Mr. MORLEY: Yes. I quite agree with the remarks which were made by Mr. Warren and I feel much indebted to him for mentioning my Paper so kindly as he did, also for pointing out the danger arising from ventilating pipes which pass through water-tight bulkheads. I have often looked at such pipes, and have felt that they were sources of danger; people would forget to shut the valves in them if any emergeney arose, and water would get through. Then with regard to your observation, Sir, about the number of valves, I should like to explain that every Kingston valve has a stop-cock inside the valve; so that, before the water can get into the ship, the valve must be opened, and also the stop-cock; and in each case these are opened by rods from the main deck. So that the rules which have been made with regard to the valves being under the stoke-hole plate, and so forth, in connection with the engine to which Mr. Spence refers, certainly were not required for the pumps of the ships which I had to speak of. It only remains for me to thank you for the attention which you have given to the Paper.

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Mr. SPENCE: It is, I must say, some satisfaction to me to find that none of you have found great fault with my arrangements. I think Mr. Merrifield was the only one to find a little fault, and that was on account of the fear he had that small fish, or something else, would be drawn into the valve. The proof of the pudding is in the eating;" and, as the valve has been fitted on a ship for three years, going to all parts of the world, and has been found to be a success in every respect, and has not swallowed up any small fish, I think there is not so much fault to be found with it. Mr. Parker stated, I think, that about eighteen months ago—when this matter was brought before him-he examined all the steamers, and put all the sea connections in proper order, so that no accident could occur. It is rather unfortunate, but just about a year ago, on the 5th of May, 1875, one of his ships came to grief in Bombay, as stated in my Paper. With regard to the Rover, I am exceedingly sorry I mentioned it; but in future, if the Admiralty would give full details to the newspapers of the accidents that happen to their engines, shipbuilders would not be apt to get a wrong impression into their minds as to the way in which they get the water into them.

The CHAIRMAN: I am very sorry that we shall be obliged to postpone till a more convenient opportunity a very interesting Paper which is left on the list. Time gallops very fast; and I am afraid we must deny ourselves that pleasure till possibly to-morrow morning.

ON THE UNEQUAL ONWARD MOTION IN THE UPPER AND LOWER CURRENTS IN THE WAKE OF A SHIP; AND THE EFFECTS OF THIS UNEQUAL MOTION ON THE ACTION OF THE SCREW-PROPELLER.*

By Professor OSBORNE REYNOLDS.

[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 very important part which the tendency of the water to follow in the wake of a ship plays in the action of the screw-propeller has often been the subject of remark. It has been very prominently brought forward by Mr. Froude and others; and is, I believe, now very generally accredited a place in all considerations of the very complicated phenomena which envelop the action of the screw. There is one effect of this wake, however, which I think has not hitherto received the attention which its importance demands, and this is the object of my present communication.

Of the various phenomena which have been developed during our experience of screws, none have given more trouble than their tendency to cause vibrations; and although certain causes have been suggested for this, it has never received a satisfactory explanation. This, I think, arises from the fact, that in the calculations and estimates which have been made hitherto respecting the screw, it has been uniformly assumed that the blades of the screw act with equal effect in all positions-that the screw acts equally on all the water through which it sweeps. If this assumption were correct there could be no tendency in the screw to cause vibrations except by throwing water against parts of the ship. But this equal action can only be assumed to exist on the supposition either that the water through which the screw moves is initially at rest, or that it is all moving with the same velocity. Now this supposition will, I think, on closer examination, be seen to be very far from true; and in recognising what is the actual condition of the water, I think we can see what are the causes of general phenomena which have been hitherto only partially explained, besides the above mentioned tendency to cause vibrations which is the péché habituel of the screw-propeller.

*Received 18th March, 1876.

Last year, while investigating the action of a screw on the steering of a vessel, my attention was drawn to the tendency which the screw had to turn the ship out of her direct course. This tendency I found was very generally recognised, and was attributed, like the tendency to cause vibrations, to the action of the water thrown by the screw obliquely against the stern-post.

That this explanation was not the true one, I was at once able to convince myself by removing the stern-post, when I found that the tendency of the screw to turn the ship out of her course was increased. I was thus led to conclude that the upper blade or blades of the screw experienced greater lateral resistance than the lower blade or blades; for the stern of the ship was always driven in a direction opposite to that in which the upper blades were moving. On looking for the cause of this resistance it appeared that it might arise from the water in which the upper blades worked following the ship faster than that below; and on comparing the various tendencies which the ship had to turn when moving at different velocities, with what might be expected to result from such an unequal motion, I found sufficient agreement to confirm me in this opinion. Being at that time concerned with the steering, I only examined this phenomenon so far as it related to the investigation in hand, the results of which investigation were contained in a Paper read before Section G at the British Association last year. Subsequently, however, it occurred to me that this difference in the speed of the following currents must play an important part in the action of the screw-propeller, particularly as regarded the vibrations.

The Relative Speed of the Upper and Lower Currents in the Wake.

As is well known, a ship imparts an onward motion to the water in its wake in two ways-by the friction of the skin, and by the wave which follows the ship. From neither of these causes does it appear that the motion imparted to the water will be equally distributed through the whole area of the wake; but, on the other hand, it appears that both causes will act to give the water near the surface a greater onward velocity than that which is on a level with the keel of the ship, and to give that water which is directly behind the stern-post a greater velocity than that which is more on one side.

When a long narrow plane is dragged through the water in the manner adopted by Mr. Froude in his experiments on surface friction, its only effect, in the way of setting the water in motion, is that of skin-resistance; but even here the upper water will be made to move faster than the lower. The motion imparted to the water in the immediate vicinity of the plane is rapidly communicated to the adjacent water, and so becomes more or less dissipated. Now at the top of the plane the only direction in which this dissipation can extend is laterally, whereas towards the bottom of the

plane the dissipation can extend downwards as well as sideways, and is therefore much more rapid, leaving the water near the bottom of the plane moving with less velocity than that near the top. So that, looking at a ship as a long narrow plane, we see that even so there would be not only a difference between the velocity of the water in the middle of the wake and that towards the outside, but that there would also be a difference in the velocity at different elevations. A ship, however, differs considerably from a plane, and its form tends further to increase the inequality in the motion of

the water.

The water which fills the opening left by the ship in large part rises up from beneath its bottom, and in rising carries up to the surface that water which has received the greatest onward motion from rubbing against the skin, supplying its place below by fresh water without any onward velocity. This would be the case even if the run of the ship were in the form of a vertical wedge, and the actual form of the ship, which is more like an inclined plane than a vertical wedge, tends greatly to increase this action, for the water moves upwards along what are called the geodetic lines.-See Rankine's Shipbuilding, p. 83.

The fact that the lines of a ship are much fuller near the surface than those below tends also to give the upper water greater forward motion.

Again, the form of a ship is such as to cause a wave to follow it, the crest of the wave being not far from the stern-post. This wave also causes a greater onward motion in the particles of water near the surface than those which are below.

We see therefore, taking all the causes together, that there is probably a very considerable difference in the relative onward velocity imparted by the ship to the water in which the upper and lower blades of the screw work. There is also a difference in the velocity of the water at different lateral distances from the middle of the wake, but this latter variation is not of any direct importance as regards the object of this communication and therefore will not be considered further.

The Actual Velocity of the Wake.

Before we can form an estimate of the probable magnitude of the actual difference in the velocity with which the upper and lower currents move, it is necessary to arrive at some conclusion as regards the proportion which the velocity of the wake bears to that of the ship. The actual motion imparted by a ship to the water in its wake has never, so far as I am aware, been experimentally investigated; there are however two ways in which estimates have been formed;-by observations on the surface, and by calculations based on the resistance of the ship.

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If one may judge from various incidental comments, one finds that the observation of the motion at the surface of the wake has led to much higher estimates of its velocity than the calculations from the ship's resistance.

Mr. G. B. Rennie remarks, "The current caused by the onward motion of the ship "has a velocity at the stern equal to that of the ship itself."*

Mr. Griffiths

66 says, The water in which the screw works is an eddy which follows "the ship at the same speed or nearly so. * If a patent log were placed in the screw opening, it would not even approximately indicate the speed of the ship."†

I would remark here that I do not make these quotations in order to show that they are wrong, but simply to show that observation of the surface has led those who have had the best opportunities of judging, to form a high estimate of the onward motion imparted to the wake for the purpose of comparing this estimate with that based on the resistance of the ship.

In his Marine Engineering, Rankine gives a rule for calculating the velocity of the wake (see p. 249); and applying this rule to the Warrior (a very long ship), he finds that the speed of the water near the stern-post is '09 the speed of the ship.

We see, therefore, how widely this estimate differs from the estimates formed from observations at the surface. The previous argument, however, regarding the difference between the velocity at the surface and that below will go a long way to reconcile these estimates.

Rankine's estimate is based on the supposition that all the water following the ship has the same onward motion imparted to it. A very different result, however, is arrived at, if instead of the entire mass of water in the wake it is only, or principally, the upper layers that are supposed to be set in motion. The speed imparted to the water must be inversely proportioned to the volume acted on, so that if the motion only extends to the bottom of the ship, and gradually dies out, instead of the velocity being 10 per cent. it will be 20 per cent. that of the ship.

This seems to me to agree with what may be observed on looking over the stern of a paddle steamer, or a sailing ship. In the case of the steamer, the inner ends of the lines of foam left by the paddles become curved forwards as they approach the stern, where they join the wake, and are violently dragged forward with a velocity of certainly more than one-tenth that of the ship.

As a rough estimate, therefore, I should conclude that in a ship with a fairly fine run the velocity of the wake, at the surface, is not less than 20 that of the ship, while