Screw propulsion

“ 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. ”

“ 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 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. ”

“ Each technology had its partisans: the sidewheel provided greater combat maneuverability, was suited to riverine warfare, and presented no problems of leakage, as did the screw’s underwater shaft hole. However, the exposed wheels were vulnerable during combat, ate up deck space needed for guns, hindered sail handling, and created more drag than a screw when the vessel was under sail. The launching of the screw warships H.M.S. Rattler in Britain and U.S.S. Princeton in the United States in 1843 signaled the coming ascendancy of screw propulsion. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ In the general theory of propulsion we are not concerned with the machinery of propulsion, i.e., the form of propeller—paddle, screw, jet, or other known or unknown mechanism; we merely take account of the fact that forces are exerted between the propelled body and certain parts of the fluid, and investigate the conditions that obtain and the proportion of power that may be utilised and lost. The theory of propulsion on this broad basis is the common foundation of propeller theory generally, and the conditions deduced from the Newtonian principle are essential to every form of propeller. ”

“ Of course, it will be understood that we are not taking into account resistances, and defects proper to the screw, from which hydraulic propulsion is free, nor are we considering certain drawbacks to the efficiency of the hydraulic propeller, from which the screw is exempt; all that we are dealing with is the waste of power in the shape of work done in moving water astern which we do not want to move, but cannot help moving. ”

“ 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 power expended in steamships, whether fast or slow, is prodigious; any marked improvement in the contour of screws will mean either a saving of fuel or an increase of speed. Of equal importance with water-propulsion is the setting in motion of air. In blast furnaces enormous volumes of air are forced at high pressure into the fuel and ore: the fans are carefully molded in screw form, any departure from the best curves entailing serious loss. Fans for less important services are seldom shaped with care and usually waste much energy. ”

“ A type of engine specially devised for airship propulsion is that in which the cylinders are arranged horizontally instead of vertically, the main advantages of this form being the reduction of head resistance and less obstruction to the view of the pilot. A casing, mounted on the top of the engine, supports the air-screw, which is driven through bevel gearing from the upper end of the crankshaft. ”

“ A careful and dispassionate study of the facts, so far as they can be known at the present time, seems to indicate clearly that of those who were concerned in successfully adapting the screw-propeller to the needs of marine propulsion and in laying the foundation for these changed conditions, especially in the United States, none was so prominent as Ericsson, or so fairly deserving of the chief credit ”

“ 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. ”

“ The “Plunger’s” main engines are used for propulsion when she is navigated at the surface of the water. As originally designed they were triple-expansion steam engines, driving triple screws, but have since been altered to gasolene internal-combustion engines driving a single screw. These engines are also used for charging electric accumulators, from which alone motive-power can be obtained when the boat is submerged. The current for charging the accumulators is obtained from a dynamo of 70 H.P., which can always be run in the awash condition to keep the accumulators fully charged. ”

“ On destroyers the screws make 400 revolutions a minute. That is to say, all the momentum of the pistons, cranks, rods, and valves (weighing tons) , has to be arrested thirteen or fourteen times every second. However well the moving parts may be balanced, the vibration is felt from stem to stern of the vessel. Even on luxuriously-appointed liners, with engines running at a far slower speed, the throbbing of the screw (i.e. engines) is only too noticeable and productive of discomfort.
We shall be told, perhaps, that vibration is a necessary consequence of 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 ”

“ 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. ”

“ When the sea is rough, and the screw leaves the water as a consequence of the ship's motions, the rotary velocity of the screw and engine increases to a dangerous degree, because the resistance that the screw was meeting in the water suddenly disappears. When the screw enters the water again, the resistance makes itself abruptly felt, and causes powerful shocks, which put both the screw and engine in danger. Ordinary regulators are powerless to overcome this trouble, since their construction is such that they act upon the engine only when the excess of velocity has already been reached. ”

“ If we should increase the diameter of the screw to 20 feet, and if it had the same pitch and revolved at the same rate, it would require four times as much power and would give four times as much screw thrust, because the area of the disc increases as the square of the diameter. Suppose, now, that we should reduce the pitch of the screw to 3 feet, we should in this case engage four times as much air, and double the screw thrust without using any more power—that is, assuming that the machine is stationary and that the full power of the engine is being used for accelerating the air. ”

“ In Continental machines, very small screw propellers are used. These screws have probably been made small because the experimenters have found that they encounter a good deal of friction in the atmosphere, but this is caused by imperfect shape and the rib of steel at the back of the blades. In order to use a small screw, experimenters have been forced to use a very quick-running engine which makes it necessary to have the cylinders very short, so, in order to get the necessary power, they are obliged to use no less than eight cylinders. ”

“ The efficiency of a propeller of any sort will depend not only upon its area, but also upon its ability to send the air away in parallel streams. If we wish to go forward, the air must be driven aft, and a forced current in any other direction will at best give us back but a fraction of its energy. Ordinary screw-propellers have not proved very effective, for the reason, probably, that revolving at great speed, they send off a large amount of air tangentially.
What, now, should be the mechanical construction of a successful flying-machine? ”

“ 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. ”

“ The experience of modern steam navigation points to the exclusive use and advantage of the screw propeller where great speed of shaft is obtainable, and the electric engine is pre-eminently a high-speed engine, consequently the screw appears to be most suitable to the requirements of electric boats. By simply fixing the propeller to the prolonged motor shaft, we complete the whole system, which, when correctly made, will do its duty in perfect order, with an efficiency approaching theory to a high degree. ”

“ 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?" ”