Tonnages

Four main rules define carrying capacity of a vessel
The origin of tonnage is from the Old English tunne, a large cask for carrying wine, ale, or beer. It had a capacity of about 252 gallons (1,145 L). In those days, a ship of 20 tons was a ship that could stow 20 tunnes in her hold.The earliest legal act dealing with ship measurement in Britain dates from 1492, but in 1694 it was officially decided that a ship’s tonnage would be decided by the simple formula

The number 94 was chosen so that the result gave the approximate number of tons of coal a boat could carry. Of course, that wasn’t an altogether satisfactory formula because it was difficult to measure the depth of a laden vessel, and the definition of length was often in dispute because of the varying size of overhangs. So the tonnage rule has been modified many times over the years until today there are four main tonnage rules:

• Gross tonnage. This is a volume, not a weight. It’s the total enclosed space (or internal capacity) of a vessel expressed in units of 100 cubic feet, each one of which is reckoned to be a ton.

• Net (or registered) tonnage. This is the gross tonnage less the volume of interior spaces that cannot be used for cargo. On pleasure boats, deductions are made mainly for engine compartments and control stations.

• Displacement tonnage. This is the actual weight of a vessel in long tons of 2,240 pounds each. It can be calculated by finding the volume of the vessel from the waterline downward. The volume in cubic feet divided by 35 equals the tonnage, because 35 cubic feet of seawater weighs 1 long ton.

• Deadweight tonnage. Like net tonnage, this is an indication of cargo-carrying capacity; but whereas gross and net tonnages are volumes, deadweight tonnage is measured in long tons.

By tradition, naval warships are referred to by their displacement tonnage; that is, their actual weight. For the practical purpose of federal documentation of pleasure boats, gross tonnage is measured as half the overall length (L), times overall breadth (B), times depth (D)—which is the internal measurement of the hull, not the draft—divided by 100. That is:½ (L x B x D ÷ 100)All the measurements are in feet. Net tonnage is taken to be 9/10 of gross tonnage, and the minimum size for federal documentation is 5 net tons.See also Net Tonnage.

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