RESISTANCE AND SPEED AND THE INFLUENCE OF THAMES MEASUREMENT It has been assumed, in the last chapter, that the qualities of the two yachts as to form, upon which their resistance to motion depended, were equal, although one by reason of inferior stability might have much greater propulsive force than the other. Of course this equality of speed as dependent on form, need not necessarily exist, and in fact, at very high speeds, the resistance, due to form and other qualities, might vary very considerably ; and consequently the speed might vary independently of relative sail-carrying efficiency. According to recent investigations there are only two principal sources of resistance, and they are consequent upon surface friction and wave-making. " Surface-friction " is due to the adhesion of the water to the, immersed surface of the hull ; and from the experiments made by Mr. Froude for Admiralty, we learn that up to the period of wave-making the whole appreciable resistance is caused by surface friction alone ; thus at low velocities any form, not having an absolutely flat or blunt end, would only have surface friction to contend with. A fairly modelled yacht of, say 50ft. in length, moved at a velocity of five knots an hour, need make scarcely any waves, and the resistance she would encounter would be almost entirely due to the friction of the water on the surface of her copper. From the foregoing it can be gathered that, so far as the resistance due to surface friction is concerned, it will be advantageous for a yacht to have a comparatively small area of surface immersed, and for that surface to be of the most uniformly perfect smoothness. The frictional resistance of perfectly clean copper (with no nail heads or other uneven protrusions), upon a vessel 50ft. long, is equal to 0.2461b. per square foot at a speed of six knots per hour ; and the resistance increases uniformly as the 1.88 power of the speed.* But if the vessel be coated with fine sand, the resistance is equal to 0-4051b.
per square foot, and increases as the 2.06 power of the speed. For medium sand, such as may be met with on the shore, the resistance is as much as 0.4881b per square foot, and the resistance increases as the power of the speed. The advantage of having a perfectly clean and smooth surface is therefore paramount, and the advantage will be most marked when sailing in light winds, which of course means low velocities.
The form of vessel which yields the least surface for friction for any fixed displacement is one of round full lines, with the dead wood for ward and aft reduced as much as practicable ; but very full lines are incompatible with high speeds, as the wave-making resistance assumes enormous proportions in bluff bowed vessels. That form which most nearly accords with the wave-line theory is the one which can be made to move at the highest velocities without any very sensible wave-making; * and it should be always remembered that directly wave-making com mences, the resistance no longer increases at about the square of the speed, but very rapidly ascends, and may reach even as high as the 6th power of the speed.
For the sake of great internal accommodation, it is considered an advantage to have a considerable parallel length of middle body, or, as shipwrights call it, " straight of breadth." Mr. Froude has recently made some experiments in this direction with a view of determining how far a perfectly straight piece of middle inserted between two good ends affects resistance. The results of his experiments were communicated to the Institution of Naval Architects in March, 1877, and are most interesting. So long as a moderate speed, not exceeding 12 knots, with a ship 160ft. long, was maintained, the insertion of 40ft. of middle body affected the speed in almost a direct ratio to the increased surface; but at higher speeds the resistance increased in a greater ratio, and presented some peculiarities.