There is yet another strong argument in favour of a raking keel, which involves a question of speed. If the triangle v were taken away from A, the surface would be reduced one-fourth ; and, consequently, the resistance to forward motion, dependent on surface friction, would be proportionately reduced. It might not be prudent to make this reduction of surface if the effectiveness of the lateral resistance were going to be thereby reduced ; but the fact is that the effectiveness of the lateral resistance would be almost unimpaired ; a comparatively useless piece of wood would be removed, and a positive gain would ensue in the matter of frictional resistance.
The necessity of keeping the centre of lateral resistance, relatively to the length of the vessel, far aft, and the fashion of much raking the stern post, led builders to by degrees advantageously increase the rake of keel, and there have been some extraordinary examples of disproportionate draught of water fore and aft ;* but at the present time, with perhaps a more intimate knowledge of the theory of lateral resistance, most designers prefer the " rockered " keel to the keel that simply rakes upward in a straight line from the sternpost, as with the " rocker " other important advantages can be secured. By having the greatest draught, or what might be termed the termination of the effective surface for lateral resistance, amidships, the after part or heel of the keel can be very much rounded up—somewhat in the fashion of the fore foot; and, as this is done the necessity for a great rake to the sternpost is removed, and the sternpost is fixed in a position nearly in accordance with the vertical. The effect is, that for the same tonnage—it is assumed that the stern post on deck is kept at the same distance from the stem piece—a longer body can be obtained, and presumably a more capable, a larger, and faster vessel.t Beyond this, a deeper middle body is practicable, which is found to be of the utmost advantage, as it admits of the weights being stowed lower without being much distributed in a fore-and-aft direction.
The effectiveness of triangular centre-boards is well known, and it is astonishing how small a piece of board will check lee way, providing the board is deep and not long. Nevertheless, one supposed disadvantage— especially in small boats—of a keel very much rockered, or of a triangular centre-plate instead of a keel, is that in very disturbed water a vessel's head, in beating to windward, gets " knocked of the wind; " but it is overlooked that if the bow is readily knocked of the wind the same facility exists for " coming-to " the wind during favourable puffs.
A vessel with a much rockered keel will probably steer wildly off the wind, and will require watching, and on any point of sailing she is likely to run off her helm. To meet these drawbacks, some eastern boats (such as those of Bombay) have cambered keels, i.e., the reverse of rockered, as the back of the arch is turned upwards ; and a few boats in America and this country have been fitted with double boards. However, we think that the balance of advantages for close-hauled sailing are overwhelmingly in favour of the rockered keel for yachts or the centre plate for small boats. If, as before said, the vessel is quick in falling off, she will be equally sensitive in coming to ; and a careful helmsman will take his " rocker" farther up to windward than any similarly careful helmsman could a " straight keel," all other things being equal ; and further, the helmsman will find the vessel with a rockered keel, when sailing by the wind, a much pleasanter one to steer ; she will readily answer her weather helm for a foul puff, or spring to quickly under a little lee helm for a free one. It is undeniable, however, that in sailing very much off the wind the craft with " drag," or a large area of dead wood aft, will require much less helm than one whose heel is rounded up—that is, she will have less tendency to yaw.