Thames measurement (or tonnage)
When an owner talks about his ‘five tonner’ he refers to a peculiar type of tonnage which has nothing to do with weight. More than a hundred years ago the Royal Thames Yacht Club adopted a formula for estimating the tonnage of yachts, and we still can’t break free of it. The formula takes no account of weight, but relates entirely to length and beam. The measurements are taken in feet. Because beam acts twice as a multiplier, fat boats have disproportionately large tonnages.
(See also: Displacement and Register tonnage.)