## 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 **burden**" (** Early Modern English: burthen**,

The formula is:

where:

*Length*is the length, infeet , from thestem to thesternpost ;is the maximum beam, in feet.Beam ^{[1]}

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

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King

where:

*Length*is the length (undefined), infeet *Beam*is thebeam , in feet.*Depth*is the depth of the hold, in feet below the main deck.

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.

*100*the divisor is unitless, so tonnage would be expressed in 'ft³ of tun'.^{[1]}

In 1678 Thames shipbuilders used a method assuming that a ship's burden would be 3/5 of its displacement. Since tonnage is calculated by multiplying length × beam × draft ×

where:

*Draft*is estimated to be half of the beam.*Block coefficient*is based on an assumed average of 0.62.*35 ft³*is the volume of one ton of sea water.^{[2]}

Or by solving :

In 1694 a new British law required that tonnage for tax purposes be calculated according to a similar formula:

This formula remained in effect until the Builder's Old Measurement rule was put into use in 1720, and then mandated by Act of Parliament in 1773.

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