Gross and net tonnages are a complicated matter and have always been so.
I'll make some general statements that may be illuminating (or not) and then refer anyone interested to the USCG/CFR here:
Electronic Code of Federal Regulations: Keep browsing as it's laid out in typical government fashion!
Gross tonnage is, except for certain exempted spaces, the entire internal volume of a vessel in cubic feet with 100 cubic feet equaling one ton.
Net tonnage is the tonnage that remains after non-revenue generating spaces are deducted from gross tonnage.
Certain fees such as dry-docking are based upon gross tonnage while others such as dock fees and canal transits are based upon net tonnage.
You'll see on board vessels signs over spaces that say such as, "certified for four seamen". The law states that each seaman's accommodation space is equal to 120 cubic feet and at least 16 square feet of deck space. While the sign may indicate a certification for four seamen, the space may actually be home to only one or two seamen in actuality.
As may be seen, gross and net tonnages are volume measurements and not to be confused with displacement.
Displacement is of two forms commonly; light ship and load. Light ship displacement is the diplacement of the ship absent cargo, crew, fuel, water, etc...basically the weight of the ship as she comes off the ways. Load displacement is the weight of the ship fully laden to her summer marks. the difference between the two is deadweight tonnage or her cargo carrying capacity in rough terms.
You'll see ships described in two ways commonly; by deadweight tonnage for a tank ship and sometimes certain freighters and, more commonly, by gross tonnage for non tank ships.
The origin of the word ton is maritime in nature. It originated in the thirteenth century when ships were carrying large amounts of wine. The wine was carried in a large cask called a
tun. It held 250 gallons of wine, had a volume of 57 cubic feet, and weighed 2,240 lbs. this measurement was codified into law in England in the fifteenth century and the taxes and fees a vessel was to pay were based upon how many
tuns she could carry. The spelling was later changed to ton and thus we get tonnage. Later yet, the volume for a ton was arbitrarily set at 100 cubic feet while the weight of a ton is still 2,240 cubic feet. The short ton, of 2000 lbs, has no standing in the maritime world. The metric ton is 1000 kilograms or 2205 lbs and is used more frequently today than in the past due to it's closeness to the traditional long ton of 2,240 lbs.
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