Screw (marine propulsion)

“ 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. ”

“ 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 ”

“ 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. ”

“ He could secure all the capital he needed for the well-tried sailing ship, but no one would advance money for what seemed to be, then, an experiment with a curious device not yet well tried out. Almost incredible as the statement may seem now, the most influential American ship-owners, during the years before the Civil War, refused to have anything to do with the screw propeller. In an essay on "Screw Propulsion in the United States," read before the American Society of Naval Architects and Marine Engineers, at a meeting held in New York in November, 1909, Mr. ”

“ 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 ”

“ 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. ”

“ The most effective screws have slightly greater pitch at the periphery than at the hub, and an increasing pitch from the forward to the rear part of the screw. The latter method of increasing pitch is more generally adopted alone. The thrust of the screw is the pressure which it exerts in driving the vessel forward. In well-formed vessels, with good screws, about two-thirds of the power applied to the screw is utilized in propulsion, the remainder being wasted in slip and other useless work. Its efficiency is in such a case, therefore, 66 per cent. ”

“ Although the ship A has a much greater resistance than B, the screw of the former is much the smaller, both in the blade area and the disk. A's screws, however, in addition to 22 per cent. more revolutions than B, have a much larger slip, and the blades have rather a fuller form at the tips. Compared with the practice in the mercantile marine, the revolutions of these screws are very high, and from the foregoing remarks it may appear that much larger screws would be required for a merchant ship than for a war ship of the same displacement and speed. ”

“ 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 ”

“ But there does not seem any reason why steam-power should be less economically convertible to the purpose of propulsion by an internal screw-pump out of the way of risk, than by an external screw-pump, exposed to risk, not only from the enemy’s shot, but from its weak attachment to the vessel and its exposure to fouling any floating substance. ”

“ The shaft must be guided vertically and horizontally in bearings at certain intervals to prevent it from breaking, and where it issues from the vessel, the leverage of the screw against the water causes it to tremble and vibrate enormously, just where the vessel is weakest; and in wooden vessels converted to screw propulsion the difficulty and cost of repairs are very great. It is found that metal linings for the screw shaft are quite inefficient, and surfaces of wood are required. If any lever movement obtains in the screw shaft, the destruction of the bearings is very rapid. ”

“ At first all screws were so constructed, that they could be lifted up through a well when sails alone were being used, so that it would not impede the ship. The funnels, too, being made to shut up like a telescope, a steamer could thus be easily turned into a sailing ship.
At the very time that the screw propeller was initiating a revolution in the method of steam propulsion, another revolution was taking place in shipbuilding material. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ We have only arrived as yet at the infancy of the process, but we have made a good beginning, for, having once proved that a machine capable of raising itself in the air, wholly unaided from without, can be made, we have overcome with this apparently small result the whole difficulty. The principle of propulsion by means of a screw is by no means a novelty. It was first utilised in windmills, whose sails are nothing more nor less than an immense screw which is turned by the action of the wind on its surface. ”

“ When direct acting engines, however, are employed, the machinery will occupy a much less space in screw vessels than is possible in paddle vessels, and the use of direct acting engines in screw propellers is necessary, therefore, for the realization of the full measure of advantage, which screw propulsion is able to afford. ”

“ A new material of construction has been introduced into shipbuilding, with entirely new methods of propulsion. Old things have been displaced by new; and the magnitude of the results has been extraordinary. The most important changes have been in the use of iron and steel instead of wood, and in the employment of the steam-engine in impelling ships by the paddle or the screw. ”

“ The first step towards the revolution was taken many years ago when the screw propeller was substituted for the paddle-wheel. The [Pg 125] latter means of propulsion caused shock and vibration not only owing to the thrusts of the piston-rod from the steam-engine itself, but also from the impact of the paddles upon the water one after the other. A great increase in the smoothness of running was attained when the screw was invented—a propeller which was entirely sunk in the water and therefore exercised its force, not in shocks, but in gentle constant pressure upon the fluid around it. ”

“ Another way of employing steam power for propulsion was needed. The propeller (called a “screw”) was the answer, allowing naval steam to come into its own. Construction of the prototype screw sloop Princeton began in 1841, before America’s first sidewheel warships even went into service. The Navy built only eight more deepwater sidewheelers before the famous 1854 class of six screw steamers (led by the Charlestown-built Merrimack) made the cumbersome vessels a footnote in naval history. ”

“ Sometimes the idea is born in one generation, followed out in the next, and perhaps perfected in the third. In an age of progress, one invention merely paves the way for another. What was the wonder of yesterday, becomes the common and unnoticed thing of to-day.
The first idea of the screw was thrown out by James Watt more than a century ago. Matthew Boulton, of Birmingham, had proposed to move canal boats by means of the steam-engine ”

“ The advantages of the double-screw over the single are so many and so manifest as to leave no question as to their superiority. The disabling of the shaft or screw of the single-screw steamer, or the derangement of her rudder renders the vessel helpless. Not so the twin-screw ship; for on such ships the screws can be used for steering as well as propelling. And it has happened many times that twin-screw ships have crossed the ocean with the steering gear disabled, or with one screw entirely out of commission. ”

“ That's all."
"That's all, d'you call it?" said the thrust-block, whose business it is to take the push of the screw; for if a screw had nothing to hold it back it would crawl right into the engine room. (It is the holding back of the screwing action that gives the drive to a ship.) "I know I do my work deep down and out of sight, but I warn you I expect justice. All I ask is justice. Why can't you push steadily and evenly, instead of whizzing like a whirligig and making me hot under all my collars?" ”

“ The screw-propeller, in fact, has assumed so important a part in all naval enterprise, that it may not be without interest to trace briefly its rise and progress to the consideration it now commands, and to review, in general terms, the various experiments by which the screw-frigate has been brought to its present high state of efficiency, excelling, for purposes of war, all other kinds of vessels. ”

“ A series of observations upon screw propulsion, made in the course of experimental voyages in the Archimedes, convinced him of the practicability of applying the system to large steamships. In 1841 Brunel was commissioned by the admiralty to conduct experiments which led to the adoption of the screw propeller in the navy in 1845. The Great Britain, an iron ship of dimensions far exceeding those of any vessel of the period, first designed by him for paddles, was the first large vessel in which the screw propeller was used. ”

“ These are all twin screw ships, and each of the twins is driven by its own set of engines and line of shafting, so that the propelling machinery of each ship is duplicated throughout. The speeds attained indicate a high efficiency with the twin screws. In all ships, but more especially in high speed ships, success depends largely upon the provision of propellers suited for the work they have to perform, and where a high propulsive efficiency has been secured, there is no doubt the screws are working with a high efficiency. ”

“ Changes in Ship Construction after 1840—Reluctance to recognise Advantages of Screw Propulsion—Gradual Conversion of Sailing Fleet to Steam—Armament practically remains unaltered—The Crimean War—Operations in Black Sea and Baltic—Assistance rendered by the Navy—Expedition to Sea of Azof.
The discovery that steam could be profitably utilised for the propulsion of ships, and the tardy adoption of the screw, did not for many years materially affect the construction of war vessels. ”

“ He is said to have employed the screw in launching a large ship, pushing it into the water as is now done by hydraulic appliances. By changing his fulcrum and making the screw a part of the ship, the modern engineer has only reversed the mode of applying propelling power; the principle is the same. The effect produced by the screw in propelling a ship will be best understood by supposing an ordinary screw of large dimensions to be revolving rapidly in a trough full of water. ”