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I see the following in historical ship records: burthen 1050 75/95 tons. What does the "75/95" mean? I don't see anywhere else on the web where this question is answered.

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The figure means that the tonnage of the ship had been calculated to be 1050 and 75/95ths of a ton.

This came from what is known as the Builder's Old Measurement method of calculating the tonnage of a ship based on the length and maximum beam of the vessel. The method was widely used from the mid-17th century to the mid-19th century.

The version of the formula that I learned when I studied the subject is the one contained on the Wikipedia page:

tonnage formula

The calculated value is expressed in "tons burden". In Early Modern English this would be written as "tons burthen", and in Middle English as "tons byrthen".

You'll notice that this formula would give a result in 94ths of a ton. This was the version of the calculation used in the UK. I was also confused when I first saw values quoted in 95ths of a ton in documents.

The Builder's Old Measurement system was adopted for use in the United States under An Act for Registering and Clearing Vessels, Regulating the Coasting Trade, and for other purposes, passed in September 1789, but the denominator in the formula was changed to 95:

"... to ascertain the tonnage of all ships or vessels, the surveyor or other person appointed by the collector to measure the same, shall take the length of every vessel, if double decked, from the fore part of the main stem to the after part of the stern post above the upper deck, the breadth at the broadest part above the main wales, and half such breadth shall be accounted the depth of every double decked vessel; he shall then deduct from the length three fifths of the breadth, multiply the remainder by the breadth, and the product by the depth, dividing the product of the whole by ninety-five"

There were also some differences in the way that length and breadth were measured. The result being that the calculated tonnage for a given vessel was lower when calculated using the American system.

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