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| {{see also|tonnage}}
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| '''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 (nautical)|beam]]. It is expressed in "tons '''burden'''" ('''{{lang-en-em | burthen }}''', '''{{lang-enm | byrthen }}'''), and abbreviated "tons bm".
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| The formula is:
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| <center><math>{{Tonnage}} = \frac {({Length}- ({{Beam}\times\frac{3} {5}})) \times {Beam} \times \frac {Beam}{2}} {94}</math></center>
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| where:
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| * ''Length'' is the length, in [[foot (length)|feet]], from the [[stem (ship)|stem]] to the [[sternpost]];
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| * ''[[Beam (nautical)|Beam]]'' is the maximum beam, in feet.<ref name=Kemp>{{cite book
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| | last = Kemp, ed.
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| | first = P.
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| | title = The Oxford Companion to Ships & the Sea
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| | publisher = Oxford University Press
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| | year = 1976
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| | pages = 876
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| | isbn = 0-19-211553-7}}</ref>
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| 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.
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| ==History and derivation==
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| King [[Edward I of England|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 (unit)|tun]]" of imported wine, equal to £{{formatnum:{{Inflation|UK|0.15|1377|r=2}}}} 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 {{convert|2240|lb|abbr=on}}, 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:
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| <center><math>{{Tonnage}} = \frac {{Length}\times {Beam} \times {Depth}} {100}</math></center>
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| where:
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| * ''Length'' is the length (undefined), in [[foot (length)|feet]]
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| * ''Beam'' is the [[beam (nautical)|beam]], in feet.
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| * ''Depth'' is the depth of the hold, in feet below the main deck.
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| The numerator yields the ship's volume expressed in cubic feet.
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| 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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| * ''100'' the divisor is unitless, so tonnage would be expressed in 'ft³ of tun'.<ref name = Kemp/>
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| 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#Metrics|block coefficient]], all divided by 35 ft³ per ton of seawater, the resulting formula would be:
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| <center><math>{{Tonnage}} = \frac {{Length}\times\ {Beam} \times \frac {Beam}{2} \times \frac {3}{5}\times {0.62}} {35} </math></center>
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| where: | |
| * ''Draft'' is estimated to be half of the beam.
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| * ''Block coefficient'' is based on an assumed average of 0.62.
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| * ''35 ft³'' is the volume of one ton of sea water.<ref>{{cite web
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| | last = Pearn
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| | first = Rodney Stone
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| | title = Tonnage Measurement of Ships
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| | work = Articles
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| | publisher = Steamship Mutual
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| | url = http://www.simsl.com/Articles/Tonnage.asp
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| | accessdate = 2007-04-23}}</ref>
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| Or by solving :
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| <center><math>{{Tonnage}} = \frac {{Length}\times\ {Beam} \times \frac {Beam}{2}} {94} </math></center>
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| In 1694 a new British law required that tonnage for tax purposes be calculated according to a similar formula:
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| <center><math>{{Tonnage}} = \frac {{Length}\times\ {Beam} \times {Depth}} {94}</math></center>
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| 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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| ==Depth==
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| * Depth to deck
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| :The height from the underside of the [[hull (watercraft)|hull]], excluding the [[keel]] itself, at the ship's midpoint, to the top of the uppermost full length deck.<ref name=Chapman>
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| {{cite book
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| | last = Schäuffelen
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| | first = Otmar
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| | authorlink =
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| | title = Chapman great sailing ships of the world
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| | publisher = Hearst Books
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| | location =
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| | year = 2005
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| | page = xx
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| | url = http://books.google.com/books?id=QgMRudqoLGQC&pg=PR20&dq=%22Depth+to+deck%22+%22depth+in+hold%22+draft&lr=
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| | doi =
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| | isbn = 978-1-58816-384-4 }}
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| </ref> | |
| * Depth in hold
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| :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.<ref name=Chapman/>
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| * Main deck
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| :[[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.<ref>{{cite web | |
| | last =
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| | first =
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| | title = Construction and Dimensions
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| | work =
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| | publisher = The Mary Rose Trust
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| | url = http://www.maryrose.org/ship/ship2.htm
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| | accessdate = 2009-07-17}}
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| </ref> In a calculation of the tonnage of ''Mary Rose'' the [[Draft (hull)|draft]] was used instead of the depth.<ref>{{cite web
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| | last = Fielding
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| | first = Andrew
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| | title = The Mary Rose - a Model
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| | work =
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| | publisher = Not published
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| | url = http://www.maryrose.org/ship/dimensions.htm
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| | accessdate = 2009-07-17}}
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| </ref>
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| ==American tons burthen==
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| 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.
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| The upshot was that American calculations gave a lower number than the British. For instance, when the British measured the captured {{USS|President|1800|6}}, their calculations gave her a burthen of 1533<small>{{frac|7|94}}</small> tons, whereas the American calculations gave the burthen as 1444 tons.<ref>Henderson, James, CBE (1994) ''The Frigates: An account of the lighter warships of the Napoleonic Wars, 1793-1815''. (London:Leo Cooper), p.167. ISBN 0-85052-432-6</ref> The British measure appears to be about 6% greater than the American.
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| ==See also== | |
| [[Thames Measurement]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| {{wiktionary|burthen|tun}}
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| * [http://www.bruzelius.info/nautica/Tonnage/Vademecum(1707)_p127.html Concerning Measuring of Ships, London, 1707]
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| * [http://southseas.nla.gov.au/refs/falc/0239.html ''Burthen'', or ''Burden'', William Falconer's Dictionary of the Marine, 1780]
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| * [http://www.bruzelius.info/Nautica/Tonnage/Steel(1805)_p249.html Of Finding the Tonnage or Burthen of Ships, &c., Steel, London, 1805]
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| {{Ship measurements}}
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| [[Category:Nautical terms]]
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| [[Category:Sailing rules and handicapping]]
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| [[Category:Volume]]
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| [[Category:Mass]]
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