Builder's Old Measurement
A tax formula that changed the shape of British ships.
Origin
In use from around 1650, formalised by statute in 1720, and replaced by the Moorsom System in 1849.
The catch
Because the formula punished beam far more heavily than length, shipbuilders responded by designing hulls that were absurdly long and narrow. The tax did not measure the ships; the ships were built to game the tax.
((Length − (Beam × 3/5)) × Beam × (Beam / 2)) / 94Length is the keel length and beam the maximum width, both in feet. The result is a tonnage: not a weight, but a notional cargo capacity in tuns of wine.
Worked examples
The word tonnage descends from tun, the largest wine cask, because medieval customs duty was levied on how many a vessel could carry. Builder’s Old Measurement turned that count into a formula, and the formula then shaped the industry: hulls grew narrow because beam was expensive. The Moorsom System of 1849 measured internal volume directly and ended the distortion, though by then a century and a half of British hull design had been bent around a division by 94.