THE RULE FOR CASTING THE TONNAGE, In the royal navy, is to take the length on a straight line :long the lower side of the rabbit of the keel, from a perpendi cular or square from the back of the main stern post, at the height of the wing-transom, to a perpendicular or square at the height of the upper deck (and middle deck of three decked ships) from the fore-part of the stern. The only difference in merchant ships is to take this length as before, from the back of the main-post, at the height of the wing-transom, to the same height forward to the foreside of the stern ; then from the length between those perpendiculars subtract three•fifths of the extreme breadth for the rake for ward, and two inches and a half for every foot the wing-transom is high above the lower part of the rabbit of the keel for the rake abaft. The remainder is the length of the keel for ton nage.
Although this is the dimension sought, yet, to show the fallacy of acquiring this tonnage, the whole of the rule shall be here subjoined.
Then multiply the length of the keel for tonnage by the extreme breadth, and the product by half that breadth, and di vide the whole by 94 ; the quotient will be the tonnage.
This extreme breadth to be taken from the outside to the outside plank or thick. stuff, in the broadest part of the ship, either above, on, or below the wales, de ducting from the said thickstuff or plank all that it exceeds the thickness of the plank of the bottom, which shall be ac counted the extreme breadth ; so that the moulding breadth, or breadth of the frame, will then be less than the extreme breadth so found. For the thickness of the bottom plank, see the foregoing di mensions.
I3y this rule, the following lengths of the keel for tonnage of the same class of ships are found, of Guns. Ft. In Tons. Ft. In.
110 . . . 158 10 1257 . . . 134 0 98 . . . 156 6 1000 . . . 131 0 74 . . . 148 8} 818 . . . 118 8 50 . . . 119 9 544 . . . 100 0 36 . . . 113 3 441 . . . 98 8 18 . . . 90 8 329 . . . 82 0 Also their burthcn in tons ; Guns. Tons. Guns. Tons.
110 . . . . 2358 50 . . . . 1044 98 . . . . 2067 36 . . . 877
74 . . . . 1873 18 . . . . 419 Teat. East India ships . 1257 Ditto 1000 Ditto 818 West India ships . 544 Ditto 441 Ditto 329 Hence it is obvious, had the length and breadth of the ships in the royal navy, and those in the merchant-service, been the same, the tonnage would also have been the same, although the construction under water is so very different ; therefore no dependence can be placed on those rules for the conformation of the real bur then of vessels. And as to builder's ton nage, it is equally as fallacious, because depth isnot taken at all into consideration, and it is easy to imagine that two vessels may, by this rule, be the same tonnage, and one some feet deeper than the other; consequently, what results to the builder is, to regulate his price accordingly. Hence, there remains scarcely one unde viating method in the construction of ships. We will allow, it is not to be ex pected to obtain any rule, in this particu lar, that would be quite exact ; because the true but-then, or tonnage, a ship should carry, not only depends upon the cubical dimensions of the ship's bottom, but her own gravity with respect to the whole of the hull; and, in short, on the weight of every article which makes a part of the ship. Therefore, the nearest rule that approximates to the burtheu different built vessels are found by ex perience to carry, should be adopted ; as the fallacy of the rule in present use discovers no one thing whatever, as may be easily seen by any person, though a novice in the art of ship-build ing.
I.astly, the depth in hold, which, in naval ships, must be always governed by the height which the guns are in tended to be above the water, and load water line ; as the depth is taken from the upper side of the limber-strake to the upper side of the lower deck beam in midships. In merchant vessels, the depth in hold is regulated for the different cargoes that each may be designed to carry ; and here, again, as there can be no certain rule observed, we will give the depth in hold of the same acknow ledged superior vessels.