enclosed space or internal capacity for transporting cargo. A gross ton represents 100 cubic feet.
Net tonnage is gross tonnage less the volume of spaces that will not hold cargo. In pleasure boats these spaces would be the engine compartment, helm station, etc. Net tonnage is also called registered tonnage. In order to document a vessel with the USCG it has to be a minimum of 5 net tons.
Displacement tonnage deals with weight in long tons which equal 2,240 pounds each. This is the actual weight of the boat. This can be calculated by finding the total volume of the boat below the waterline expressed in cubic feet. Divide this number by 35. (35 cubic feet of seawater weighs one long ton)
Deadweight tonnage is to displacement tonnage what net tonnage is to gross tonnage. Deadweight tonnage represents a boats cargo capacity in weight or long tons.