at the level of the keel the water is practically stationary. And, in the cases of ships, moving at an abnormal speed, carrying a high stern wave, or having full sterns, the difference may be increased to almost any extent.

The Effect on the Screw.

Having now shown that there is a considerable difference in the onward velocity in different parts of the ship's wake, it only remains to consider what effect this difference would have on the action of the screw.

Compared with the speed of the ship the difference is after all but small, and if the thrust of the screw depended on the speed of the ship, this small difference might well be neglected; all that would result would be a difference of some 20 per cent. in the pressure on the upper and lower blades. I cannot help thinking that it is owing to some such confusion as this between the action of the screw and the speed of the ship that the unequal motion of the water in the wake has remained unattended to for so long. When we realize the fact that the thrust of the screw does not depend on the speed of the ship, but on the difference between the speed of the ship and the geometrical speed of the screw, as the speed at which it would have to move forward were the water unyielding; and that this difference, called the slip, is somewhere between one-tenth and one-fourth the speed of the ship, we see at once what an important influence an increase in the speed of the water anything like one-fourth the speed of the ship would have. Hence, although the inequality in the motion of the water is small as compared with the speed of the ship,-as compared with the slip it is very large, and this is the essential comparison.

The slip of a screw would be somewhere between one-tenth and one-fourth the speed of the ship if it were equally distributed over the entire area through which the screw acts. But if there is a difference in the rate at which the water is following the ship at different parts of the section of the screw race, then the slip, and consequently the pressure on the blades of the screw, will be greatest at those places where the water is following the ship fastest.

Taking the mean slip at 2, and supposing the upper blades to be working in a current which has an onward velocity 2 greater than that in which the lower blades are working, then the slip at the tops of the upper blades would be 3, and that at the tops of the lower blades only 1; so that the resistance at the tips of the upper blades would be three times as great as that at the tips of the lower blades. Or, to put it roundly, the area of the water on which the screw acts to drive the ship forward would be virtually reduced; it would be the blades above the shaft that principally drive the ship, the lower blades merely passing through the water.

The Tendency to cause Vibrations.

Under these circumstances it is clear that the lateral resistances which the upper and lower blades encountered would no longer balance each other. For example, on a twobladed screw the pressure on the blades would only be equal when they were both on a level with the shaft. As the one rose towards the vertical position the resistance on it would increase, while that on the lower blade would diminish. The action of the screw would, therefore, be to cause an intermittent force, urging the stem in the direction opposite to that in which the tips of the upper blades were moving.

The magnitude of this intermittent force would be very considerable; under the circumstances assumed above, it would, while it acted, be comparable to the entire lateral resistance encountered by the screw. It would, therefore, afford sufficient explanation of the screw's tendency to cause vibrations, which the shock caused by the water thrown by the screw against the stern-post does not. It would also fully accord with what experience has shown respecting the effect of the screw on the steering of the ship.

Effect on the Efficiency.

Such an inequality in the action of the screw as that described above, would not necessarily reduce its efficiency as a propeller. So long as there was some small slip left to the bottom blades there could be no actual retardation of the ship. But if the inequality in the motion of the water should at any time bear such proportion to the mean slip that the lower blades could not, as it were, screw themselves through the water fast enough to keep up with the ship, then they would have to be dragged through the water, and would retard the ship. Such a result would only be experienced when the inequality of motion in the water was more than double the mean slip of the screw. Such a state of things, it would appear, could only be brought about by a vessel moving at an abnormal speed and carrying a large stern wave, or by a vessel having a very full stern-conditions which are invariably found to result in loss of efficiency and excessive vibration, and very often in what is called negative slip. The loss of efficiency which usually attends negative slip has received what appears to be a satisfactory explanation as being due to the back suction, or reduction of pressure which the action of the screw causes on the stern of the ship. And that it is in some part at least due to this cause has been proved by Mr. Froude by actual experiment. But, considering that when this action occurs, all the conditions which would cause the lower blade to drag back are known actually to exist, it would seem to be highly probable that at least in part the loss of efficiency, as well as the excessive vibration, is due to the unequal motion of the water on which the upper and lower blades of the screw act.

Disadvantage of Large Screws.

It can be easily seen that the effects which have been attributed to the unequal motion of the water would be greater with screws, which are larger in proportion to the draught of the ship than with those which are smaller.

There are two reasons for this. In the first place the larger the screw the smaller must be the mean slip, and consequently the greater would be the proportion which the inequality of the motion of the water would bear to it. On the smaller would be the margin allowed for the difference in the slip at the top and bottom of the blades. And, secondly, the larger the screw the greater would be the difference in the motion of the water in which the upper and lower blades worked.

Now I believe it has been found, as a matter of experience, that there is a limit to the size of screws which give the best results for each ship. This limit is doubtless in part owing to the increased friction which large screws experience, owing to their increased surface; but the friction must be much larger than what we have reason to suppose it is, if this alone can account for the limit. It seems probable therefore that this limit is another result of the inequality of the motion of the following waters.

A few years ago a large Atlantic steamer was fitted with a screw, which could be lowered until its blades extended below the bottom of the ship. Various advantages would appear as likely to result from such an arrangement. But it seems to me to be probable that the disadvantages resulting from the inequality in the motion of the wake would be considerably increased, for the lower blades of the screw would descend into the water with no following motion at all, while the upper blades would still be high up in the wake. I do not know what was the result of the experiment, but I have heard that the plan had to be abandoned on account of the excessive vibration.

Conclusion.

It is not my object in this Paper to enter upon the question as to what modification in the construction or dispositions of screws might be suggested by the recognition of the unequal motion of the wake and its effects. Any suggestions I might make would be premature. My endeavour has been solely to elucidate further the actual conditions or circumstances of the problem of screw propulsion, it being my conviction that a complete knowledge of the conditions of any problem must be conducive to its eventual solution. My opportunities of studying the action of screws are limited; and in venturing to come before you, my inducement has been that my ideas would be.

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ON THE UNEQUAL MOTION OF CURRENTS IN THE WAKE OF A SHIP. criticised by those who have much better opportunities. If, through ignorance, I have been occupying time by dilating upon what is unimportant or already known, I can only hope that I may have your indulgence; a claim which I feel entitled to make, as it is only the importance which you were pleased to attach to my former communication which has emboldened me to come forward again.

DISCUSSION.

Mr. J. SCOTT RUSSELL, F.R.S. I should be sorry if this Paper passed without comment upon it. I think it is a very valuable Paper, and very much that Professor Reynolds has said is profound and valuable truth. The only element I would call his attention to very strongly is this, that the distribution of those currents in the stern of the vessel is very materially altered by each form of stern of the ship, What those peculiarities are I think he will be able to guess and to calculate. At all events I think it a very valuable Paper, that the direction of thought is very important, and with a great deal of what he has said as to what takes place in the stern of the ship by the action of the screw I entirely agree.

ON STEEL FOR SHIPBUILDING AS SUPPLIED TO THE ROYAL NAVY.*

By JAMES RILEY, Esq., Manager of the Landore Siemens-Steel Works.

[Read at the Seventeenth Session of the Institution of Naval Architects, 7th April, 1876; the Right Hon. LORD HAMPTON, G.C.B., D.C.L., President, in the Chair.]

THE purpose of the present Paper is to show what has been done by the Company (of whose works the author is the Manager), towards producing a material for shipbuilding which shall meet the requirements indicated by Mr. Barnaby in his Paper on "Iron and "Steel for Shipbuilding," read before the Members of your Institution twelve months ago. This being so, the closing sentences of that Paper may well be quoted at this point. They are as follows:

“The uncertainties and treacheries of Bessemer steel, in the form of ship and "boiler plates, are such that it requires all the care which it has bestowed upon it at "L'Orient to avoid failure. The question we have to put to the steel makers is, What แ are our prospects of obtaining a material which we can use without such delicate "manipulation, and so much fear and trembling? We have gone on for years, using "iron plates which are a compound of impure irons, of different and unknown "qualities, welded together imperfectly in the rolls. We want a perfectly coherent and 'definitely carburized bloom, or ingot, of which the rolls have only to alter the form “in order to make plates with qualities as regular and precise as those of copper and 66 gun metal, and we look to the manufacturers for it.

"I am ready, for my part, to go further than the French architects have gone, and "build the entire vessel, bottom plates and all, of steel; but I know that at present the "undertaking will involve an immense amount of anxiety and care. We ought not to "be behind any other country in this matter, and it shall not be my fault if we are.'

Long previous to the date when these words were spoken, it had been the desire of Dr. Siemens that the Landore Siemens-Steel Company should turn their attention to

*Received 23rd March, 1876.