Builder's Old Measurement

{{Short description|Estimate of the cargo capacity of a sailing vessel (c. 1650–1849)}} {{Use British English |date=May 2025}} {{Use dmy dates |date=May 2025}} '''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'''" ({{langx |en-emodeng| '''burthen''' }}, {{langx |enm| '''byrthen''' }}), and abbreviated "tons bm".

The formula is:

<math display="block"> \text{Tonnage} = \frac {(\text{Length}- (\text{Beam}\times\frac{3} {5})) \times \text{Beam} \times \frac {\text{Beam}}{2}} {94}</math>

where: * ''Length'' is the length, in feet, from the stem to the sternpost; * ''Beam'' is the maximum beam, in feet.<ref name=Kemp>{{cite book | editor-last = Kemp | editor-first = P. | title = The Oxford Companion to Ships & the Sea | publisher = Oxford University Press | year = 1976 | pages = [https://archive.org/details/oxfordcompaniont00kemp/page/876 876] | isbn = 0-19-211553-7 | url-access = registration | url = https://archive.org/details/oxfordcompaniont00kemp/page/876 }}</ref>

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 {{convert|100|cuft}} of capacity per {{vague |date=May 2025 |reason=Are these short tons (ST), long tons (LT), or what else? Define "gross". |text=gross ton,}} resulting in a tonnage expressed in {{vague |date=May 2025 |reason=Short tons (ST), long tons (LT), or ''tons '''burden'''''? |text=tons.}}

==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, roughly {{Inflation|UK|0.15|1377|fmt=eq|cursign=£|r=0}}.{{efn|Inflation, taking the last year of Edward III's reign, 1377, as the base year, and a shilling as one-twentieth of a pound sterling.}} At that time a tun was a wine container of 252 wine gallons, approx {{cvt|210|impgal|L|sigfig=3}} 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 tuns for tax purposes, an early formula used in England was:

<math display="block"> \text{Tonnage} = \frac {\text{Length}\times \text{Beam} \times \text{Depth}} {100}</math>

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. (The divisor of 100 is dimensionless, so tonnage would be expressed in 'ft3 of tun'.<ref name = Kemp/>)

In 1678 Thames shipbuilders used a method assuming that a ship's burden would be {{frac|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:

<math display="block"> \text{Tonnage} = \frac {\text{Length}\times \text{Beam} \times \frac {\text{Beam}}{2} \times \frac {3}{5}\times {0.62}} {35} </math>

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.<ref>{{cite web | last = Pearn | first = Rodney Stone | title = Tonnage Measurement of Ships | work = Articles | publisher = Steamship Mutual | url = http://www.simsl.com/Articles/Tonnage.asp | access-date = 2007-04-23 | archive-date = 2012-08-28 | archive-url = https://web.archive.org/web/20120828131720/http://www.simsl.com/Publications/Articles/Articles/Tonnage.asp | url-status = dead }}</ref>

Or by solving:

<math display="block"> \text{Tonnage} = \frac {\text{Length}\times\text{Beam} \times \frac \text{Beam}{2}} {94} </math>

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

<math display="block"> \text{Tonnage} = \frac {\text{Length}\times\text{Beam} \times \text{Depth}} {94}</math>

This formula remained in effect until the Builder's Old Measurement rule (above) 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.<ref name=Chapman> {{cite book | last = Schäuffelen | first = Otmar | title = Chapman great sailing ships of the world | publisher = Hearst Books | year = 2005 | page = xx | url = https://books.google.com/books?id=QgMRudqoLGQC&q=%22Depth+to+deck%22+%22depth+in+hold%22+draft&pg=PR20 | isbn = 978-1-58816-384-4 }} </ref> ; 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.<ref name=Chapman/> ; 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.<ref>{{cite web| title =Construction and Dimensions| publisher =The Mary Rose Trust| url =http://www.maryrose.org/ship/ship2.htm| access-date =2009-07-17| url-status =dead| archive-url =https://web.archive.org/web/20090416181350/http://www.maryrose.org/ship/ship2.htm| archive-date =2009-04-16}} </ref> In a calculation of the tonnage of ''Mary Rose'' the draft was used instead of the depth.<ref>{{cite web | last = Fielding | first = Andrew | title = The Mary Rose - a Model | publisher = Not published | url = http://www.maryrose.org/ship/dimensions.htm | access-date = 2009-07-17 | url-status = dead | archive-url = https://web.archive.org/web/20090416061826/http://www.maryrose.org/ship/dimensions.htm | archive-date = 2009-04-16 }} </ref>

==American tons burthen== The British took the length measurement from the outside of the stem to the outside of the sternpost, whereas the Americans measured from inside the posts. The British measured breadth from outside the planks, whereas the Americans 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 ones. The British measure yields values about 6% greater than the American. For instance, when the British measured the captured {{USS|President|1800|6}}, their calculations gave her a burthen of {{frac|1533|7|94}} tons, whereas the American calculations gave the burthen as 1444 tons.<ref>Henderson, James (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 US system was in use from 1789 until 1864, when a modified version of the Moorsom System was adopted.<ref>{{cite web |first1=Phil |last1=Essex |first2=Craig S. |last2=Mork |last3=Pomeroy |first3=Craig A. |title=An Owner's Guide to Tonnage Admeasurement 1998–2003 |website=Jensen Maritime Consultants |url=http://www.jensenmaritime.com/content/download/7782/49839/file/Guide%20to%20Tonnage.pdf |access-date=2014-05-29 }}</ref>

==France== A similar method was used in France to assess the cargo capacity of ships. This was standardised in 1681 at 1 tonne burthen per 42 French cubic feet. Prior to metrification this unit was called a {{lang |fr |port}} (from {{lang |fr |porter}}, {{translation |to carry}}) {{lang |fr |tonneau}} (plural {{lang |fr |tonneaux}}).<ref>{{cite book |last1=Winfield |first1=Rif |last2=Roberts |first2=Stephen S. |title=French warships in the age of sail 1626-1786: design, construction, careers and fates |date=2017 |publisher=Seaforth Publishing |location=Barnsley, South Yorkshire |isbn=978-1-4738-9351-1 |page=2}}</ref>

==See also== *{{annotated link|Thames Measurement}}

==Notes== {{Notelist}}

==References== {{Reflist}}

==Further reading== {{wiktionary|burthen|tun}} {{Refbegin |30em}} * [http://www.bruzelius.info/Nautica/Tonnage/Vademecum%281707%29_p127.html "Concerning Measuring of Ships"], ''The Sea-Man's Vade Mecum'', London, 1707. pp 127–131. * [http://www.bruzelius.info/Nautica/Tonnage/Steel(1805)_p249.html "Of Finding the Tonnage or Burthen of Ships, &c."], David Steel, ''The Shipwright's Vade-Mecum'', London, 1805. pp. 249–251. * [http://southseas.nla.gov.au/refs/falc/0239.html "Burthen", or "Burden"], William Falconer's ''Dictionary of the Marine'', London, 1780, page 56 {{Refend}}

Category:Mass Category:Nautical terminology Category:Sailing rules and handicapping Category:Ship measurements Category:Volume