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Builder's Old Measurement
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Builder's Old Measurement
Builder's Old Measurement (BOM, bm, OM, and o.m.) is the method used in England from approximately 1650 to 1849 for calculating the cargo capacity of a ship. It is a volumetric measurement of cubic capacity. It estimated the tonnage of a ship based on length and maximum beam. It is expressed in "tons burden" (Early Modern English: burthen, Middle English: byrthen), and abbreviated "tons bm".
The formula is:
where:
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 the United Kingdom. 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 (2.8 m3) of capacity per gross ton,[vague] resulting in a tonnage expressed in tons.[vague]
King Edward I levied the first tax on the hire of ships in England in 1303 based on tons burthen. Later, King Edward III levied a tax of 3 shillings on each 'tun' of imported wine, roughly equivalent to £150 in 2023. At that time a 'tun' was a wine container of 252 wine gallons, approx 210 imp gal (955 L) weighing about 2,240 lb (1,020 kg), a weight known today as a long ton or imperial ton. In order to estimate the capacity of a ship in terms of 'tun' for tax purposes, an early formula used in England was:
where:
The numerator yields the ship's volume expressed in cubic feet.
If a 'tun' is deemed to be equivalent to 100 cubic feet, then the tonnage is simply the number of such 100 cubic feet 'tun' units of volume.
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Builder's Old Measurement
Builder's Old Measurement (BOM, bm, OM, and o.m.) is the method used in England from approximately 1650 to 1849 for calculating the cargo capacity of a ship. It is a volumetric measurement of cubic capacity. It estimated the tonnage of a ship based on length and maximum beam. It is expressed in "tons burden" (Early Modern English: burthen, Middle English: byrthen), and abbreviated "tons bm".
The formula is:
where:
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 the United Kingdom. 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 (2.8 m3) of capacity per gross ton,[vague] resulting in a tonnage expressed in tons.[vague]
King Edward I levied the first tax on the hire of ships in England in 1303 based on tons burthen. Later, King Edward III levied a tax of 3 shillings on each 'tun' of imported wine, roughly equivalent to £150 in 2023. At that time a 'tun' was a wine container of 252 wine gallons, approx 210 imp gal (955 L) weighing about 2,240 lb (1,020 kg), a weight known today as a long ton or imperial ton. In order to estimate the capacity of a ship in terms of 'tun' for tax purposes, an early formula used in England was:
where:
The numerator yields the ship's volume expressed in cubic feet.
If a 'tun' is deemed to be equivalent to 100 cubic feet, then the tonnage is simply the number of such 100 cubic feet 'tun' units of volume.