Ships tend to be described and compared in terms of gross registered
tonnage (GRT). According to Branch, GRT is calculated by dividing the volume in cubic feet of a vessel’s closed-in spaces by 100. A vessel ton is 100ft3. Tonnage is frequently made use of by port authorities when calculating charges when a ship requires a pilot and for harbour fees. The word ‘tonnage’ is derived from ‘tun’, a medieval term meaning barrel"