Great Facts / A Popular History and Description of the Most Remarkable Inventions During the Present Century

by Frederick C. Bakewell

The steam-boats employed in this country were, almost from the first, and continue with few exceptions to be, on the low-pressure condensing principle; the whole of the machinery being placed below the deck. This renders it necessary to diminish the height of the engines as much as possible; and in all marine steam engines, till within the last twenty years, instead of having a working beam over the cylinders, a cross-head was placed at the top of the piston-rod, the action of which was conveyed by parallel motions to cross beams on each side, which were situated at the bottom part of each engine. The motion, compared with that of an ordinary land engine, was thus inverted. The proportions of the cylinders were also different; the length of stroke being shorter, to diminish the height, and the diameter consequently greater. The valves, and the gearing connected with them, the air pump, the condenser, and other subsidiary parts, do not differ essentially from those of land engines; but the governor is omitted, as it is found impracticable to work a marine engine with great regularity.

Latterly, many engineers have introduced, with much success, arrangements for communicating the action directly from the piston-rod to the crank, without the intervention of the beam and parallel motions. This is generally done by causing the piston-rod to work between guides, and a jointed arm connects it with the crank. One method of producing the same effect is to make the cylinders oscillate on pivots, as contrived by Mr. Murdoch, in the first model steam carriage, made in 1784. This principle has been successfully carried into operation by Messrs. Penn, of Greenwich. The oscillating cylinders accommodate themselves to the varying directions of the cranks, and the strain occasioned by guide rods is diminished; but when very large cylinders are required, the friction and the pressure on the pivots must tend to counterbalance the advantage otherwise obtained.

In the ordinary paddle-wheel steam-boats, the floats of the paddle-wheels are fixed at equal distances round the rim, radiating from the centre; therefore they enter and come out of the water obliquely. There is, consequently, a considerable loss of power attending the use of such paddle-wheels, as only one float at a time can be acting vertically on the water, and exerting the propelling force in a direct line. Several attempts have been made to remedy this defect, and to produce what is called "feathering" floats, every one of which will act against the water at right angles. The mechanism required for making this adjustment is, however, liable to get out of order, and the introduction of vertically acting floats has consequently been very limited.

The large projecting paddle-boxes are objectionable in sea-going ships, as they present so large a surface to the action of the wind, and either impede the course of the ship, or make it unweatherly. This inconvenience was experienced in the early progress of Steam Navigation, and many attempts were made to overcome it, by substituting a different kind of propeller. Recourse was had to the inventions of the ancients, from whom the paddle-wheel was taken, to find some other means of propulsion. A method of propulsion, similar in principle to the action of sculls at the back of a boat, had been contrived long before the inconvenience of paddle-wheels in Steam Navigation was experienced. In 1784, Mr. Bramah obtained a patent for a propeller similar in its forms to the vanes of a windmill, which by acting obliquely on the water as it revolved, pushed the boat forward. Ten years afterwards, an "aquatic propeller" was patented by Mr. William Lyttleton, a merchant in London. It consisted of a single convolution of a three-threaded screw, and may be considered to be the first screw propeller invented. Numerous other ingenious persons, among whom were Tredgold, Trevethick, Maceroni, and Millington, afterwards invented propellers on the screw principle; but none of them were sufficiently satisfactory in their results to come into practical use.

In 1836, Mr. Smith and Mr. Ericsson obtained a patent for a screw propeller, which nearly resembled Mr. Lyttleton's original contrivance; and by perseverance in struggling against the many obstacles with which he had to contend, Mr. Smith succeeded, though all previous efforts had failed. His partner, however, became disheartened by the obstacles thrown in their way, and left this country for America before the success of the screw was established.

The first ship fitted with the screw propeller was called the "Archimedes." It was a vessel of 237 tons burthen, with a draught of water of 9 feet 4 inches. The screw projected at the stern, and being turned rapidly round by the steam engine, the oblique action of the thread of the screw against the water impelled the vessel forward.

The "Archimedes" was originally fitted with a single-threaded screw, the threads of which were 8 feet apart, and there were two convolutions of the screw round the shaft. One convolution of the screw having been accidentally broken off, the ship was found to go faster in consequence; and, following the course of investigation suggested by the accident, Mr. Smith at last adopted a double-threaded screw, with only half a convolution. The average performance of the engines was 26 strokes per minute, and the number of revolutions of the screw in the same time was 138½. The "pitch" of the screw was 8 feet; that is, the space across one entire convolution of the thread would have measured 8 feet; consequently, had it been acting against a solid body, as a cork-screw when entering a cork, one revolution of the shaft would have advanced the vessel 8 feet, and the speed would have been 12½ miles an hour; but the utmost speed the "Archimedes" obtained was 9¼ nautical miles. The difference was owing to the screw "slipping" in the water, because the fluid yielded to the oblique action of the blades.

The results of the working of that experimental ship were so satisfactory, that other ships were soon built, with modifications of the form of the propeller. It was found disadvantageous to have an entire convolution of the thread of the screw; for one part of it worked in the wake of the other, and resistance was produced by the backwater. After numerous experiments, in which the dimensions of the screw were successively diminished, the propeller was at length reduced to two oblique blades. Experiments on a large scale were conducted by Captain Carpenter, to determine the size and angle of inclination best adapted for the purpose of propulsion; and nearly all the ships now built for the Royal Navy are fitted with propellers on his principle. The annexed diagram represents on a scale of one-eighth of an inch to a foot, the form of the propeller of the "Agamemnon," of 606-horse power, which was recently engaged in successfully laying down the Atlantic Telegraph cable. The diameter of the screw is 18 feet, and the pitch 20 feet.

The screw propeller possesses great advantages in ships of war, as it is not exposed to damage by shot, and it leaves the entire deck clear for mounting guns. It has also the further advantage of not interfering with the working of sails, and is, therefore, admirably adapted for sea-going ships that economize fuel by alternately steaming and sailing, as the wind is adverse or favourable. The commotion in the water made by paddle-wheels, which is an objection to their use in narrow rivers, is avoided by screw propellers, which being immersed under the water, make little agitation on the surface, and the ships move along without any apparent impelling power.