For instance, a screw with a large positive slip in smooth water is frequently inefficient at sea against a head wind, which increases the resistance, and necessitates an increase of slip. I venture to say that these statements, taken in a general manner, are not at variance with experience obtained from the performances of screw ships. Before it is possible to satisfactorily decide if this curve applies in a general manner to full sized screws propelling ships, we require the results of trials of various ships where the screws are working about the region of no slip. Model experiments teach that the scale for the slip varies with the design of the screw, and that with a given screw the speed of the wake (which decides the point of no apparent slip) varies with the type of ship and with the position of the screw with respect to the hull. Remembering these disturbances, it is not improbable that it may be possible to account for or explain what at first sight may appear departures from the curve.

The diameters of the screws in the table are not compared with the diameters given by the method explained by Mr. Froude in his paper last year, for there are differences in the slips, the proportions of blade area to disk, and, to some extent, in the shapes of the blades, which are not taken into account in that method. Assuming, however, as Mr. Froude does, a constant proportion of blade area to disk, and a uniform pattern of blade, the determination of the diameter for a given set of conditions may, as a rule, be a complete solution of the problem of the design of a screw, but these assumptions do not cover all the necessities of actual practice, which make it extremely desirable to know something about the influence or efficiency of various proportions of blade area to disk, and of the form or distribution of a given area.

During the discussion which followed, Mr. John said that, both as regarded the mercantile marine and the Royal Navy, there were few data to work upon, but few ships having been built with twin screws. Mr. Linnington's proportions of pitch to diameter of 1.2 to 1.34 was not invariably adhered to. He mentioned a couple of small twin screw vessels where the proportion of pitch to diameter came nearly to 1.5, and he remembered a few years ago the propellers in one of these vessels being changed and the pitch increased, the result being a very considerable improvement. He believed they might go with quick running twin screw engines to a larger proportion of pitch to diameter than they could with a single screw. He might instance the change in the Iris. She was first engined with the pitch equal to the diameter, and she gained two knots or thereabout when the diameter was reduced 2 ft. and the pitch increased 2 ft.

Admiral De Horsey said that he tried experiments with the single screw in the Aurora. She had a feathering serew, and when the sails were used to assist, they commonly altered the pitch of the screw according to the strength of the wind. The screw could be altered while it was revolving, and as the wind freshened they coarsened the pitch, and when they wanted to stop the engines they coarsened the pitch so as to bring the screw right fore and aft, so that they never altered the way of the ship in changing from steam to sail alone. The reason why twin screws had been adopted in the navy was that if one was damaged there was the other still available. But it gave them a still further advantage, as it enabled them to have a fore and aft bulkhead, which with a single screw was difficult. The mercantile marine had not as yet looked favorably on twin screws. Their finest and fastest ships were single screws, probably because, in very bad weather, the single screw was better.

Mr. Spyer said that in designing propellers for ships of war, they were obliged to attempt to obtain the highest possible speed, and that was not necessarily coincident with a propeller of maximum efficiency. On the other hand, for mercantile purposes, coal consumption was obviously of paramount importance, and the speed of any particular vessel must be obtained with the smallest possible amount of indicated horse power, and a propeller of maximum efficiency. Regarding the position of the propellers in a small pinnace, the propellers were shifted six or seven inches further out, and with about ten per cent. less indicated horse power she obtained three tenths of a knot more speed.

Mr. Barnaby asked Mr. Linnington whether, in designing twin screws for a vessel of 8,000 i.h.p., he would make each screw, which would have to take 4,000 i.h.p., of the same diameter as a screw for a single ship of 4,000 i.h.p., of the same speed. Unfortunately in high speed vessels, from one point of view, the faster they went for a given power the smaller the diameter of the screw had to be, and the larger the pitch, so that in very high speed twin screw vessels the ratio of pitch to diameter would be found to come out very great indeed. In a twin screw torpedo boat, to be tried shortly, they had a ratio as high as 1.64. In the case of the Inflexible it was found, owing possibly to the position of the screw, that the whole of the plates immediately over the screws were damaged. Mr. Beckett Hill had been using, during the past three or four years, the twin screw steamers the Ludgate Hill, Richmond Hill, and Tower Hill. These were all over 4,000 tons register, and indicated, when at work at full speed, 2,500 h.p. Before he and his friends built these steamers, they built some very large tug boats on the twin screw principle. At the present moment, four of the fastest steamers building for the Atlantic service were to have twin screws.

The great obstacle to the extension of the twin screw in the mercantile navy had been the fear that the projection of these screws would make the vessels very difficult to handle, but he had found no such difficulties. He had found it an advantage to put the point of the propeller as near the deadwood as he could, without actually touching it, and in the large steamers, as well as in the tugs, the distance was a few inches. As to the point of safety, he thought it a great advantage to have twin screws, and on two occasions twin screw vessels had met with accidents which, but for the twin screws, would have necessitated their putting back to New York for repairs. The Richmond Hill, on one occasion, met with an accident to her machinery two days after leaving New York; but she was able to come on with the second set of engines, and was only one day late in the passage. No difficulty had been found in the docking and undocking of these vessels, either in London or Liverpool, and while with single screw vessels they had sometimes to employ one or two dock boats to dock and undock them, they never had to do so with the twin screw vessels.

These vessels were 400 ft. long, with 48 ft. breadth of beam - a very large size to handle in a river like the Thames. He noticed in the paper a propeller with a diameter of 15 ft. 6 in. to indicate 11,110 h.p., so that a great Atlantic steamer, which should indicate 11,000 or 12,000 h.p., and have a beam of about 65ft., would have her screws very well protected.

Mr. White said that as soon as it was found that with twin screws they lost nothing in efficiency, ship owners generally were contemplating their adoption, an admirable example of which had been set in the vessels of the Hill line. In adopting twin screws, the question whether they should overlap was one that deserved very serious consideration, and it was interesting to know, from experience gained by the vessels of the Hill line, that there was no difficulty in the way of the projection of the screws. With a moderate power, and with vessels of considerable size, the screws were well sheltered: but in the large ships which were contemplated, where there must necessarily be larger screws, this might be different, and become a difficulty.

Mr. Linnington, in reply, said there was no reason to think that the twin screw at sea might not be as satisfactory, in comparison with the single screw, as it appeared in smooth water. As a matter of fact, one of the great advantages of twin screws was that at sea the condition of weather which would bring the single screw out of the water, and make it extremely inefficient, would have no appreciable effect on the twin screws. In vessels of deep draught especially, they were well immersed, and they were really more efficient at sea than in smooth water. In ships of full form, the longitudinal position of the screws was of importance; but in the ships referred to in this table the run was very fine, and the screws were well covered by the hull. He did not think, in such a case, any small difference in longitudinal position would affect the performance. If any alteration were made, it would probably be better to put the screws farther off. When the rudder was hard over, the blades of the screw should be about a foot clear of the rudder. - Industries.

[1]A paper recently read before the Institution of Naval Architects, London.