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Builder's Old Measurement

From Wikipedia, the free encyclopedia

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:

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 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.

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>> DESIGNING SOMETHING LIKE THE HULL OF A SHIP CAN BE AN EXPENSIVE PROCESS. SO HOW DO YOU RIGOROUSLY TEST OUT A DESIGN BEFORE YOU EVEN BUILD IT? A UNIQUE RESEARCH PROGRAM AT THE UNIVERSITY OF IOWA MAKES THAT POSSIBLE, USING SOPHISTICATED SOFTWARE, MODEL SHIPS, AND AN ENORMOUS INDOOR POOL. PROFESSOR FRED STERN LEADS THE UNIVERSITY OF IOWA'S SHIP-HYDRODYNAMICS PROGRAM, THE HOME OF THIS HIGH-TECH FACILITY. >> THESE ARE CALLED WAVE BASINS. YOU CAN SEE THE WAVE-MAKERS AT THE FAR END HERE. THEY'RE PLUNGER TYPE. THEY GO UP AND DOWN. THEY GENERATE EITHER REGULAR OR IRREGULAR WAVES IN THE BASIN. THE NEXT PART FOR US, THEN, IS THE CARRIAGE. THIS IS A CUSTOM-DESIGNED CARRIAGE. IT FOLLOWS THE MODEL. THERE'S TARGETS ON THE MODEL THAT THE CAMERAS AND THE TRACKING SYSTEM USE. OURS IS UNIQUE INSOFAR AS IT'S DESIGNED SPECIFICALLY TO CARRY MEASUREMENT SYSTEMS TO MEASURE THE LOCAL FLOWS AROUND THE FREE-RUNNING MODELS. >> BELIEVE IT OR NOT, THIS NEARLY $5 MILLION FACILITY WAS DESIGNED AND BUILT PRIMARILY FOR THE PURPOSE OF VALIDATING OR TESTING COMPUTER CODE. THAT CODE, CFDSHIP-IOWA, IS A SIMULATION SOFTWARE DEVELOPED AT THE UNIVERSITY OF IOWA FOR THE OFFICE OF NAVAL RESEARCH. IN THE PAST, STERN AND HIS RESEARCH TEAM VALIDATED THE CODE BY RUNNING THEIR MODELS IN A 300-FOOT-LONG TOWING TANK, BUT AS COMPUTER POWER GREW AND THE CODE BECAME ABLE TO DO MORE ADVANCED SIMULATIONS, SO IT REQUIRED A MORE SOPHISTICATED TOOL FOR VALIDATING. >> OUR CODES DEVELOPED VERY RAPIDLY OVER THE LAST, REALLY, 5 TO 10 YEARS, TO THE POINT THAT WE'RE ABLE TO PREDICT THE EXACT TRAJECTORY OF THE MODEL, INCLUDING RUDDERS AND ROTATING PROPELLERS, AND NO LONGER RESTRICTED TO CAPTIVE SIMULATIONS. THEREFORE, TO VALIDATE IT, WE NEEDED TO SEE, "WELL, HOW ACCURATE WOULD THE TRAJECTORIES BE?" BY TRAJECTORIES, I MEAN MODELS. FOR EXAMPLE, YOU START OUT STRAIGHT AHEAD, YOU GIVE THE RUDDER A 35-DEGREE ANGLE, AND IT'S GONNA GO IN A CIRCLE. WE WANT TO KNOW, WHAT'S THE RADIUS OF THAT CIRCLE? ANOTHER TYPE IS A ZIGZAG TEST WHERE YOU PUT THE RUDDER 10, 10, 20, 20, AND THEN YOU PREDICT WHAT'S KNOWN AS THE OVERSHOOT. THERE'S HALF A DOZEN OR SO OF THESE TRADITIONAL TESTS, BOTH IN CALM WATER AND WAVES, THAT SHIPS HAVE TO BE ABLE TO DEMONSTRATE THE CAPABILITY OF PERFORMING TO A SATISFACTORY STANDARD IN ORDER TO BE BUILT. AND SO OUR FOCUS IS NOT ACTUALLY DESIGN. OUR FOCUS IS DEVELOPMENT OF THE TOOLS. >> WHILE THE WAVE BASIN WILL ENABLE NEW ADVANCES IN HYDRODYNAMICS, PROFESSOR STERN SAYS IT INDICATES THE DIRECTION MUCH OF SCIENCE IS ALREADY HEADING. >> YOU KNOW, THE FUTURE OF ENGINEERING IS SIMULATION-BASED DESIGN, AND, IN FACT, WE CAN SIMULATE THINGS WE CAN'T MEASURE. SO YOU BEGIN TO SEE A MAJOR SHIFT IN THE SCIENTIFIC METHOD, WHERE WE WENT FROM ARISTOTLE, OKAY, WHO BASED EVERYTHING ON LOGIC -- HE DIDN'T BOTHER TO -- YOU KNOW, HE THOUGHT MEN AND WOMAN HAD A DIFFERENT NUMBER OF TEETH IN THEIR MOUTHS. IT'S ONE OF MY FAVORITE STORIES. OF COURSE, THEY DON'T, AND HE COULD HAVE FIGURED THAT OUT EASILY BY JUST LOOKING IN HIS WIFE'S MOUTH, BUT -- THEY'RE TEETH. BUT, NO. SOME LOGIC AND NO VALIDATION. GALILEO SAID, "OKAY, VALIDATION. EXPERIMENTS. PHYSICAL OBSERVATIONS." BUT THERE'S LIMITS THERE, TOO. NOW WE COME BACK TO, IF YOU THINK OF SIMULATION TECHNOLOGY AS REALLY LOGIC, SO NOW WE'RE COMING BACK TO LOGIC -- LOGIC, THOUGH, COUPLED WITH THE EXPERIMENTS. THIS IS THE ENGINE OF SIMULATION-BASED DESIGN, AND, IN THAT SENSE, EACH OF THESE STEPS ARE JUST LITTLE, YOU KNOW, STEPS ALONG THE WAY. WE'LL ALWAYS NEED THE SMALLER FACILITIES, THE MEDIUM-SIZE, AND THE LARGER FACILITIES. THIS FACILITY IS HALF THE SIZE IT SHOULD BE FOR ACTUAL INDUSTRIAL USE, BUT FOR BASIC SCIENCE, IT'S JUST PERFECT. >> TO LEARN MORE ABOUT SHIP HYDRODYNAMICS AND THE SCIENCE OF ALL THINGS FLUID, VISIT...

Contents

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 £108.95 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:

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'.[1]

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.[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 until the Builder's Old Measurement rule 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, excluding the keel itself, at the ship's 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]

American tons burthen

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 1533794 tons, whereas the American calculations gave the burthen as 1444 tons.[6] 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.[7]

See also

References

  1. ^ a b Kemp, ed., P. (1976). The Oxford Companion to Ships & the Sea. Oxford University Press. p. 876. ISBN 0-19-211553-7.
  2. ^ Pearn, Rodney Stone. "Tonnage Measurement of Ships". Articles. Steamship Mutual. Retrieved 2007-04-23.
  3. ^ a b 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.

External links

This page was last edited on 2 November 2018, at 14:38
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