H. B. M. S. Polyphemus at Full Speed—185⁄8 knots.

[Larger image] (148 kB)

In order to produce motion at all, the inertia of the ship, or that quality which every concrete body possesses of remaining at rest until disturbed, has to be overcome, and when the ship is in motion through the water there is resistance of a two-fold kind—that due to the disturbance of the water, and that due to the frictional resistance of the immersed surface. If a thin sheet of metal is moved edgewise through water it offers a decided resistance, even if its surface be smooth and bright; it will also be noted that this resistance increases very rapidly as the speed is increased, and that the larger the area the greater is the resistance. If this sheet of metal is moved in a direction at right angles to its surface the resistance is of course great: in fact, it is very great compared with that of the previous experiment, and the disturbance of the water is considerable. If a log of timber is to be towed from one place to another, it is a common observation that an experienced boatman causes it to move with its big end first, because he finds it easier work that way than with the smaller end first; in the latter case he has the same section of timber offering resistance to the log’s passage, but owing to its wedge-like form the pressure on its long sides is greater than when towed the other way, and the friction of the water past these sides—which are generally more or less rough—causes very great resistance; no doubt, for the same reason, those forms of ships adopted for centuries by some European nations, and known to mariners as “cod’s-head and mackerel-tail” shape, were such good sailers; and if to-day we were content with the maximum speed attained by such vessels, it is possible we might copy their form with advantage. If, however, we attempted to move them, either by sail or mechanical power, at a higher rate, we should find the increase in speed to be of no account, but the increase in wave disturbance would be great; in other words, the greater portion of the additional power would be used up in producing this water disturbance, or waves, instead of propelling the ship.

When the propeller of a steamer is first set in motion it does little else than project a stream of water in the direction opposite to that in which it is desired to move the vessel; it is presently seen that the latter begins to move, indicating that the inertia of the ship has been overcome by the reaction of that stream of water from the propeller; the propeller still continues to project the stream, the ship in the meanwhile increasing in speed, or, as sailors term it, “gathering way,” showing that the power expended is still in excess of the resistance of the ship, inasmuch as something is producing an augmentation of speed; it is afterward noticed that the ship continues to move at a uniform rate, and that the stream of water is still projected by the propeller, but at a lower velocity compared with the surrounding still water than was the case when the vessel was at rest. This means that the power and the resistance are evenly balanced, and that the work done by the ship in moving forward is exactly equal to that of the water moving in the opposite direction through the surrounding water. The vessel has now stored up in herself what is called energy, which is the power developed in overcoming the inertia, so that if the engine stops she still progresses forward and does not come to a standstill until the whole of that stored-up power is expended. If the vessel is a large and heavy one, its speed will be, when under way, virtually uniform, in spite of casual changes of resistance due to wind and waves; and this is one of the reasons for large ships being a necessity for successful passages on stations like the North Atlantic, and it is likewise one of the reasons why light craft like torpedo-boats show such a poor performance in stormy weather.

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 171⁄4 knots when laden much deeper than the designed draft [[p. 64]].

As before mentioned, when speaking of the experiment with a thin sheet of metal, the resistance to passage through the water increases very rapidly with the increase of speed, and careful observation has shown that such increase is proportionate to the square of the speed, so that an immersed body has four times the resistance when moving at twice the speed, and since it will travel double the distance in the same time the power required is eight times as great; that is, the power needed to propel a ship varies as the cube of the speed. It was also discovered that the power varied with the cube root of the square of the displacement; although more correct modern experiment has shown that this variation is not strictly true, it is sufficient for the purpose of this article to assume that it is so.