the launch moored, it was about 0.55 pounds with 240 revolutions of the screw; and when the launch was propelled by her screw at 6 knots, the mean pressure was 1.65 pounds per square inch, and 250 revolutions per minute, and 0.85 pounds outside the launch (as shown on diagram). I may here mention that in neither the Bruiser nor the launch was the supply of water drawn in as far forward as to avoid the suction from the stern of the ship, which is one of the greatest advantages to be derived by my system. There is no gain in speed to be obtained by the application of the casing when the power is less than 4 indicated horse-power to each square foot of the screw's disc; for in the launch where the indicated horse-power was 3 to each square foot of the screw's disc there was a loss of about of a knot, and no doubt the loss in speed would have increased as the horse-power was reduced in proportion to the area of the screw's disc; for the screw when worked in a fine run in a small vessel obtains all the water it requires for that proportion of power without drawing it away from the stern so as to retard the speed of the ship.

Fig. 3 is a curve showing the quantity of water the screw requires, according to the power that is applied to work it. This curve does not extend beyond 16 indicated horse-power to each square foot of the screw's disc, though I have no doubt it might be extended much further with safety, since the Waterwitch could be propelled nearly as effectively by a screw with about 75 indicated horse-power to each square foot of the area of the discharge pipes.

DISCUSSION ON THE TWO PRECEDING PAPERS.

Mr. J. FORTESCUE FLANNERY: There is one question which, with great respect for Mr. Griffiths's able Paper and what he has said with regard to screw-propellers, I would wish to ask him. I find in the opening of the Paper he makes this remark: "I do not intend to enter into the question of what is termed "the loss by the slip of the screw-propeller or the configuration of the screw, in neither of which do I 66 expect any naval architect or engineer at the present time to believe. Much valuable time has been "wasted in reading Papers on and discussing such erroneous ideas." The interpretation which this paragraph bears is that Mr. Griffiths believes that, no matter what may be the form of the screw, the result in the economy of propulsion and consumption of coal will be entirely the same. I am really much surprised that a gentleman, who was the very first to call attention to the difficulties of the ordinary system of screws, should be the very first to go back upon his own labours and condemn them in such a wholesale manner. It may be, however, that I have misunderstood the remark. Mr. Griffiths will perhaps explain that in his reply. This brings me to notice that Mr. Pendred's Paper includes a large number of differently-formed screws, and on looking over the diagrams there is one which I must confess seems to bear within it the promise of a much more economical performance than some of the others.

I allude to the screw with the curved blade. Perhaps I can explain my reason better by reference to the black board. Suppose this [illustrating] to be the centre of the shaft, and we have a straight blade as the common screw usually is formed, then as the screw goes round there is a large amount of centrifugal action set up and a large amount of power lost by that centrifugal force falling away from the centre. If, however, we curve the blade in this direction, as is shown upon the diagram, the other blade being curved so, then we confine the centrifugal force much more towards the centre, and it is converted into a forward motion. Then again, what is the cause of vibration in screw-propellers? I believe that, this being the stern-post of the ship, as the screw goes round, if the blade be straight, there is a lateral blow struck upon the stern-post at the top, a corresponding lateral blow struck at the bottom, and that results in the vibration of the whole ship; whereas, if you curve the blade in this direction, so that by following this dotted line the tip of the blade will pass through the aperture before the next portion, and so on, then the blades will pass through gradually, giving a gradual lateral blow to the stern-post, and you do not have the same chance of vibration as you have with the straight blade. Then there is just one other point. The author says he does not know the reason why the Britannic was condemned and why she is now being altered-or rather why she has been altered, because her alterations are complete-at Belfast. I happen to be in a position to inform him of the reason. The bottom part of the propeller being below the keel line and the upper part of the propeller being above the keel line, as the screw went round the upper blade gave a lateral blow to the stern-post in the same way as I have described; but there was no counteracting blow on the under side of the shaft, and the result was vibration to an enormous extent; and this was further increased by the propeller being so much lower down, and having so much more leverage. The arrangements for water-tight work were shaken completely to pieces, and great expense was necessarily incurred at the end of each voyage to make them tight. Then there is another point in Mr. Pendred's Paper which, I think, it is only justice to Mr. Wimshurst I should take notice of. That refers to the scheme for raising the screw out of the water. The scheme I allude to was for raising the screw-propeller out of the water in sailing ships when the vessel was under sail alone. This was effected by a universal ball and socket joint in the shaft. I am informed, and know quite well it is the case, that this invention was anticipated and patented by Mr. Henry Wimshurst in 1863, and it has been most valuable in some cases. I think it is only just that the right inventor should have the credit of his ingenuity.

Mr. J. MAC FARLANE GRAY: I was about to say what this gentleman has already said as to the gentleman by whom this invention was patented. I think Mr. Henry Wimshurst is here. He is undoubtedly the inventor of the lowering shaft if there is anything in it.

Mr. HENRY WIMSHURST: I may perhaps be allowed to say a word or two upon this subject. I have heard a great deal said about screws. I built the first two ships in which screws of this kind were applied; and since then I have carried out a series of experiments at my own cost. The first was in the Archimedes. That was not done at my own cost, but at the cost of the Ship's Propeller Company. The second was at my own cost for two years. But there is one great drawback to the screw-propeller which has not been named at present, and that is, as you all know, that the water passing under the ship's stern goes at the same rate on both sides. The screw when the arm is coming up with the current of water is coming with the current at the same rate. Say the screw is 15 and the vessel is going 15-that goes up as it were smooth, but the moment it turns it meets the water on the opposite side of the post at 15 coming up and 15 going down, giving an enormous blow. Now my object in the arrangement in the Archimedes was to give plenty of room for the screw at either end. It only wants room-nothing before

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and nothing behind, and then there is no shock; but the moment you contract the openings you must have the shaking of the stern. I would recommend in all cases where it can be done—I had to do it with the Archimedes screw fitted in quite close-that it should be cut in half. Then we got over the shaking of the stern, but not altogether. Then with regard to the novelty of it we all know that things have gone on progressing since the time I refer to. I put the first engine to drive the vessel on the screw principle, and all I would say at the present time is that anybody can make a screw and anybody can talk about a screw, but I had the honour of building the first two successful screw steam ships.

Mr. H. W. PENDRED: I do not know that I need take up your time by making any remarks. One gentleman has made an observation that he believed the curved-bladed screw was better than a straightbladed screw. I am disposed to agree with him on the principle of the water being scooped in at the centre; but at the same time I would observe that as to the exact curve which would give the best results there is no rule as far as I know by which it can be established. With regard to the question about the Britannic, I have to thank the gentleman who spoke on this subject very much for the information. I felt a good deal of interest in that lowering gear, and was very much at a loss to understand why that was removed. With regard to the observations of Mr. Wimshurst I am quite disposed to agree with him also; and I think that the less water there is above the propeller the better. I think it only remains for me to thank the Members for the courtesy and attention which they have given me.

Mr. R. GRIFFITHS: I would make one observation upon the remarks which have been made by Mr. Pendred in his Paper as to the Bruiser. She was sent away from Plymouth to the Scilly Islands to try her effect. When she came back I asked Mr. Watson, the chief engineer, what he thought of her. He said, "I will tell you candidly what I think. I hope they will not put me in a ship again without "casing over the screw; we were out in nasty weather, she never raced at all, and there was no vibration "whatever." With regard to the screw question I may remark that upwards of 20 years ago I made a good many experiments at Portsmouth on the Fury. At last we took a blade that was perfectly flat and had no twist in it; and it came, within a very trifling difference, to the same as that which Mr. Wright said. There was not near half a knot extra speed. When I first went with my screw to Mr. Lloyd the first thing he asked me was, "What speed do you expect to get?" I said, "I do not expect to get much; "but if you get the right pitch on the screw, you may manage to get half a knot." The remark he made was, "I believe you, because every one that comes here with a new screw never gives us less than 4 or 5 "knots extra speed." My conviction after all my experiments is, that if you take four strips of plate iron and set them up at an angle that will hold the engines at the speed you require, it will give you within half a knot of the best screw that was ever brought out.

The CHAIRMAN: I am sure you will all thank the two authors for the Papers which they have just read; and I cannot help saying that Mr. Griffiths' allusion to what the inventors of screws always say they will gain in speed reminds one of the old formula with regard to every new invention relating to steam engines, which was always a case of a saving of 75 per cent. of fuel.

ON WATER-TUBE BOILERS.*

By J. FORTESCUE FLANNERY, Esq., Member.

[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.]

MUCH public and professional attention has lately been drawn to the official enquiry held at Liverpool by the Board of Trade into the loss of life caused by the explosion of portions of the water-tube boilers of the steam ship Propontis. This is rightly regarded as a most important case; and as other attempts to improve the construction of marine boilers, although very fully discussed in the Press, have never been directly brought under the notice of this Institution, I venture to offer a short résumé of some of the principles and facts involved in the question.

The almost universal adoption of high-pressure steam at sea has brought with it much inconvenience, notwithstanding its great economy of fuel. The anxiety attending upon the construction and the working of the marine boiler is much greater now than formerly; its first cost is largely increased, the expense of repair is much more, and the duration of its life is much less. I think I shall have the concurrence of the Members in saying that one of the gravest, if not the very gravest and most important, of the questions now arising in connection with the machinery of steam ships is the improvement of the marine boiler. And not only does the present practice leave much to be desired, but the feeling in favour of still higher pressure is checked chiefly by the difficulty of designing a boiler which will sustain it safely and efficiently. It is an axiom laid down by theory, and confirmed by practice, that the higher the boiler pressure and the greater the ratio of expansion the greater is the economy of fuel. Theoretically, this axiom is sustained by the fact that to generate steam of the pressure of 30 pounds per square inch about 1,190 degrees of heat are required, while to generate steam of 120 pounds per square inch about 1,218 degrees of heat are required, that is to say, to produce steam of four times the boiler pressure less than 10 times the heat is required. Again, a boiler pressure of 240 pounds per square inch requires about 1,235 degrees of

* Received 16th March, 1876.

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heat, or 11 times the heat of steam at 30 pounds, and contains eight times the initial force. Practically these differences are so small that the same consumption of coal may be taken to evaporate the same quantity of water, whatever the pressure. Theoretically, therefore, we have only to increase our boiler pressure to reduce our consumption of fuel, were it not that two well-ascertained considerations intervene. First, we have not hitherto been practically able to construct a boiler which, under usual conditions of steam-ship requirements, will stand a pressure greater than is now in use; and second, the heat of very highly-pressed steam burns up the lubricant in the engine cylinder, quickly destroying the working parts. With the second consideration, however, we have not now to deal; and if we could overcome the difficulty of making a boiler to bear even 120 pounds with safety, much improvement would be effected.

To start from the beginning, let us remember that the thickness of the shell of a cylindrical vessel containing high-pressed steam must, in order to maintain the necessary strength, be increased directly in proportion to increase of pressure and diameter, and that the thickness of the shell may, consistently with the strength necessary for any given pressure, be reduced, if its diameter be also reduced. A very usual size for modern high-pressure cylindrical boilers is say 12 feet diameter; and the thickness of the shell of a boiler of this diameter, double riveted to bear a pressure of 120 pounds per square inch, would be fully 12 inches. I am not aware of any examples of boiler construction where the thickness of the shell plates has exceeded 14 or 13 inches; and it is almost doubtful if, considering the practical difficulties of manufacture, a perfectly good job is made with shell plates of this thickness; at all events, it is generally accepted that to make sound workmanship with a thickness greater than this is practically impossible. On the other hand a cylindrical vessel or tube say 12 inches diameter may, for this same pressure of 120 pounds per square inch, be made if welded only inch thick, and still possess the same proportionate strength; or, for a pressure of 150 pounds per square inch, it would be made practically about inch thick, and this would give a factor of safety nearly three-fourth times greater than that possessed by the cylindrical boiler shell 12 feet diameter 11⁄2 inches thick and pressure of 120 pounds. It is practically certain, therefore, that the limit of pressure with the present type of boiler has been reached; and when we consider how much economy of fuel has been effected by increasing the pressure to its present height, and how much more economy may be effected by still further increasing the pressure, it will be seen that the adaptation of an efficient high-pressure marine boiler would meet a great and increasing want.

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Many look for the solution of this problem to the type known as the water-tube or tubulous boiler-that is, the system by which the cylindrical portions of the boiler subject to internal pressure are reduced in diameter and increased in number, the result