Screw propeller

“ The speed of ships with the best constructed screw propellers is fully equal to that of paddle-wheel vessels; and when two vessels of the same size, and with engines of equal power, one fitted with paddles, and the other with the screw, are fastened stem and stern together, in a trial of strength, the screw propeller has been found to have the advantage, and to pull its antagonist along at the rate of one or two miles an hour. ”

“ At the same time it must be admitted that the screw-propeller as a possibility for marine propulsion was known in a vague way to the engineering practice of the day, and it is at this time of course quite impossible to say how much may have been known by Ericsson, Smith, or others concerned in later developments, or to what extent they may have been dependent for suggestion on what had preceded them. The question of who invented the screw-propeller in the absolute sense is entirely futile and without answer. No one could ever have reasonably advanced any such unique claim. ”

“ Upon consulting an English experimenter, who had made a “life-long study” of the question, he assured me that I should find the screw propeller very inefficient and very wasteful of power, and that all screw propellers had a powerful fan-blower action, drawing in air at the centre and discharging it with great force at the periphery. ”

“ The first practical use of the screw was made by John Stevens, who in 1804 launched a steamboat eighteen feet long by fourteen feet beam with a direct acting high-pressure engine having a tubular boiler—and driving a screw with four blades. Although the principle of the screw for ship propulsion was thus recognised at this early period it was not till the thirties (of the nineteenth century) that the screw-propeller succeeded in attracting the attention of the engineering world. ”

“ But the achievements of screw steamers are not always satisfactory at first, and time has shown some curious instances where what appeared at first sight a little thing prevented great results. To-day we know somewhat of the screw propeller, but it is very difficult, if not impossible, for the cleverest and most experienced engineer to define his knowledge or to classify his facts so as to deduce any rules from them that shall enable him to lay down fixed laws for the practical guidance of others. ”

“ While the last few years have seen great advances made in the designs of steamships and of their engines, little or nothing has been done in the way of improving the screw propeller. As a general rule it would appear to be taken for granted that no radical improvement could be made in the form of the propeller, although various metals have been introduced in its manufacture with the view of increasing its efficiency. For sea-going steamers, however, the shape remains the same, the variation chiefly relating to the number of blades employed. ”

“ Paddle, it was suggested that the screw-propeller might probably be used as an auxiliary power, for occasional use during calms and contrary winds; the vessel to be a sailing-vessel under ordinary circumstances; but to have a marine engine and a screw for exigencies at times when the ship would be brought to a stand-still or even driven backwards. About seven years ago, an American packet-ship, the Massachusetts, a complete sailing-vessel in other respects, was provided with a screw and a steam-engine powerful enough to keep the ship moving when winds and tides were adverse ”

“ If the screw penetrates wood or metal nut it will advance a certain distance known as the pitch which is always the same, namely, the distance separating two consecutive turns of the threads. The revolving blades of the propeller cut their way through the air in identically the same manner. But since air is a very thin medium as compared to wood or iron the propeller slips a little just like a screw going into an unsteady nut and does not advance the distance it theoretically should considering the angle of the blades. The distance lost in each revolution is called the slip. ”

“ Every designer is seeking for the same result—the securing of the greatest possible thrust, or air displacement, with the least possible energy.
The angles of any screw propeller blade having a uniform or true pitch change gradually for every increased diameter. In order to give a reasonably clear explanation, it will be well to review in a primary way some of the definitions or terms used in connection with and applied to screw propellers. ”

“ Sporadic attempts were made in the early days of the nineteenth century both in this country and in America to drive ships by means of screws, both manually and by the medium of steam, some of which were attended with a certain measure of success.153 Yet some time was to elapse before screw propulsion gained recognition. Doubt as to the efficiency of a screw’s action, ignorance as to the shape of the vessel required and as to the best position for the propeller, difficulty in accommodating the early long-stroke steam engine to drive direct an under-water propeller shaft ”

“ He further contended that the proportion of stroke to diameter should be greater in an engine that is to drive a screw propeller direct than what is required for applying the same power to a paddle-wheel, and it would soon be found that as an instrument of propulsion, even for great speed, the screw would not be inferior to the most approved patent paddle-wheel. ”

“ A screw propeller operates on exactly the same principle as an electric fan, and ships may have one or more of these [Pg 103] propellers, which are fastened to shafts projecting through the hull beneath the water at the stern. If the ship were tied up strongly to a pier, so that it could not move, and the propellers were turned by the engines, the result would be to set in motion a column of water away from the propeller just as an electric fan sets in motion a column of air. ”

“ Turning to the means of propulsion, we find sails as the main, or almost only, reliance during the early years of the century. The steam-engine operating paddle-wheels had come to be recognized as a possibility, and under certain conditions as a commercial success. The screw-propeller as a means of propulsion was known only as a freak idea, and was without status or recognition as a commercial or practical means for propelling ships. ”

“ Although the Lords of the Admiralty would not further encourage the screw propeller of Ericsson, an officer of the United States Navy, Capt. R. F. Stockton, was so satisfied of its success, that after making a single trip in the experimental steamboat from London Bridge to Greenwich, he ordered the inventor to build for him forthwith two iron boats for the United States, with steam machinery and a propeller on the same plan. ”

“ Nevertheless, no modern ship of the British navy is without the means of raising her propeller, and the best opinion of commanders and engineers of that service, of longest experience in screw-ships, goes to establish the conviction, that, for men-of-war, the advantages of being able to lift the propeller far more than outweigh the objections urged against lifting. In this connection we mention the fact, that all screw-ships "by the wind" have a strong tendency to gripe. ”

“ However, after the construction of the Great Britain had been commenced, the steamer Archimedes, fitted with Smith’s screw propeller, arrived at Bristol during her tour of the ports and demonstrated once and for all that the screw propeller could be used in seagoing vessels, and that, provided engines of sufficient power were installed, the screw propeller was more suitable for large hulls built to make ocean voyages than the best paddle-wheels then designed. ”

“ Twist the ends of this fly askew like the fans of a screw propeller, so that it will catch the wind in revolving. Now fix a block of wood to the bottom of the case and fix the fly to it by a small brass screw passing through its centre, so that it works freely and is 31⁄2 in. from the centre of the driving pulley R and level with it. Fasten a block of wood to the back of the case, in which you must fix the screw N (Fig. 5) for the lever O to pivot on. You must next make the starting gear. ”

“ In order for a model to fly really well the propeller must usually be out of all proportion to the rest of the machine. In fact its size will make the machine appear very awkward and unsightly.
FIG. 38. Quasi-helical propeller. The enormous slip of small screw propellers when turning rapidly makes them very inefficient. The thrust of the propeller is dependent upon the volume of air sent backwards. A large propeller naturally deflects more air than a small one and so in order for the latter to equal the work of a large propeller it must either have an increased pitch or revolve more rapidly. ”

“ It is never less than ten or twelve per cent in marine work, and with aerial screws is much greater. Within certain limits, the less the slip, the greater the efficiency of the propeller. Small screws have relatively greater slips and less efficiency, but are lighter. The maximum efficiency of a screw propeller in water is under 80%. According to Langley’s experiments, the usual efficiency in air is only about 50%. This means that only half the power of the motor will be actually available for producing forward movement—a conclusion already foreshadowed. ”

“ While engineers, however, are prepared to admit its advantages so far as greater security from total breakdown is concerned, there is by no means thorough agreement as to whether single or twin screws have the greater propulsive efficiency. What is required to form a sound judgment upon the whole question is a series of examples of twin and single screw vessels, each of which is known to be fitted with the most suitable propeller for the type of vessel and speed ”

“ It was revived again in 1785, by Joseph Bramah, a wonderful projector and inventor. [5] He took out a patent, which included a rotatory steam-engine, and a mode of propelling vessels by means either of a paddle-wheel or a "screw propeller." This propeller was "similar to the fly of a smoke-jack"; but there is no account of Bramah having practically tried this method of propulsion.
Austria, also, claims the honour of the invention of the screw steamer. At Trieste and Vienna are statues erected to Joseph Ressel, on whose behalf his countrymen lay claim to the invention ”

“ He did not attach much importance to the particular form of the blade either in single or twin screws, as he believed so long as the disk area, the surface, and pitch were properly adjusted to the speed of the vessel, and to enable the engines to use, at the maximum speed, just the full quantity of steam that the boilers can make, we have got pretty nearly as far as we can get. To fix these dimensions of the propeller accurately at the present time, and without further knowledge of the action of the screw on the water, was, he thought, impossible. All the rules and formulæ are empirical. ”

“ Jet propulsion had been suggested by Benjamin Franklin in 1775 and was tried several times with some success. But the greater success of the screw-propeller, perfected by Colonel John Stevens and Captain John Ericsson, soon caused jet-propulsion to be abandoned. The screw-propeller made possible—and was quickly followed by—great improvements in engines; the gearing used with paddles was soon given up for direct-acting engines—compound about 1854, triple-expansion in 1874. ”

“ On the Marine Propeller.—The marine propeller, which has already been made the subject of comparison, is of especial interest as representing in a concentrated form the experience of over half a century. Beyond the advantages to be gained from an examination of established practice as a quantitative check on our investigation, there is some reasonable probability that we may arrive at facts having a bearing on the future evolution of screw propeller design. ”

“ Another step in the right direction was the suggestion of paddle wheels, projecting from each side of the car, and beating the air as they revolved. This was coming very close to the correct solution, that of a revolving propeller.
But unfortunately at this early date the mechanical sciences were in their infancy, and although soon afterward the idea of a screw propeller did come up, the inventors were handicapped by the fact they knew of no other power than “hand-power” with which to drive it. ”

“ In all long sea-going voyages, also, the screw is now the favourite mode of propulsion. Screw ships of prodigious size are now built and launched in all the ship-building ports of Britain, and are sent out to navigate in every part of the world.
The introduction of iron as the material for shipbuilding has immensely advanced the interests of steam navigation, as it enables the builders to construct vessels of great size with the finest lines, so as to attain the highest rates of speed. ”

“ The intervening years have witnessed the transition from wooden ships to iron, and from iron to steel; from the paddle-wheel to the single screw-propeller, and then to the twin-screw; from the simple side-lever engines to the compound, and from the compound to the triple and quadruple expansion engines of the present time. These successive changes, common to all the other important lines of ocean steamers, have resulted in greatly increased speed with economy of fuel. ”