Builder's Old Measurement

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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 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".

Contents

The formula is:

where:

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 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 £121.37 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 210 imp gal (955 L) 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:

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.

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:

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 (above) was put into use in 1720, and then mandated by Act of Parliament in 1773.

Depth

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]
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, 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, 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, their calculations gave her a burthen of 1533794 tons, whereas the American calculations gave the burthen as 1444 tons. [6]

The US system was in use from 1789 until 1864, when a modified version of the Moorsom System was adopted. [7]

See also

Related Research Articles

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Ton Unit of mass or volume with different values

The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight. Its original use as a measurement of volume has continued in the capacity of cargo ships and in terms such as the freight ton. Recent specialized uses include the ton as a measure of energy and for truck classification. It is also a colloquial term, "ton" is the heaviest unit of weight typically used in colloquial speech. It is also used informally to mean a large amount of something, material or not.

Volume Quantity of three-dimensional space

Volume is a scalar quantity expressing the amount of three-dimensional space enclosed by a closed surface. For example, the space that a substance or 3D shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre. The volume of a container is generally understood to be the capacity of the container; i.e., the amount of fluid that the container could hold, rather than the amount of space the container itself displaces. Three dimensional mathematical shapes are also assigned volumes. Volumes of some simple shapes, such as regular, straight-edged, and circular shapes can be easily calculated using arithmetic formulas. Volumes of complicated shapes can be calculated with integral calculus if a formula exists for the shape's boundary. One-dimensional figures and two-dimensional shapes are assigned zero volume in the three-dimensional space.

Tonnage is a measure of the cargo-carrying capacity of a ship, and is commonly used to assess fees on commercial shipping. The term derives from the taxation paid on tuns or casks of wine. In modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Although tonnage (volume) should not be confused with displacement, the long ton of 2240 lb is derived from the fact that a "tun" of wine typically weighed that much.

Twenty-foot equivalent unit Unit of cargo capacity

The twenty-foot equivalent unit is an inexact unit of cargo capacity, often used for container ships and container ports. It is based on the volume of a 20-foot-long (6.1 m) intermodal container, a standard-sized metal box which can be easily transferred between different modes of transportation, such as ships, trains, and trucks.

Gross register tonnage or gross registered tonnage, is a ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m3). Replaced by Gross Tonnage (GT), gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume. Typically this is used for dockage fees, canal transit fees, and similar purposes where it is appropriate to charge based on the size of the entire vessel. Internationally, GRT may be abbreviated as BRT for the German "Bruttoregistertonne".

Beam (nautical) Width of a ship at its widest point

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Thames Measurement, also known as Thames Tonnage, is a system for measuring ships and boats. It was created in 1855 as a variation of Builder's Old Measurement by the Royal Thames Yacht Club, and was designed for small vessels, such as yachts. It was originally used for calculating the port dues for yachts; the formula was also used in some early handicapping systems for yacht racing.

Gross tonnage Nonlinear measure of a ships overall internal volume

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Net tonnage Ship cargo space volume

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The 1719 Establishment was a set of mandatory requirements governing the construction of all Royal Navy warships capable of carrying more than 20 naval long guns. It was designed to bring economies of scale through uniform vessel design, and ensure a degree of certainty about vessel capability once at sea, and was applied to all vessels from the first-rate to the fifth-rate. Once in effect, it superseded the 1706 Establishment, which had specified major dimensions for ships of the second-rate, third-rate and fourth-rate only.

Ship measurements consist of a multitude of terms and definitions specifically related to ships and measuring or defining their characteristics.

Ton classes are categories used to identify classes of yachts.

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<i>Merlin</i>-class packet boat

The Merlin-class packet boat of 1838 was a Sir William Symonds design that was approved on 2 April 1838. The vessels were to be built for steam mail packet service on the Liverpool to Dublin route. The initial two ships were ordered in the fall of 1838 from Pembroke Dockyard. The third vessel (Medina) was ordered in 1839, also from Pembroke. When completed all vessels were used on the Liverpool station. In 1848 all three were in the Mediterranean on mail packet service. Merlin went to the Baltic during the Russian Was as a survey ship. Medusa went to the Black Sea. Medina remained in the Mediterranean being converted to a survey ship for Mediterranean service. Merlin served on the West Coast of Africa and was sold for commercial use in 1863. Medina was sold at Malta in April 1864. Medusa was converted to a tugboat in 1861 and remained in that service until sold in 1872.

References

  1. 1 2 Kemp, P., ed. (1976). The Oxford Companion to Ships & the Sea . Oxford University Press. pp.  876. ISBN   0-19-211553-7.
  2. Pearn, Rodney Stone. "Tonnage Measurement of Ships". Articles. Steamship Mutual. Retrieved 2007-04-23.
  3. 1 2 Schäuffelen, Otmar (2005). Chapman great sailing ships of the world. Hearst Books. p. xx. ISBN   978-1-58816-384-4.
  4. "Construction and Dimensions". The Mary Rose Trust. Archived from the original on 2009-04-16. Retrieved 2009-07-17.
  5. Fielding, Andrew. "The Mary Rose - a Model". Not published. Archived from the original on 2009-04-16. Retrieved 2009-07-17.
  6. 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
  7. Essex, Phil; Mork, Craig S.; Pomeroy, Craig A. "An Owner's Guide to Tonnage Admeasurement 1998-2003" (PDF). Jensen Maritime Consultants, Inc. Retrieved 2014-05-29.