Builder's Old Measurement

Builder's Old Measurement (BOM) is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. The BOM estimated the tonnage of a ship based on length and maximum beam. The formula is:

where:

• Length is the length, in feet, from the stem to the sternpost;
• Beam is the maximum beam, in feet.^[1]

Thus, BOM estimates the weight of the cargo carrying capacity of a ship measured in tons, a weight that is also termed deadweight. The Builder's Old Measurement formula remained in effect until the advent of steam propulsion. Steamships required a different method of measuring 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. Instead of calculating deadweight, the Moorsom system calculates the cargo carrying capacity in cubic feet, a volumetric measurement rather than a weight 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

The first tax on the hire of ships in England was levied by King Edward I in 1303 based on tons of burthen (an archaic term for burden). Later, King Edward III levied a tax of 3 shillings on each "tun" of imported wine. At that time a "tun" was a wine container of 252 gallons weighing about 2,240 lb (1,020 kg). In order to estimate the capacity of a ship 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.
• 100 the divisor is undefined, but the units would be ft³ per tun^[1]

In 1678, Thames shipbuilders used a deadweight method assuming that a ship's burden would be 3/5 of its displacement. Since displacement is calculated by multiplying Length x Beam x Draft x Block Coefficient, all divided by 35 ft³ per ton of seawater, the resulting formula for deadweight would be:

where:

• Draft is estimated to be ½ Beam.
• Deadweight of cargo is assumed to be 3/5 of displacement.
• Block Coefficient is based on an assumed average of 0.62
• 35 ft³ is the volume of one ton of sea water.^[2]

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 in until the Builders Old Measurement rule was put into use in 1720, and then by Parliamentary law in 1773.

Depth

• Depth to deck

The height from the underside of the hull, excluding the keel itself, at the ships midpoint, to the top of the uppermost full length deck.^[3]

• 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.^[3]

• 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.^[4] In a calculation of the tonnage of Mary Rose the draft was used instead of the depth.^[5]

See also

Thames Measurement

References

External links

• Concerning Measuring of Ships, London, 1707
• BURTHEN, or BURDEN, William Falconer's Dictionary of the Marine, 1780
• OF FINDING THE TONNAGE OR BURTHEN OF SHIPS, &c., Steel, London, 1805