Builder's Old Measurement, often printed as tons burthen or tons BM, is a historical capacity estimate derived from hull dimensions. Its formula uses length and beam, and the measure appeared in sailing-ship specifications from the seventeenth through nineteenth centuries. Engines, fuel, and other machinery make it unsuitable for comparing modern vessels. The explanation helps decode eighteenth- and nineteenth-century ship specifications.
Notable Point · 13:25-16:35
Builder's Old Measurement and sailing-ship tonnage
A technical aside explains the BM tonnage notation used for sailing ships and why its cargo-capacity formula does not translate cleanly to powered vessels.