It is not possible to go below the surface of such a subject in a popular paper, and it must suffice to speak of Mr. Froude’s deductions, in which he divided the resistances met by ships into two principal parts: the surface or skin friction, and the wave-making resistance (which latter has no existence in the case of a totally submerged body—only begins to exist when the body is near the surface, and has its full effect when the body is only partially submerged). He showed that the surface friction constitutes almost the whole resistance at moderate speeds, and a very great percentage at all speeds; that the immersed midship section area which formerly weighed so much in the minds of naval architects was of much less importance than was supposed, and that ships must have a length corresponding in a degree to the length of wave produced by the speed at which they are to be driven.
[Illustration: The Chilian Cruiser Esmeralda.]
He showed that at high speeds waves of two different characters are produced: the one class largest at the bow, which separate from the ship, decreasing in successive undulations without afterward affecting her progress; the other, those in which the wave-crests are at right angles to the ship’s course, and the positions of these crests have a very telling effect upon the resistance.
As the ship’s speed is increased the spaces between the crests of these lengthen in unison with the speed, and it has been shown that when the speed is such that a wave-crest would be at the middle point of the after body (or quarter) the wave-making resistance is least, and that it is greatest when the hollow appears at this point.
A ship must therefore be of a length that depends largely upon the length of wave which at a high speed she will tend to produce in order that she may be driven at such a speed without an expenditure of power disproportionate to the effect produced. This length, if very high speeds are desired, is best wholly taken up in fining the entrance and run, leaving no parallelism of middle body, and broadening and deepening the ship to keep the necessary displacement. The wave-action at several speeds is well shown in the illustrations, which are from instantaneous photographs, showing the Chilian cruiser Esmeralda at her full speed of 18 knots, when on her trial off Newcastle-upon-Tyne, the Giovanni Bausan, of the Italian navy (almost a sister ship to the Esmeralda), at a moderate speed, and H.M.S. Impérieuse, at about 17-1/4 knots. [See illustration, p. 64.] The following are the principal details of the Esmeralda and Impérieuse: