Builder's Old Measurement

Builder's Old Measurement (BOM) is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. The BOM estimated the tonnage of a ship based on length and maximum beam. The formula is:

$$Tonnage = frac {({Length}- Beam} imesfrac{3} {5) imes {Beam} imes frac {Beam}{2 {94}
where:
*"Length" is the length, in feet, from the stem to the sternpost;
*"Beam" is the maximum beam, in feet.cite book
last = Kemp, ed.
first = P.
authorlink =
coauthors =
title = The Oxford Companion to Ships & the Sea
publisher = Oxford University Press
date = 1976
location =
pages = 876
url =
doi =
id = ISBN 0-1921-1553-7]

Thus, BOM estimates the weight of the cargo carrying capacity of a ship measured in tons, a weight that is also termed deadweight. The Builder's Old Measurement formula remained in effect until the advent of steam propulsion. Steamships required a different method of measuring 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. Instead of calculating deadweight, the Moorsom system calculates the cargo carrying capacity in cubic feet, a volumetric measurement rather than a weight 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.

History and Derivation

The first tax on the hire of ships in England was levied by King Edward I in 1303 based on tons of burthen (burden). Later, King Edward III levied a tax of 3 shillings on each "tun" of imported wine. At that time a "tun" was a wine container of 252 gallons weighing about 2240 lbs. In order to estimate the capacity of a ship for tax purposes, an early formula used in England was

$$Tonnage = frac Length} imes {Beam} imes {Depth {100}

where:
**"Length" is the length (undefined), in feet
**"Beam" is the beam, in feet.
**"Depth" is the depth of the hold, in feet below the main deck.
**"100" the divisor is undefined, but the units would be ft³ per tun

In 1678, Thames shipbuilders used a deadweight method assuming that a ship's burden would be 3/5 of its displacement. Since displacement is calculated by multiplying Length x Beam x Draft x Block Coefficient, all divided by 35 ft³ per ton of seawater, the resulting formula for deadweight would be:

$$Deadweight = frac Length} imes {Beam} imes frac {Beam}{2} imes frac {3}{5} imes {0.62 {35}

where:
**"Draft" is estimated to be ½ Beam.
**"Deadweight" of cargo is assumed to be 3/5 of displacement.
**"Block Coefficient" is based on an assumed average of 0.62
**"35 ft³" is the volume of one ton of sea water.cite web
last = Pearn
first = Rodney Stone
authorlink =
coauthors =
title = Tonnage Measurement of Ships
work = Articles
publisher = Steamship Mutual
date =
url = http://www.simsl.com/Articles/Tonnage.asp
format =
doi =
accessdate = 2007-04-23]

Or by solving :
$$Deadweight = frac Length} imes {Beam} imes frac {Beam}{2 {94}

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

$$Tonnage = frac Length} imes {Beam} imes {Depth {94}

This formula remained in effect in until the Builders Old Measurement rule was put into use in 1720, and then by Parliamentary law in 1773.

References

* [http://www.bruzelius.info/nautica/Tonnage/Vademecum(1707)_p127.html Concerning Measuring of Ships, London, 1707.]
* [http://southseas.nla.gov.au/refs/falc/0239.html BURTHEN, or BURDEN, William Falconer's Dictionary of the Marine, 1780]
* [http://www.bruzelius.info/Nautica/Tonnage/Steel(1805)_p249.html OF FINDING THE TONNAGE OR BURTHEN OF SHIPS, &c., Steel, London, 1805.]