Tonnage

In England sailors until fairly recently described a yacht as, for example, a "seven ton cutter." That didn't mean a yacht weighing seven tons, but rather a yacht with a useful interior space of about 700 cubic feet, a "ton", as used in that context, equaling 100 cubic feet.

Lots of folks are confused by tonnage terms. Here are the four fundamental measurements.

Gross tonnage measures the interior space of the ship, including most non-cargo spaces. One of these "tons" equals 100 cubic feet. As I understand it, the term is derived from "tun", a great fat barrel holding 256 gallons of water or wine, and requiring about 100 cubic feet of stowage space per tun.

Net tonnage is the useful cargo space of a ship, with again one ton equaling 100 cubic feet. Generally, net tonnage is the ship's gross tonnage minus engine spaces, shaft alleys, crew quarters etc. There are, as the normally very dry Knight's Modern Seamanship puts it, "some annoying variations" among nations in what spaces are disregarded to find net tonnage, but these variations are disappearing in the face of international conventions, including the International Convention on Tonnage Measurement of Ships. Net tonnage is important not only because it describes the cargo carrying capacity of the ship, but because canal fees, harbor charges etc. turn on net tonnage.

Displacement is the weight of the water displaced by the ship, which is, as Archimedes realized, equal to the ship's weight. Generally, not always, yachts are described by their displacement.

Deadweight tonnage (DWT) is a measure of how much weight a ship can safely carry, disregarding the unloaded, unfueled, unprovisioned weight of the ship ("lightship weight") but including not only cargo, but also stores, fuel, water etc. The term is sometimes confused with displacement but the two terms mean entirely different things.