all consideration of the enormous momentum of the reciprocating parts of the engine in that short-stroke high-speed engine which is the modern screw engine. I would just ask the Members to tell me if they see their way out of it? I observe that on the first half of the stroke the indicator diagram tells you that this steam is giving out an enormous power, not only in doing work but as we know in striking a heavy piston. I would ask the gentlemen present whether they know any part of the performance of an indicator diagram which shows how or measures the manner in which any of this power is afterwards given out? I do not know. Perhaps some of you may tell me. If, as far as I see, there is no indication given on the diagram how this buried power is restored, then I think we have a place in which we can find out how it is that so much more of the power of a steam engine is swallowed up in the engine, according to the indicator diagram, than we can possibly believe it is in reality. I believe that in the second half of the stroke of a quick-driving engine, to which there is a large cylinder and heavy masses, a great deal of the power which is put into it in the shape of acceleration comes out of it in the shape of giving out power to the engine; and I do not think that that is recognised anywhere in the indicator diagrams. ON SOME PERFORMANCES OF THE SCREW-PROPELLER, AND CERTAIN PROPOSED SCHEMES FOR INCREASING ITS EFFICIENCY AND REDUCING ITS STRAINING EFFECTS UPON THE HULLS OF SHIPS.* By HAMILTON W. PENDRED, Esq., C.E. [Read at the Seventeenth Session of the Institution of Naval Architects, 7th April, 1876; Vice-Admiral Sir F. W. E. NICOLSON, Bart., C.B., Vice-President, in the Chair.] THE subject matter of the present Paper is divided into three heads, namely: first, a statement of the respective performances of screw-propellers of different shapes, and called individually after their respective inventors or designers; secondly, a general descriptive review of some novelties invented by various engineers in connection with screw machinery, each contrived by its originator either to fulfil in a better manner some functions already performed by other means in an imperfect fashion, or else to improve the working and increase the durability of existing machinery; and lastly, a consideration, in connection with the other two heads, of the straining injury done to the hulls of steam ships by their machinery, when the power of the latter is great in proportion to the displacement of the former; the fracture of shafts, and what means yield the best promise of mitigating such evils. In putting before the Members certain particulars of the performance of screwpropellers, the author has to comment on the great obstacles the collector of data on this subject encounters in his search. The patentees of propellers, one and all, unite in a complaint that the owners, companies, and others using this or that pattern, persistently decline to furnish any particulars of its performance. The only explanation the author can give as a reason for such secrecy is that probably an owner who has tried a new form of screw and found it a failure does not care to publish the fact that he has made a mistake; while, on the other hand, if the trial is successful he will not want a rival to hear of the good things he has got. Whatever the cause, however, the * Received 28th February, 1876. fact remains, and it is one to be regretted, because it is by extended and systematic comparison of the performances of different forms of propeller under various conditions of working that alone we can hope to secure the best results. It is impossible within the limits of a single Paper to consider at all exhaustively the process of development which the screw has undergone even during the last ten years, and the author hopes that this will exonerate him from the charge of dealing superficially with his subject. As a matter of course, the efficiency of the screwpropeller is perhaps the most important of all the questions bearing on the subject. In using the word "efficiency" the author desires to impress upon his audience that he does not do so to denote the relative merits of two different patterns or forms of screw, contrasted with each other per se, but as referable to their respective capabilities to utilise the power of the engines to the best advantage. Marine engineers know that little more than half the total indicated horse-power of the engines is absolutely available for the propulsion of the ship by means of the screw. The most accurate way to ascertain the actual amount of the indicated horse-power available for the propulsion of the ship is to measure the thrust of the screw-shaft in pounds, and this multiplied by the speed of the ship in feet per minute will give it. A very concise and practical method of estimating the efficiency either of screw or of machinery is generally adopted now both by owners and engine builders; it consists simply in gauging the number of knots a given set of boilers and machinery and a given type of propeller will drive a certain ship with a fixed number of tons of coal; the factor of time may or may not be introduced into the trial. The author will, however, remind his hearers that the smallest consumption of coal per horse-power is not a complete test in itself; the boilers and engines may be very economical, and the screw, either from inherent defects or from its ill adaptation to the hull and machinery, be very wasteful. A comparison between the consumption of fuel and what the author will call "thrust horse-power," he thinks would be a good test of economy, because it would at once show, if taken in conjunction with the indicated horse-power of the engines, the real quantity of fuel used to propel the ship and the quantity wasted in loss. Such a test has very seldom been applied to determine the efficiency of this or that propeller. The engineer who has bestowed most attention on such tests is Mr. Isherwood, formerly chief of the engineering department of the United States Navy, and Tables I. and II. give general particulars of some experiments instituted by him in 1858 on some gunboats originally built for the British Government to attack the forts of Cronstadt during the last Russian war, but which were subsequently employed in Chinese waters. The Tables set forth the particulars of the boats and of their engines. Half the number of boats were engined by Maudslay, the remainder by Penn. The distribution of power was computed as follows:-First, the following deductions were made from the gross horse-power of the engines: (a) power to work engines and shafting, the remainder (b) being the nett power applied to the screw-shaft. Of this nett power, judged by Morin's experiments, 7.5 per cent. is first absorbed by the friction of the load, and this is additional to the friction of the engine itself, and is directly as the load or pressure under which the engine works. This being deducted, the residue is partly absorbed in overcoming the cohesive resistance of the water to the surface of the screw, sometimes erroneously called the friction of the water on the screw-blades; it is in the direct ratio of the surface and as the square of the velocity; it equals about 0-45 pounds per square foot with a velocity of 10 feet per second. This then, as well as the power required to overcome the cohesive resistance of the water to the screw surface, being deducted, the residue of the engine power is apportioned between that expended in the slip of the screw, the propulsion of the vessel, and the ratio of speed of ship to speed of vessel. The Tables I. and II. are compiled on this analysis; in both equal weights of steam and fuel were used, and the speed of Maudslay's boats was 8.38 knots and that of Penn's 8.26 knots. Efficiency of working as measured by fuel consumption was greatest in Maudslay's engines, but owing to the better utilization of the power by Penn's screw a gain of 8 per cent. of economy was realized as compared with Maudslay's. Penn's was the most wasteful engine, but the superiority of his screw more than compensated for this. Both boilers were alike. Mr. Isherwood states that from his investigations he finds that the weight of steam required to propel a vessel at different rates of speed is always in a much lower ratio than the cubes of the speeds, but its real value is difficult to determine accurately. Table III. gives the results of some experiments made by Mr. Isherwood at Hong Kong, in 1858, with the British transport, Sydney; and Table IV. gives her performances as contrasted with those of her sister ships, the Ireland and Scotland, and the relative economical efficiency of application of power of each of the screws was-Sydney, 118; Ireland, 1.00; Scotland, 1∙146. Table V. shows the relative performances of six different propellers tried by Mr. Isherwood on H.M.S. Conflict. One of the screws tried was Sir Thomas Mitchel's "Boomerang," so called from its resemblance to a weapon used by the Australian savages; its shape resembles that of a strip of paper first twisted in a spiral and then bent like the letter S. Table V. also gives the general dimensions of the screws, and these experiments are especially valuable because so many different forms of propeller were tried with one and the same hull and machinery; though, as all the propellers were of the two-bladed type, no knowledge is gained as to the performances of three or four-bladed screws. The dimensions of the ship were as in Table VI. The attention of all inventors dealing with the screw-propeller has been principally directed to two points-the one being to devise a propeller that will drive a ship at the greatest speed with a given power, and which shall also have good reversing or backing power, and give the ship also as straight a course as possible with the helm amidships; the other being the greatest reduction of vibration. No one has bestowed more attention to the study and improvement of screw propulsion than Mr. Robert Griffiths, and one of his earliest ideas was to find out what radius of circle on a screw disc was the limit of maximum efficiency. He experimented on the principle of filling up the central part of the screw with a spherical boss, making these bosses of different sizes or various proportionate diameters to that of the screw disc. The author need not dilate on this point as it is one familiar to his audience, suffice it to say that Mr. Griffiths found the best size of central sphere to be between one-third and one-fourth of the diameter of the screw. Next he dealt with the shape of the blades, and he found the best width for them just over the boss to be one-third the diameter of the screw, and at the outer extremities one-ninth thereof. screw. Mr. Griffiths's more recent experiments have been directed, not to the shape of the screw itself, but to the determination of the best way of feeding it with water. It is perfectly evident that the action of the screw in driving a ship forward depends on the relative resistance opposed by her hull and by the water to the screw; or, to put it in another way, it depends on the resistance opposed by the water to the movement of the hull in one direction and to the repelling action of the propeller on the water in the opposite one. If both were equal of course the ship if then at rest would remain so; as it is, her rate of speed is exactly that due to the difference between the two statical forces, namely, that opposed by the water to the hull, and that opposed by it to the The density of the water is alike in both cases so long as the hull and screw are at rest, but is so no longer when screw and hull are moving onwards; then the case is quite different, for the water resistance to the hull increases rapidly while that opposed to the repellent action of the screw diminishes, because to some extent the screw is going from the reaction of the water. Besides this too the rotation of the screw tends to a centrifugal action, and throws the water partly sideways, the power expended in doing so being totally wasted, as the repellent action of the screw should be only in a line with the vessel's keel; and the author is of opinion that if the actual longitudinal area of the blades of a screw equal say one-fourth of the disc area, that then one-fourth of the power of the screw is wasted in centrifugal action on the water, with the attendant evil that there is the less water left to be repelled backwards. Thus if we suppose, for sake of argument and illustration, that the screw of a ship of given size must repel or away G G |