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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 Cargo consists of bulk goods conveyed by water, air, or land. In economics, freight is cargo that is transported at a freight rate for commercial gain. ''Cargo'' was originally a shipload but now covers all types of freight, including tra ...
capacity of a
ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...
. 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" ( en-em , burthen , enm , byrthen ), and abbreviated "tons bm". The formula is: : \text = \frac 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 the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...
. 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 Edward III (13 November 1312 – 21 June 1377), also known as Edward of Windsor before his accession, was King of England and Lord of Ireland from January 1327 until his death in 1377. He is noted for his military success and for restoring r ...
levied a tax of 3 shillings on each "
tun TUN or tun may refer to: Biology * Tun shells, large sea snails of the family '' Tonnidae'' * Tun, a tardigrade in its cryptobiotic state * Tun or Toon, common name for trees of the genus '' Toona'' Places * Tun, Sweden, a locality in Västra ...
" 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 wine gallons, approx 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: : \text = \frac 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: : \text = \frac 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 : : \text = \frac In 1694 a new British law required that tonnage for tax purposes be calculated according to a similar formula: : \text = \frac 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 Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affi ...
, excluding the
keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...
itself, at the ship's midpoint, to the top of the uppermost full length deck. * 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. * Main deck :
Main deck The main deck of a ship is the uppermost complete deck extending from bow to stern. A steel ship's hull may be considered a structural beam with the main deck forming the upper flange of a box girder and the keel The keel is the bottom-mos ...
, that is used in context of depth measurement, is usually defined as the uppermost full length deck. For the 16th century ship ''
Mary Rose The ''Mary Rose'' (launched 1511) is a carrack-type warship of the English Tudor navy of King Henry VIII. She served for 33 years in several wars against France, Scotland, and Brittany. After being substantially rebuilt in 1536, she saw her ...
'', main deck is the ''second'' uppermost full length deck. In a calculation of the tonnage of ''Mary Rose'' the draft was used instead of the depth.


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 , their calculations gave her a burthen of 1533 tons, whereas the American calculations gave the burthen as 1444 tons. The US system was in use from 1789 until 1864, when a modified version of the Moorsom System was adopted.


See also

* Thames Measurement


References


External links


"Concerning Measuring of Ships"
''The Sea-Man's Vade Mecum'', London, 1707. pp 127–131.

David Steel, ''The Shipwright's Vade-Mecum'', London, 1805. pp. 249–251.

William Falconer's ''Dictionary of the Marine'', London, 1780, page 56 {{Ship measurements Mass Nautical terminology Sailing rules and handicapping Ship measurements Volume