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".
The formula is:
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.
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 £116.61 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 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:
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:
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.
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.
The upshot was that American calculations gave a lower number than the British. For instance, when the British measured the captured USS President, their calculations gave her a burthen of 15337⁄94 tons, whereas the American calculations gave the burthen as 1444 tons. The British measure yields values about 6% greater than the American.
The US system was in use from 1789 until 1864, when a modified version of the Moorsom System was adopted.
A hull is the watertight body of a ship or boat. The hull may open at the top, or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, derrick, or mast. The line where the hull meets the water surface is called the waterline.
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 mass. 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 specialised uses include the ton as a measure of energy and for truck classification. It is also a colloquial term.
Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance or 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. 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. Tonnage should not be confused with displacement, which refers to the actual weight of the vessel. Tonnage is commonly used to assess fees on commercial shipping.
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). 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.
The beam of a ship is its width at the widest point as measured at the ship's nominal waterline. The beam is a bearing projected at right-angles from the fore and aft line, outwards from the widest part of ship. Beam may also be used to define the maximum width of a ship's hull, or maximum width including superstructure overhangs.
The International rule, also known as the Metre rule, was created for the measuring and rating of yachts to allow different designs of yacht to race together under a handicap system. Prior to the ratification of the International rule in 1907, countries raced yachts under their own national rules and international competition was always subject to various forms of subjective handicapping.
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 is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage. Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weight such as deadweight tonnage or displacement.
The Moorsom System is a method created in Great Britain of calculating the tonnage or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees. It was put into use starting in 1849 and became British law in 1854.
Net tonnage is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula. Defined in The International Convention on Tonnage Measurement of Ships that was adopted by the International Maritime Organization in 1969, the net tonnage replaced the earlier net register tonnage (NRT) which denoted the volume of the ship's revenue-earning spaces in "register tons", units of volume equal to 100 cubic feet (2.83 m3). Net tonnage is used to calculate the port duties and should not be taken as less than 30 per cent of the ship's gross tonnage.
HMS Falmouth was a 50-gun fourth-rate ship of the line built for the Royal Navy in the first decade of the 18th century. The ship participated in several battles during the War of the Spanish Succession (1701–15) and the War of Jenkins' Ear (1739–48).
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.
Roanoke was one of the largest wooden ships ever constructed.
HMS Tartarus was a paddle steamer gunvessel, the name ship of her class, built for the Royal Navy during the 1830s.
HMS Merlin was a 2-gun paddle packet boat, the name ship of her class, built for the Royal Navy during the 1830s. She was converted into a survey ship in 1854 and then into a gunvessel two years later.
HMS Medina was a 2-gun Merlin-class paddle packet boat built for the Royal Navy during the 1830s. She was converted into a survey ship in 1856 and was broken up in March 1864.
The Merlin-class packet boat was a trio of 2-gun paddle packet boats built for the Royal Navy during the 1830s. Two of the three ships were immediately commissioned for packet service at Liverpool and the third was not commissioned until 1848 for packet service in the Mediterranean. Merlin and Medina were later converted into service as survey ships and Medusa was converted into a tugboat. Merlin and Medina also saw service off the West African coast during the 1850s.
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