vided that the draught of water of the ship is great enough to permit the use of a screw, or twin-screws, of sufficiently large diameter, it is preferable to use the screws. When the draught is too limited even for twin-screws, multiple screws have been used, but ordinarily the paddle would be employed. The efficiency of the screw, as compared with the paddle or jet, in smooth water results from the fact that it operates upon much larger quantities of water in a unit of time. The race of a well-immersed screw-propeller has a sectional area approximately equal to the screw-disc; and this is very large even as compared with the largest paddle-race. It is scarcely necessary to illustrate this statement; let one contrast suffice. The famous paddle-steamer Scotia and her Majesty's ship Volage both attained a speed of about 15 knots on the measured mile. For both paddles in the Scotia the sectional area of the race equalled about 130 square feet, and the speed of the floats was about 34 feet per second, that of the ship being 26 feet. For the screw of the Volage, the disc area was about 280 square feet, the speed of the screw 304 feet per second, and that of the ship 25 feet. On calculation it will be found that the paddles operated on about 4500 cubic feet of water per second, the indicated horse-power of the engines being about 4800; whereas in the screw the quantitity of water operated on per second exceeded 7500 cubic feet, the engines indicating 4500 horse-power. It has been shown that the thrust of a propeller depends upon the product of the quantity of water operated upon in a unit of time, by the sternward velocity imparted to it; and since the screw has so large an excess over the paddle in the quantity of water in its race, the waste work due to obliquity of action, screw friction, and centrifugal action is more than compensated for. Smooth-water performances are not the true test of efficiency; in a seaway the screw is far more superior to the paddle than it is on the measured mile. Rolling motions, which would seriously affect the paddle, leave the screw almost uninfluenced. Pitching oscillations of course affect the screw more than the paddle; but if the screw is well immersed, or, still better, if twin-screws are employed, the loss of efficiency on account of pitching does not appear to be at all serious in large ships. Considerable variations in the draught of water may also take place, yet leave the screw efficient; whereas it has been shown that this is not equally true of the paddle. The screw lends itself much more readily than the paddle to the association of steam with sail power; the absence of projecting paddle-boxes is a great advantage in steaming head to wind and in general service; and, finally, in war-ships the screw is much less exposed to damage in action. The most convincing argument in favour of the superiority of the screw under all conditions of service is, however, to be found in the fact that it has almost entirely replaced the paddle in sea-going ships of the mercantile marine, wherein economical propulsion is of the highest importance. From the foregoing considerations it will appear that two conditions are essential to the successful application of screwpropellers: (1) the disc area must be made large; (2) the water must be permitted to flow freely to the screw. With a single screw a large disc means a large diameter; and the superior propelling effect of large screws is now so generally recognised that it is unnecessary to say much in their favour. The well-known trials made twenty years ago with her Majesty's ship Flying Fish proved that the larger screw, even if not wholly immersed, might still be used with advantage; and since that time the largest screws that could be employed conveniently have been adopted. Considerable trim by the stern has also been given to many ships, mainly for the purpose of obtaining an extreme draught aft that would permit the use of a large well-immersed screw. In many cases, however, the draught is too limited to permit the employment of a single screw of sufficiently large diameter, and then recourse is had to twin or multiple screws. Multiple screws have not been used in many cases, nor in any vessels attaining high speeds. Some of the shallow-draught vessels built for service on the Mississippi during the American civil war had four screws; but their speed was low. The Russian circular ironclads have no less than six screws. In the Novgorod each of these screws is 10 feet in diameter; the aggregate disc area exceeding 500 square feet. No authoritative explanation has been published of the considerations which led to the choice of the number and diameter of these propellers; but an endeavour appears to have been made to secure a collective disc area bearing about the same ratio to the total immersed midship section as would hold in vessels of ordinary form. The area of immersed midship section in ordinary screw-ships, when fully immersed, varies from two to four times the disc area, three times being a good average value; in the Novgorod it is about 24 times the aggregate disc area. Such a large number of screws, situated close together, must, however, have some prejudicial effect upon each other, the race of any screw almost certainly being less defined than it would be if that screw acted alone. And further, although they are placed at some distance abaft the hull proper, their position is such that they must produce, when at work, considerable disturbance of the stream line-motions. Twin-screws were first used in shallow-draught vessels, but their employment has since been extended to deep-draught vessels with very considerable advantage. The performances of recent twin-screw ships belonging to the Royal Navy show that they gain on single-screw ships of similar size, form, and speed, being driven with a less expenditure of power. Taking, for example, the ratio of the indicated horse-power to (displacement) as an index of the relative economy of power, which may be done without unfairness under the conditions stated, and comparing the performances of a * * See the remarks on page 486 as to the use of (displacement) as a measure of the frictional resist ance. What is here assumed is that in very similar vessels the total resistance will vary as the frictional resistance for the same speeds. number of ships when steaming at the uniform speed of 14 knots, the average ratio for the single-screw ships is about 17.5 against 15.5 for the twin-screw ships, or about 11 per cent. in favour of the latter. This superiority in propelling effect is obtained in association with other important advantages. The engines and propellers being duplicated, it is possible to make use of a middle-line watertight bulkhead (as shown in Figs. 18-25, page 30), and to greatly increase the safety of the ship against foundering. There is also far less risk of entire disablement than with a single screw; and, with either screw at work, a twin-screw ship is not merely under control, but able to make fair headway. The Vanguard, for example, with one screw steamed 11.4 knots, while with both screws she attained 14.9 knots; and in many cases long passages have been made by twinscrew ships, with a single screw at work and a small angle of helm to keep the course. So manifold are the advantages of twin-screws that all the large ships now building for the Royal Navy are to be propelled in that manner, the armed despatch vessels Iris and Mercury, which are intended to have a speed of 17 or 18 knots on the measured mile, being among the number. Single screws are now used only in cases where cruising qualities are of great importance and the sailpower is good; it is then desirable to lift the screw when under sail alone, and single screws are used chiefly because no satisfactory plan has yet been devised for lifting twinIn view of these facts it seems a matter worthy of the gravest consideration of merchant ship owners whether twin-screws might not be introduced into ocean steamers, which now almost without exception have single screws, any slight accident to which or to the machinery may disable the vessels as steamers, and throw them back upon their sailpower. It has been objected that twin-screws are more exposed to injury by collision, fouling of wreckage, &c., than single screws, which are sheltered under the stern; but while there is undoubtedly force in the objection, it can scarcely screws. be regarded as a counterbalance to all the advantages obtainable with twin-screws. The second condition essential to the efficiency of screws is that they shall have a good supply of water, in order that the race may have its full sectional area, and the whole of the propeller-disc may be creating momentum and exercising thrust. Amongst the more important circumstances influencing the supply of water to the screw may be mentioned the form of the stern of the ship, the distance of the screw abaft the stern, and the immersion of the upper blades when they are passing through the vertical position. If the screw is not sufficiently immersed, it will create considerable surface disturbance, have a less compact and well-defined race, and do more waste work. If the stern is bluff or very full, the efficiency of the screw will be decreased because the water cannot flow freely to certain parts of the screw-disc, which are masked by the sternpost and body of the ship. If the screw is close under the stern, as it usually is in single-screw ships, it has to act at some disadvantage in the "wake” of the ship, as compared with what it would have to do if placed further astern. Fineness in the "run" of single-screw steamships is now recognised as a desirable and necessary feature. In the earlier periods of steam propulsion, this was not so well understood, and in many of the bluff-sterned vessels of the Royal Navy, converted from sailing into steamships, the prejudicial effect of their forms on the action of the screw was most marked. One case alone can be cited out of the many on record. The screw-frigate Dauntless, built in 1848, was first tried with a full stern, and her performance being unsatisfactory, she was lengthened aft about 10 feet, and made of much finer form in the run. In her earlier trials, when the displacement was 2300 tons, she was driven at a speed of 73 knots with 836 horse-power (indicated). After the alteration, with the same screw and nearly the same displacement, the ship attained a speed of 10 knots with 1388 horse-power; but |