Builder's Old Measurement (BOM or bm) is the method of calculating the cargo capacity of a ship used in England from approximately 1650 to 1849. It estimated the tonnage of a ship based on length and maximum beam. It is a volumetric measurement of cubic capacity and NOT of weight. It is expressed in "tons burden", (Early Modern English: burthen, Middle English: byrthen ), and abbreviated "tons bm".
The formula is:
where:
- Length is the length, in feet, from the stem to the sternpost;
- Beam is the maximum beam, in feet.
Thus, Builder's Old Measurement estimates the cargo-carrying capacity of a ship in "tons", a measurement that has nothing to do with weight. For example, one might say, "She's rated 300 tons burthen."
The Builder's Old Measurement formula remained in effect until the advent of steam propulsion. Steamships required a different method of estimating tonnage, because the ratio of length to beam was larger and a significant volume of internal space was used for boilers and machinery. In 1849 the Moorsom System was created in Great Britain. The Moorsom system calculates the cargo-carrying capacity in cubic feet, another method of volumetric measurement. The capacity in cubic feet is then divided by 100 cubic feet of capacity per gross ton, resulting in a tonnage expressed in tons.
Read more about Builder's Old Measurement: History and Derivation, Depth
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