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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. It is expressed in "tons burden" (, ), 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. 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. ==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 £ 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 , 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'.〔 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Builder's Old Measurement」の詳細全文を読む スポンサード リンク
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