Builder's Old Measurement
Builder's Old Measurement (BOM or bm) 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:
 Length is the length, in feet, from the stem to the sternpost;
 Beam is the maximum beam, in feet.^{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 Moorsom System was created in Great Britain. The Moorsom system calculates the cargocarrying 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.
Contents
History and derivation
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, equal to £100.52 today (using the last year of Edward III's reign, 1377, as the base year). At that time a "tun" was a wine container of 252 gallons 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:
 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.
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 × block coefficient, all divided by 35 ft³ per ton of seawater, the resulting formula would be:
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.
Depth
 Depth to deck
 The height from the underside of the hull, excluding the keel itself, at the ship's midpoint, to the top of the uppermost full length deck.^{3}
 Depth in hold
 Interior space; The height from the lowest part of the hull inside the ship, at its midpoint, to the ceiling that is made up of the uppermost full length deck. For old warships it is to the ceiling that is made up of the lowermost full length deck.^{3}
 Main deck
 Main deck, that is used in context of depth measurement, is usually defined as the uppermost full length deck. For the 16th century ship Mary Rose, main deck is the second uppermost full length deck.^{4} In a calculation of the tonnage of Mary Rose the draft was used instead of the depth.^{5}
American tons burthen
The British took the length measurement from the outside of the stem to the outside of the sternpost; the Americans measured from inside the posts. The British measured breadth from outside the planks, whereas the American measured the breadth from inside the planks. Lastly, the British divided by 94, whereas the Americans divided by 95.
The upshot was that American calculations gave a lower number than the British. For instance, when the British measured the captured USS President, their calculations gave her a burthen of 1533^{7}⁄_{94} tons, whereas the American calculations gave the burthen as 1444 tons.^{6} The British measure appears to be about 6% greater than the American.
See also
References
 ^ ^{a} ^{b} Kemp, ed., P. (1976). The Oxford Companion to Ships & the Sea. Oxford University Press. p. 876. ISBN 0192115537.
 ^ Pearn, Rodney Stone. "Tonnage Measurement of Ships". Articles. Steamship Mutual. Retrieved 20070423.
 ^ ^{a} ^{b} Schäuffelen, Otmar (2005). Chapman great sailing ships of the world. Hearst Books. p. xx. ISBN 9781588163844.
 ^ "Construction and Dimensions". The Mary Rose Trust. Retrieved 20090717.
 ^ Fielding, Andrew. "The Mary Rose  a Model". Not published. Retrieved 20090717.
 ^ Henderson, James, CBE (1994) The Frigates: An account of the lighter warships of the Napoleonic Wars, 17931815. (London:Leo Cooper), p.167. ISBN 0850524326
External links
Look up burthen or tun in Wiktionary, the free dictionary. 
 Concerning Measuring of Ships, London, 1707
 Burthen, or Burden, William Falconer's Dictionary of the Marine, 1780
 Of Finding the Tonnage or Burthen of Ships, &c., Steel, London, 1805

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