The primary condition for high speed is fineness of form, so that the water at the bow of the vessel may be separated and thrown to one side, and brought to rest again at the stern and behind the vessel with the least possible disturbance, and the measure of efficiency of form for the maximum speed intended is inversely as the height of the waves of disturbance. A ship that has been designed to attain a speed of 15 knots will, when moving at 12 knots, show a very slight disturbance indeed, and in one designed for 18 knots, when moving at this lower speed, it will be scarcely observable; but however fine the lines of a ship may be, she must at every speed produce some disturbance, although it may be very slight, as the water displaced by her must be raised above the normal level and replaced at the normal level; hence, at or near the bow of a ship there is always the crest of a wave, and at or near the stern the hollow of one. When a vessel is going at its maximum speed, and is properly designed for that speed, the wave should not be very high, nor should it extend beyond the immediate neighborhood of the bow; likewise the wave of replacement should be the same at or near the stern of a ship, and the “wake,” or disturbance of water left behind in the track of the ship, should be narrow. Among naval architects and others it is usual to judge of the forms of ships by the relation they bear to rectangular blocks of the same dimensions; that is to say, a ship whose dimensions are—length, 100 feet; breadth, 20 feet, and draft of water, 10 feet, and whose displacement is 12,000 cubic feet, would be said to have a coefficient of fineness of 0.6, or that her fineness was sixty per cent., inasmuch as that of a rectangular block[10] of the same dimensions would be 20,000 cubic feet. Modern experience has shown that for speeds not exceeding 9 knots, and with ships of the tonnage now common for general ocean work, the bow may be very bluff and the stern only sufficiently fine to allow free access of water to the propeller, so that the coefficient of such vessels is frequently 0.78, whereas that of our fastest warships is only 0.5, and of our large modern passenger steamers 0.55. As already stated, in the ship whose coefficient is 0.78 any increase of power produces very little gain in speed, and if such a ship were fitted with engines and boilers of the same size and developing the same power as those of a 20-knot Atlantic greyhound, the increase in speed would be very insignificant, but the disturbance in its immediate neighborhood would be very great; in fact, if any vessel is driven beyond a speed for which her form is suitable, she produces waves[11] both numerous and high, as may be seen by reference to the illustration of H. B. M. S. Impérieuse being driven at her full speed of 17-1/4 knots when laden much deeper than the designed draft [p. 64].