WATER AND AIR RESISTANCE
It takes very little power to move a boat slowly because the resistance that has to be overcome is merely the parting of the water at the bow and closing in of the water at the stern and the skin friction along the sides of the hull. In addition to this there is a similar resistance offered by the air. At very low speeds the resistances of the water and the air are practically negligible. In perfectly quiet water with no air stirring the pull of a cord will move a ship weighing hundreds of tons, but the motion will be very slow indeed. Unfortunately the speed of a ship does not increase directly in proportion to the power that drives it. Doubling the power does not double the speed. If it takes ten horsepower to drive a vessel at a speed of ten knots it will take not 2 but 2³ or 8 times as much power to drive it at a speed of twenty knots. In other words, the horsepower goes up as the cube of the speed. This is an average condition for ordinary speeds. For very high speeds the horsepower may have to be increased as the 4th and even the 5th power of the speed. The shape of the bow and the stern is of utmost importance. The parting and displacement of the water at the bow and the replacement at the stern produce waves and the forming of these waves represents so much wasted energy. The swell that is kicked up by a steamer is evidence of power uselessly expended. Much of this loss can be overcome by careful design of the ship’s lines. A vessel that kicks up a high bow wave—one that sails with a “bone in its teeth”—may present a very pleasing spectacle and may seem to be traveling at high speed, but the best designed vessel—the one that slips through the water with no fuss—is much more economical of power. It is easy to understand that the bow must be carefully designed to cut through the water, but it is not so apparent that the stern must also be shaped to permit the water to flow in readily and fill in the void behind the ship. If the stern is not carefully shaped, there will be a serious drag on the vessel. The skin friction of the vessel is greatly increased by fouling of the hull with marine growths. At high speeds the wind pressure on the superstructure is considerable. Every spar and line adds its quota. A boat that is traveling in still air at a speed of twenty-two knots or twenty-five miles per hour is encountering the equivalent of a twenty-five mile wind which will exert a pressure of over three pounds per square foot of frontage.