By the Tonnage is generally understood the burthen of a vessel as computed by an established but very defective rule, which we shall presently give, producing what is usually called Builder's Tonnage, in contra-distinction to the true tonnage.
By this rule, all vessels, whether their bodies be extremely full or extremely sharp, will appear to be precisely of the same burthen or capacity, if the length of keel and extreme breadth be similar. Thus, the sharpest cutter will seem to carry as much as the fullest merchant-ship of the same length and breadth extreme. This method is, of course, exceedingly detrimental to that principle which promises velocity; as the ship which is narrowest above, and widest and deepest below, will measure least in proportion to her real capacity; the very reverse of which is necessary for fast sailing.
In order to ascertain the true burthen of a ship, we ought to find the place of the light-water line, and thence calculate the number of cubic feet below the line of floatation: as the product, deduct from the number of cubic feet contained at the load-draught, would shew the real capacity by which the tonnage may be computed: and, if the difference be multiplied by the weight of a cubic foot of sea water, 64 3/8 lbs., the product, divided by 2240 (the number of lbs. in a ton), will give the true burthen in tons.
Or, in other words, by deducting the weight of the ship at her lightwater mark from her weight when brought down to the load-water mark, the remainder will be the tonnage.
1. Let fall a perpendicular from the foreside of the stem, at the height of the hawse-holes*, and another perpendicular from the back of the main post, at the height of the wing transom.
2. From the height between these perpendiculars, deduct three-fifths of the extreme breadth†, and likewise as many 2½ inches as the wing transom is high from the upper edge of the keel, and the remainder is accounted the length of the keel for tonnage.
Then multiply the length of the keel for tonnage by the extreme breadth, and that product by half the extreme breadth; then, dividing by 94, the quotient will be the burthen in what is denominated Builder's Tonnage.
Or, Multiply the length of the keel for tonnage by the square of the extreme breadth, and divide the product by 188, the quotient will be the burthen in tons.
The Rule made use of by the Officers of the Customs, for the computation of Tonnage Duties, for all vessels, excepting coal-vessels, is established by the act of parliament 13 Geo. III; c. 74, as follows:
The length shall be taken on a straight line along the rabbet of the keel, from the back of the main stern-post to a perpendicular line from the fore part of the main stem under the bowsprit; from which, subtracting three-fifths of the breadth, the remainder must be esteemed the just length of the keel to find the tonnage; and the breadth shall be taken from the outside plank in the broadest place in the ship, be it either above or below the main wales, exclusive of all manner of doubling planks that may be wrought upon the sides of the ship; then, multiplying the length of the keel by the breadth so taken, and that product by half the breadth, and dividing the whole by 94, the quotient will be deemed the true contents of the tonnage*.
* In the merchant-service, this perpendicular is let fall from the foreside of the stem, at the height of the wing transom, by reason of the hawse-holes being generally so very high, and their stems also having a great rake forward.
† By the extreme breadth, is meant the breadth taken from timer to timber outside, with the thickness of the bottom on each side added; or, which is the same thing, the thickness of the bottom on each side added to the moulded breadth.
* For the method of determining the exact tonnage of a ship, by her estimated weight, &c. see "Elements and Practice of Naval Architecture", page 211.
Steel: The Shipwright's Vade-Mecum
David Steel, London, 1805. pp 249-251.
Transcribed by Lars Bruzelius
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Copyright © 1999 Lars Bruzelius.