SHIP-BUILDING. 



heel, back included ; fet forward from thence lo inclies, or 

 reduce the breadth to 3 feet 5 inches. The upper hance may 

 be at one-third t!ic heipht of the lower hance, and the breadth 

 of the rudder there ihould be five-fevenths of the breadth 

 at the lower hance, or 3 feet i inch; there reduce the breadth 

 5 inches, which makes it 2 feet 8 inches, from whence a 

 itraight line may be continued to the head, which is 2 feet 

 4 inches fore and aft, or larger, if the piece will admit of it : 

 each hance fliould be reduced with mouldings, as in fliecr- 

 draught, Plate I., and 3^ inches drawn parallel to the att- 

 fide, to reprefent the thickncfs of the back. The heel of 

 the ruddtr, at the fore part, ihould be 9 inches fliort of tlie 

 under fide of the falle keel, and 1 1 inches at the aft-part, 

 the fole included, which is 6 inches deep. 



The pintles and braces may be now reprefented, placing 

 the upper brace about four inches above the wing-tranfom, 

 that the Itraps may clalp round the ftandard on the gun- 

 deck. The lecond brace (hould be fo placed as to fallen 

 on the middle of the gun-deck tranfom. The lower brace 

 may be placed 15 inches above the upper fide of the keel, 

 and the intermediate ones, four in number, to be equally 

 placed between the two latter, making feven in all. The 

 length of the brnces may be governed by a ftraight hue 

 drawn from the third brace, which (hould be 4 feet 6 inches 

 afore the rabbet of the poll to the lower one, which is to 

 be i'lx feet. 



The length of all the ilraps of the pintles, which come 

 upon the rudder immediately above the braces (except the 

 thicknefs of tiie bur or laucers), may extend within four 

 inches of the aft-fide : the pintles are 3! inches in diameter, 

 and all 14 inches long, except the lower one, which is 2 

 inches longer. The ilraps of the braces and pintles are 

 five inche.- broad. 



The length and breadth of the rudder being reprefented 

 in the (heer-plan, Plate I., it is evident the breaks or 

 hances are merely to reduce the breadth as it rifes towards 

 the head, the greatell breadth being only required below 

 the water, where it feels the motion of the fhip. 



The fluid, in paffing to the rudder, exatlly follows the 

 outlines of the bottom ; and fuppofing the rudder to make 

 an angle of 45 degrees with the keel, it may be readily feen, 

 by the water-lines abaft in the half-breadth plan, that the 

 immediate fliock it receives from the water increafes as it 

 approaches to the load-water-line, where they become nearly 

 at right angles with the fide of the rudder in tliat pofition, 

 and this holds good, whatever angle the rudder makes with the 

 keel ; hence fome are of opinion, that the rudder fhould be 

 made broader near the line of floatation, and narrower towards 

 the keel ; but the prefent method of making the rudder 

 with increafing breadth downwards, is only in proportion 

 to the obliquity of impulfe the water afts againil it near the 

 keel. It mull be obferved, that the above force llrikes the 

 rudder obliquely, and only ilrikcs it with that part of its 

 motion which, according to the fine of incidence, forces it in a 

 contrary direftion, with a momentum which not only depends 

 on the velocity of the fliip's courfe, by which this current 

 of water is produced, but alfo upon the extent of the fine 

 of incidence. This force is by confequence compofed of 

 the fquare of the velocity with which the (hip advances, 

 and the fquare of the line of incidence, which will iiecef- 

 farily be greater or fnialler aceordii:g to circumftances ; fo 

 that if the vertel increafes her velocity three or four times 

 falter, the abfolute (hock of the water upon the rudder will 

 be nine or fixteen times ilronger, under the fame incidence ; 

 and if the incidence is increafed, it will yet be augmented in 

 .T greater proportion, becaufe the fquare of the fijic of inci- 

 4ence is more enlarged. 



Amongft the feveral angles that the rudder makes with 

 the keel, there is always one pofition more favourable than 

 any of the others, as it more readily produces the defired 

 effeft of turning the fliip, in order to change her courfe. 



If the angle of the rudder with the keel is greater than 

 45 degrees, the aftion of the water upon the rudder will in- 

 creafe, and at the fame time oppofe the courfe of the fliip 

 in a greater degree ; becaufe the angle cf incidence will be 

 more open, fo as to prefe:it a greater furface to the (hock 

 of the water, by oppofing its pafifage more perpendicularly. 

 If, on the contrary, the angle is leilened to 30 degree?, 

 the rudder will receive the impreffion of the water too ob- 

 liquely, for the angle of incidence will be more acute, fo 

 that it will only prefent a fmall portion of its breadth to the 

 (hock of the water, and by confequence will onlv receive a 

 feeble effort. Thus it appears, that between the effecls which 

 refult from the water's abfolute effort, there is one which 

 always oppofes the (hip's courfe, and contributes lefs to her 

 motion of turning, whilft the other produces only this move- 

 ment of rotation, without operating to retard her velocity. [j_ 

 Hence we may cor.clude, that when the water either ftrikes 

 the rudder too diredllv, or too obliquely, it lofes a great 

 deal of the effeft it ought to produce. Between the two 

 extremes there is, therefore, a mean pofition, which is the 

 moil favourable to its operation, tis. the angle 45, or 

 between that and 42 degrees. See Watfon's Euler, p. 130. 

 See alfo Rudder. 



It is evident, that the fore part of the rudder, as high up 

 as the head of the poll, mull be trimmed on each fide, to 

 the middle of its thicknefs, (which is the fame thicknefs as 

 the aft-fide of the llern-po(l, or rather lefs, as it need not 

 projeft the pod when tlie helm is bird over,) to the greatell 

 angle the rudder is propofed to make with the keel : how- 

 ever, the common method is to fet off two-fifths the thick- 

 nefs of the rudder from the fore part on each fide, and 

 from thence trim it llraight through to the middle of its 

 thicknefs, or, what is better, to leave the middle to the con- 

 vexity of the pintles, rather than a Iharp edge. By this 

 method the rudder may be put over to the angle of 50 de- 

 grees, which is more than is neceffary, and it is very leldom 

 that the tiller, owing to its length, can be put over fo far 

 to the fide as to allow of the rudder making an angle of 

 more than 45 degrees, which angle is quite fufficient. 



When the above angle, or what is technically called the 

 hearJiri^, is wholly taken from the fore part of the rudder, 

 the main piece is very much wounded by letting on the upper 

 pintle ; but this of late years has been greatly remedied, by 

 taking half the bearding fro.-n the aft-fide of the ilern-poil 

 at the head, and from one to two inches on the heel ; of 

 courfe the rudder is bearded fo much the lefs. This, alfo, 

 will greatly affifl the converfion of the ilern-poll. 



The bearding on the rudder is reprefented by the (hading 

 on the fore part of the rudder, and the bearding on the poU 

 by the ticked line in (heer-plan, Plate I. 



The rudder, which is reprefented in the fheer-draught, 

 Plate I., and is as at prefent ufed in the navy, having its 

 axis of rotation in the centre of its pintles, which are paral- 

 lel to the aft-fide of the Ilern-poll, caufes a fpace, confider- 

 ably greater than its tranfverfe feftion, to be cut in the coun- 

 ter for the rudder to revolve in, which would be impervious 

 to the waves, were it not defended by a coating of tarred 

 canvas, nailed in fuch a manner to the rudder and counter, as 

 to cover the whole fpace required. But the ill effefts of 

 having fo large a fpace fo ill guarded, have proved very dan- 

 gerous. 



It was to remedy this defeft that rourul-heaJrd rudders 

 of late years have been adopted in many merchant-fhips, 



particularly 



