SHIP-BUILDING. 



»e of the water is exaftly at the light water-line ; and let 

 11 be fufpended until the water drains off, and then weig^hed. 

 Now, fince the weights of fimilar bodies are in a triplicate 

 ratio, or as the cubes of their homologous dimenfions, the 

 weight of the fhip, when light, is, therefore, equal to the 

 produft of the cube of the number of times the fhip ex- 

 ceeds its model by the weight of the model, which is to be 

 reduced to tons. Hence, if the model is conftrufted to a 

 quarter of an inch fcale, multiply the weight of the model 

 by the cube of 48, (one-fourth of an inch being equal to 

 ^ih of a foot,) or 11059Z, which will give the weight of 

 the (hip. If the multiplier be ounces, the produA will be 

 ounces ; if pounds, it will be pounds : and it is to be re- 

 duced to tons accordingly. 



E.vami>!e. — Suppofe the weight of a model of the 74 

 {Plate I.) to be 32 lbs. 13 drachms, when brought down 

 to the light water-line. 



The cube of 48 

 Multiplied by - 



Produces 



1 10592 



32 lbs. 13 drachms 



3544560 lbs. — 1582 tons 880 lbs. 



The weight of the (hip at her light water-mark, within 

 118 lbs. 



Again, let the model be loaded, until the furface of the 

 Water is cxaftly at the load water-line. Now the model 

 being weighed, the weight of the (liip is to be found by the 

 preceding rule ; then the difference between the weights of 

 the (hip, when light and loaded, is the tonnage required. 



Upon the Efforts of the Water to lend the Veffel. 



Here we can do no better than quote Watfon's tranfla- 

 tion of Euler upon the Theorie, &c. des Vaiffeaux ; to 

 which book we refer our readers for a further illuftration of 

 the foregoing particulars, and to Atwood on the Stability 

 of Vefftls. 



" When we fay, that the preffure of the water upon the 

 immerfed part of a veffel counterbalances its weight, we 

 fuppofe that the different parts of a veffel are fo clofely 

 connefted together, that the forces which aft upon its fur- 

 face are not capable of producing any change ; for we cafily 

 conceive, if the connedlion of the parts was not fufficicntly 

 ftrong, the veffel would run the rill< either of being broken 

 in pieces, or of fuffering fome alteration in its figure. 



" The veffel is in a fituation iimilar to that of a rod 

 AB { Plate XIV. Jig. 2.), whicii, being afted upon by 

 the forces A a, C c, D </, B i, may be maintained in equi- 

 librium, provided it has a fulHcient degree of (tiffiiefs ; but 

 as foon as it begins to give way, we fee that it muit bend 

 in a convex manner, fince its middle would obey the forces 

 C c and D d, whillt its extiemities would be aftually drawn 

 downwards by the forces A a and B b. 



" The vcli'el is generally found in inch a fituation ; and 

 fince fimilar efforts continually aft, whilll the vellcl is im- 

 merfed in the water, it happens but too often that the keel 

 experiences the bad effeft of a ftrain. It is, therefore, very 

 important to inquire into the true caufe of the accident. 



" For this purpofe, let us conceive the veffel divided into 

 two parts, by a tranfvcrfe feftion through the vertical axis 

 of the ve.Tcl, in which both the centre of gravity, G, of the 

 whole vcllel, and that of the immerfed part O, arc fituated ; 

 fo that one of them will reprcfent the head part, and the otiier 

 that of the (lern ; each of wiiich we fliall confider fcpar.iuly. 

 Let g be, therefore, the centre of gravity of the entire 

 weight of the firft, and that of the immerfed part cor- 

 refponding. In the fame manner, let y be the centre of 



3 



gravity of the whole ffcrn part, and v) that of its immediate 

 portion. 



«' Now it is plain that the head will be afted upon by the 

 two forces _f m and n, of which the firil will prels it down, 

 and the latter pufh it up. In the fame manner, the (lern 

 will be preffed down by the force j> u, and pulhed up. by the 

 force iu X : but thefe four forces will maintain themfelvcs in 

 equilibrium, as well as the total forces reunited in the points 

 G and O, which are equivalent to them ; but whilft neither 

 the forces before nor thofe abaft fall in the fame dircftion, 

 the veffel will evidently fultain efforts tending to bci.d the 

 keel upwards" (called hogging) " if the two points 0, iu, are 

 nearer the middle than the two other forces g m zv.d y u. A 

 contrary effeft would haopen if the points eand tu were more 

 diilant from the middle than from the points g and_y ;" called 



" But the firft of thefe two cafes ufually takes place in al- 

 moft all veffels ; fince their hollow has a greater breadth to- 

 wards the middle,and becomes more and more narrow toward* 

 the extremities ; whilft the weight of the veffel is, in propor- 

 tion, much more confiderable towards the extremities than 

 at the middle. From whence we fee, that the greater this 

 difference becomes, the more alfo will the veflel be fubjeft 

 to the forces which tend to bend its keel upwards : it is, 

 therefore, from thence that we muft judge how much ftrength 

 it is neceffary to give to this part of the veffel, in order to 

 avoid fuch a confequence. 



" If other circumftances would permit, either to load the 

 veffel more in the middle, or to give to the part immerfed a 

 greater hollow towards the head and ftcrn, fuch an effeft 

 would no longer be feared : but the deftination of moft 

 veffels is entirely oppofite to fuch an arrangement ; by which 

 means we are obliged to ftrengthen the keel as much as may 

 be neceffary, in order to avoid fuch a difafter." 



Having now inveftigated the centre of gravity of the 

 difplacement, meta-centre, and centre of gravity of the 

 whole (hip, with other particulars, and laid down all that 

 is requifite to be attended to, in that refpeft, for the con- 

 ftruftion of a (hip's body under water, we (hall, in the 

 next feftion, proceed to complete the remainder of the (heer- 

 draught. 



To complete the Con/IruSion of the Sheer-draught. Plate I. 



Having found that the difplacement of the (hip at the load- 

 water-line gives the (hip fufiicient liability to keep the lower 

 ports 5 feet 10 inches above the water, we may proceed to 

 draw all the decks in the (heer-plan, beginning with the 

 lower, or gun-deck. The height of the lower fills of the 

 gun-deck ports (liouldbe 2 feet 4 inches above the gun-deck 

 plank, which is four inches thick ; confequently the upper 

 fide of the beam along the fide muft be 2 feet 8 inches below 

 the fills ; add fix inches to that for the round-up of the 

 beam ; and the under fide of the n-un-deck at tlic middle lint 

 in midftiips will be 22 feet 2 inches above the upper edge 

 of the keel ; at the foremoft-perpendicular fet up 24 feet, and 

 at the after-perpendicular 24 feet 8 inches ; then a fegmcnt 

 of a circle drawn through thefe three heights will rcprefent 

 the under fide of tlie gun-deck at the middle line. (Thefe 

 kinds of fwccps are drawn by thin veneers of pear-tree wood, 

 called fwcep-moulds, llriick from a long radius on purpofe, or 

 by a drawing-bow. ) Now fet up four inches, the thicknefs of 

 the gun-deck plank, above the line laft drawn, and let another 

 line be drawn parallel thereto, and the gun-deck will be de- 

 fcribcd at the middle line in the flieer-plan. 



Next proceed to draw the upper deck ; fet up 7 feet 

 2 inches, being the height from the uppjr fide of the gun- 

 deck plank to the under fide of tiic upper deck plaok, alsng 



the 



