LATERAL resistance is the resistance the water offers to a vessel being moved sideways or laterally; this sideway or broadside motion when a vessel is underway is at right angles, or nearly at right angles, to her forward or direct motion, and is usually termed leeway ; thus the resistance to leeway is properly described as " lateral resistance." The centre of lateral resistance is usually understood to mean the centre of the vertical longitudinal section of the immersed portion of the vessel, including the rudder. In other words, this immersed longitudinal section is assumed to be a plane ; and if this plane be moved through the water in a direction at right angles to its own (the plane's) surface, then the resultant of the resistance it will meet with will act through its centre. For instance, let Fig. 8 be the immersed longitudinal section of a vessel with its centre at x. If a towing line were attached to the point x, the vessel or plane would be towed laterally or "broadside on" through the water, without exhibiting any tendency to turn one way or the other; in fact, the plane representing the longitudinal section of the vessel would keep normal, or at right angles to the towing line. But if the towing line be attached farther aft, then on being towed the stern would come round towards the line ; or if attached farther forward, the bow would turn round towards the line.* In calculating the centre of lateral resistance of a ship or yacht, it is always assumed that a plane has to be dealt with, and the immersed longitudinal section is taken as that plane. As a matter of fact, the centre of this plane would not be the centre through which the resultant pressure on the side of the ship would act. Owing to the varying form of a ship or yacht, it is almost impossible to determine by calculation the point through which the resultant of the horizontal pressure of the water actually acts ; and, moreover, if the exact point could be readily determined, the knowledge of it would be of small .practical value, for the reason that, owing to the forward motion of the vessel in a line with her keel, there is an excess of pressure on the bow and a constantly decreasing pressure towards the stern ; the bow is continually entering " solid " water, whilst towards the stern the water becomes more and more disturbed ; and beyond this there will be an accumulation of water rising on the lee bow which has the effect of altering the form of the immersed portion of the vessel, and this of itself carries the centre farther forward. And farther, even supposing the centre of pressure could be accurately calculated for the upright position, it would be useless for any other position of the vessel, as a different portion of her hull would be immersed, or its position relative to the horizon altered, each time the vessel rolled or was heeled.
The use of knowing approximately the position of the centre of lateral resistance is that the " handiness " of a vessel can be regulated thereby; and for this use the centre of the plane described by the immersed longitudinal section of a vessel is, fortunately, a sufficiently determinate point, as will hereafter be shown.
It need scarcely be pointed out that a flat surface is more effective in resisting lee way than a convex one ; hence a vessel with a large area of dead wood aft, a very sharp flat entrance, and a deep keel will make less lee way than one that has a less flat surface immersed, all other things being equal. It must always be remembered, however, that the upper part
of the dead wood aft, owing to the disturbed water it passes through, meets with less pressure than does the dead wood forward, as the bow is always entering new or undisturbed water. Hence " drag," or a much greater draught aft than forward, has been found of great use in keeping the centre of lateral resistance in a required distance aft, as the lower parts of what may be termed a raking keel are continually being moved into solid or undisturbed water. This matter can be illustrated in this way :— In the diagram (Fig. 9) let A - be an immersed plane moving in the direction of the arrow a; and let it be assumed that the plane has also a sideway or lateral motion, as indicated by t. Next, k and a are points or spots on the plane, and x and xi are particles of water. As the plane moves forward and glides past z and the spots k and a will push them severally on one side, it being always remembered that the plane has sideway motion, and it is resistance to this sideway motion which we are considering. When any other indefinitely near spots on the plane, as b h, arrive abreast of x and they find the latter receding, in consequence of the push they received from k and a ; the result is that b and h meet with less resistance to sideway or lateral motion than did k and a; and so on for g and c, &c. It is thus evident that what is required for an effective surface of lateral resistance is not a number of spots in the horizontal direction, a b c and k h g, but a number in the vertical direction, k a. It would be found inconvenient to so increase the depth at the fore end to obtain an effective surface, but fortunately it is found to be an advantage to have an increase in depth at the aft end. Assume a triangular piece v to be cut off the fore-end of the plane A, and to be placed underneath aft as shown by v in the diagram B ; the area of the surface remains exactly the same, but a double number of spots as n m o and p are obtained that will enter solid water to meet particles of water as x x, &c., as the plane advances. It is quite patent that, although of equal surface, the plane B would more greatly resist lateral motion, if attended by a simultaneous forward motion, than would the plane B ; and if the ends of the plane had been reversed so that the deep end came forward, similar results would accrue, but as there would then be such an accumulation of pressure about the anterior edge, it would be almost impossible to give a vessel with such a form a satisfactory sail spread. With the sloping edge turned forward, a quantity of what may be termed perfect pressure, is graduated aft ; this feature, coupled with the fact that the centre of gravity of the figure is relatively aft, instead of relatively forward, as it would be with the ends reversed, admits of a convenient and satisfactory arrangement of sail.