FROM OARS TO PROPELLERS
Although a century has elapsed since the first steam-driven vessel made its way across the Atlantic Ocean, sails have been as yet by no means swept off the face of the sea. Nevertheless, even when sailing vessels had no competitors they did not furnish a perfectly satisfactory means of transportation. The fickleness of wind power was felt in this application as well as in that of windmills, and inventors racked their brains for some more certain means of propelling ships. Naturally, when the steam engine was a proven success, efforts were made to apply this newly discovered power to ships. How to make steam drive a ship was a problem. At first it was proposed to use a system of oars which would be moved back and forth in imitation of oarsmen and John Fitch’s first steamboat in 1786 was driven by a set of paddles operated in a manner similar to that of paddling a canoe. It was a very natural evolution from oars to paddle wheel, which consists of a series of oars mounted in a wheel so that they will come into play one after the other. The propeller, although not invented by Col. John Stevens (as has been popularly supposed), was first applied by him to steam navigation when he constructed a small steamboat on the Hudson River in 1804. But the simplicity of the paddle wheel and its high efficiency, particularly in quiet harbors and shallow inland waters, gave it preference over the propeller. In rough seas, however, the paddle wheel was far from ideal. It was too easily broken by heavy waves and between 1855 and 1865 the propeller displaced it completely for ocean-going vessels.
WATER JET PROPULSION
Another curious form of propulsion, which dates back to the eighteenth century and is still periodically revived by inventors, is the water jet. The idea was to have the engine operate a pump which would drive a stream of water out of the stern of the boat and drive the boat by reaction. The British Government actually built two jet-propelled steamers. One of them, called the Waterwitch, was a 1,100-ton vessel and the other, the Squirt, was a small torpedo boat. The latter attained a speed of but twelve knots while a sister ship of the same steam power driven by a propeller attained a speed of seventeen knots. The Waterwitch was even less efficient. Some years ago experimental water-jet vessels were built in New York in which a jet only ⅝ inch in diameter with a pressure of 2,500 pounds per square inch was used, but the experiment proved a failure. The propelling force of a jet is the reaction of the stream of water against the orifice from which it issues. The action is just like Hero’s reaction steam turbine referred to on page 143. The propulsion would be the same were the jet discharged in the open air or in a vacuum or against a solid stone wall.
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.
FLYING ON WATER
Instead of cutting through the water modern speed boats are designed to ride over it. The boats have flat bottoms which are arranged as a series of flat planes known as hydroplanes. These planes form steps and are set at such an angle as to make the boat rise up on the water in the same way that a kite rises in the air. The higher the speed the higher the boat rises so that at full speed it skims on the surface. Hence there is comparatively little power wasted in displacing the water. Some of these boats are driven by air propellers so that water resistance to the propeller gear is avoided. These hydroplanes (they must not be confused with hydroaeroplanes) almost fly over the water.
FIRST OCEAN STEAMER
It was in 1807 that Robert Fulton built the Clermont and established steam navigation by running a regular service between New York and Albany, and it was twelve years later that the Atlantic Ocean was first crossed by a steam-driven vessel. It was an American vessel, the Savannah, that made the trip. She was a 380-ton ship equipped with steam power to help her along when the wind failed. Seventy-five tons of coal and twenty-five cords of kindling wood were taken aboard to feed her furnace. This was thought to be ample for the voyage, but before the trip was completed the fuel was all gone. The log of the Savannah bears this entry the night before sighting the Irish coast: “2 A. M. Calm. No cole to get up steam.” However the captain did raise steam just before reaching Kinsale, Ireland, by burning wood. Watchers ashore beholding the smoke issuing from her stack were convinced that the vessel was afire and boats were dispatched to the rescue. The Savannah made the trip from Savannah to Kinsale in 23 days and was under steam propulsion for only 80 hours of this time.