Now the moment of the water pressure on the rudder varies as the length of the couple a o and the area of the rudder; and the radius of gyration is dependent upon the length of the ship and the stowage of her weights, and thus a long vessel would have her already slow turning further diminished by the stowage of weights in her ends.* If the arm of the couple on which acts the pressure on the rudder be shortened, the steering efficiency will be diminished ; and it has been contended that a raking sternpost so shortens the arm of the couple. But, so far as we can judge, this is a mistaken contention, as generally the centre of lateral resistance is carried farther forward by the raking sternpost than is the centre of effort of the rudder.
Beyond this, there is usually a much greater length of sternpost when it rakes, and generally the area of the rudder is thereby increased, inasmuch as the breadth only of the rudder appears to be regarded as a matter of importance, and not its depth, so far as yachts are concerned. Some yachts with raking sternposts have enormous rudders, and, although there may be some danger in using them in the case of sternway and in scending, there is no doubt that they are efficient. But a rudder hung on a raking sternpost is not wholly effective, inasmuch as a component of the pressure on it is exerted in a vertical direction, and tends to drag the vessel's stern under. This can only be regarded as a disadvantage, and a further disadvantage is that the rudder is difficult to put over, as it has to be lifted every time ; but the latter difficulty is overcome by making the tiller longer than would be required for a similar rudder hung on an upright sternpost.
With regard to resistance to rotation, this mainly depends upon the area of the immersed longitudinal section, and particularly upon the amount of dead wood fore and aft. By reducing the dead wood fore and aft 'the resistance is proportionately decreased, and, moreover, the radius of gyration would be somewhat shortened by the reduction of the fore and aft weight ; but almost equal effects would be produced by taking away from the dead wood forward and aft, and concentrating it in the middle of the vessel under the keel. The effective surface for lateral resistance would be maintained, and the radius of gyration would be still shortened.* To sum up, the quickness of a vessel in answering her helm and the smallness of the circle in which she will turn depend : 1. Upon the smallness of the weight of the vessel and her radius of gyration.
2. Upon the area of the rudder, and the length of the couple upon which it acts, and upon the time it takes to put the rudder over.
3. Upon the area and forin of the immersed longitudinal-vertical section of the vessel.
The double-boarded boat as depioted in Fig. 14, affords peculiar advantages for lengthening the arm of the turning couple, as by lifting the board (h) the centre of lateral resistance is thrown very much forward, Fie. 14.
and the area of dead wood at the after end (which might be necessary in a sailing vessel to deck leeway or to balance sails) is considerably reduced. In tacking, if the vessel's "way " were stopped as she came head to wind, and the rudder thereby became useless, the fore board a could be raised and the after board lowered, and thus the head of the vessel, by aid of the fore sails, would readily fall off the wind.
All kinds of fanciful forms have been given to rudders, and a practice came in a few years ago of putting the greatest breadth near the surface of the water. We believe this practice was owing to a vessel once having the lower half of her rudder accidentally carried away, and the subsequent report that she steered better with the part than with the whole. We are
inclined to think that in this case the inefficiency of the entire rudder depended on its being too big for the crew to use ; at any rate, an experiment made by Mr. Fronde with a model of H.M.S. Encounter clearly proved that the lower half of a rudder is more effective than the upper half. The rudder was in two parts, i.e., squares of equal dimensions, and it was found that the upper half required to be put over to 20° to balance the lower half at 10°, in order that the vessel might follow a straight line. In the case of a very raking sternpost there may be a small advantage in having the broadest part of the rudder in the top half, as its centre of effort would thereby be carried farther aft; but generally there can be no doubt that the deeper the main area of the rudder is immersed, clear of the fulness of the after body, the more effective it is. As a clean run aft is a great assistance to the effectiveness of the rudder, it is quite possible, if an experiment similar to that tried on the Encounter were tried with a yacht, that a less difference in the effectiveness of the two halves might be found. The advantage of keeping the rudder well immersed—both in smooth and disturbed water—is so well understood by small boat sailers that we find many small and shallow craft with their rudders dropping considerably below the keel. The only disadvantage of this arrangement is that, if the boat carried much weather helm, and if the centre of effort of the rudder were much below the centre of lateral resistance, the pressure on the rudder would tend to increase the boat's heel, although not to a considerable extent. (These rudders are so hung that in shallow water they lift without unshipping.) There appears to be no definite rule for determining the breadth of rudders in sailing yachts, but generally we find the breadth to be one twentieth of the length on the load line, and about one-thirtieth of that length in steam yachts. With regard to the efficiency of broad and narrow rudders, it would appear, from experiments made for the Admiralty some years ago, that a rudder of, say, 3ft. in breadth, put over to an angle of 30°, would have double the efficiency or turning power of one 6ft. in breadth put over to half the angle, or 15'; and the force required to move the rudder would be the same in either case. Thus there can be no increase in the efficiency of a rudder by the mere addition of breadth without an increase in the power to use it, and a very large rudder, if used so as to obtain its greatest efficiency—which is found to be when put over to an angle of about 35°—means a great retardation of speed ; or, the smaller the circle a vessel is made to turn in, the more speed will be retarded. So far as small steam yachts are concerned, no rudder offers greater advantage for facility in using it and efficiency than Gumpel's, as, owing to its construction, it does not take an equal force on the tiller to balance the water pressure on the rudder. (This rudder was fully described in The Field of June 20, 1874, and Oct. 23, 1875.) A variety of steering gear has from time to time been introduced, which, by the aid of the wheel, has enabled one man to do the work of two or three. Of these contrivances none promise better results than that of Mr. G. L. Watson ; but, so far as racing yachts are concerned, we think a long tiller and relieving tackle must be relied upon.