Mr. H. C. Ashlin, Superintendent Engineer, as to S.S. " Federico" (240 nominal horse-power).—“ We “had a first-rate opportunity of trying it astern as we had to take her across the river Mersey from dock to dock under her own steam, and to leave the dock here we have to go out stern first; and on both "occasions when crossing the river and leaving again for Havana she gave entire satisfaction."

WORKING OF ENGINES.

Captain of S.S." Louisiane" (500 nominal horse-power), to Manager of Company.-"The movement "of the engines has gained enormously in regularity."

Report of S.S." New Pelton" (80 nominal horse-power).-" Working of engines improved.".

Report of W. Welsh Thornton, Esq., on his S.Y. "Avalanche" (60 nominal horse-power).—“ This "absence of vibration and smooth effect upon the working of the engines are in our opinion invaluable." Report of Captain as to S.S." Memphis" (150 nominal horse-power).—“ The effect of this absence of “vibration can best be seen by the small repairs of our engines which after three years' running are as good as new."

Particulars of the engines (compound):-Diameter of high-pressure cylinder, 57 inches; diameter of low-pressure cylinder, 90 inches; stroke 4 feet; pressure of steam in boilers, 60 lbs. ; cut-off at about two-thirds stroke in small cylinder.

ON THE PROPULSION OF BODIES THROUGH WATER.*

By ROBERT GRIFFITHS, Esq.

[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.]

FIG 1.

In bringing before the Institution this Paper on the propulsion of bodies through water, 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 expect any naval architect or engineer at the present time to believe; and much valuable time has been wasted in reading Papers on, and discussing such erroneous ideas at Scientific Institutions.

My object at present is to explain what I have already partially proved, that bodies of different forms can be propelled through the water at high speeds, in proportion to the power that is exerted to propel them, whether those bodies be of a ship-shape or any other form, such as oval, round or square. Prior to the introduction of the screw-propeller, the proportions of our sailing frigates were such as the talent of scientific and experienced men of former ages had proved to be the best for safety, steadiness, and seaworthy qualities; and I believe they will yet be found the best proportions for screw ships generally. When the screw-propeller was introduced for propelling ships naval architects and engineers found, by

FIG 2.

Curve of requisite Supply of Water
for Screws in Casings
FIG 3.

16 16

Supply due to

Screws Diso.

8 7o 12
LHP, Sq, Foot of the Screw's Disc,

experience, that to obtain speed in proportion to the power exerted to drive the screw

* Received 17th March, 1876.

propeller, the ship required to be constructed with very fine lines, narrow, deep, and of great length in proportion to the beam, which instead of improving the stability. and safety of the ships, made them weak, crank and dangerous; but I find that equal speeds can be obtained from a ship which is three to four times her beam in length, and the draught of water one-quarter to one-third times the beam.

FIG 4.

The model, Fig. 4, by which I think I have proved that this can be accomplished, is 3 feet 3 inches long, 1 foot 2 inches wide, 31 draught of water, and was tried against another model (Fig. 5) 5 feet long, 71⁄2 inches wide, 31⁄2 draught of water, the weight and displacement of each being the same, and they were propelled by the same spring and screws. The resistance of the models when drawn through the water, instead of being propelled by the screws, was as five is to three, and yet the speed was rather in favour of the short model though it offered the greatest resistance. This is overcome in the short model by the screw obtaining its supply of water from underneath at a distance forward, before it begins to flow into the space left by the ship, and also by the way the water is delivered from the propeller; while the long model, which is fitted with the screws in open water in the ordinary manner, is subject to two unavoidable defects, the first being that the screws draw their supply of

FIG 5.

water from the ship's quarters, which deprives her of the water that is required there to fill the space left by the vessel; and secondly, the screw drives a column of water backward, which meets and retards the water which would then flow forward after the ship if propelled by sails, and causes a serious resistance to the speed of the vessel. I believe it will be generally admitted that there is a considerably greater resistance to the ship when the screw is in operation, which is attributed solely to the screw drawing the water from the stern of the vessel; but I find the resistance is divided between the suction and the delivery of the water by the screw. By the present system of placing the screw at the stern of the ship, the loss occasioned from the suction of the screw can be modified by fining the lines at the stern to allow a better supply of water to the screw, or by removing the screw further back; but I can see no remedy that can be made for the water that is forced back from the screw as long as it is worked in open water in the ordinary way. When however the screw is worked in a casing, if the discharge

ends of the tunnels are formed at an angle of about 45° horizontally with the line of screw shaft towards the rudder, 40 per cent. more area will be allowed; so that the water that is forced from the screw meets the still water at the stern and mixes with it, while the remainder fills up the space left by the area of the screw tunnel, leaving the water at the stern of a screw ship in nearly the same condition as it would have been at the stern of a sailing vessel.

I will now explain how any other than the ship form can be propelled at moderate speeds through the water. I will first take an elongated body of say 30 feet wide and 7 feet deep and 50 feet long, or upwards, according to the amount of displacement required, with either square or round ends, drawing 6 feet 6 inches of water; the immersed ends would be 195 feet area, and if fitted with four screws of 6 feet 3 inches diameter, in tunnels at each end, they would take about 200 feet of water through them, or just in proportion to the amount of power applied to drive the screws. The tunnels for the screws to work in would require funnel-mouths of such capacity as to admit the quantity of water the screws required.

According to Colonel Beaufoy's experiments there does not appear to be much difference in the resistance of bodies propelled through the water, whether their ends are made in a wedge form vertically or horizontally, provided the angle of incidence of that wedge is made to give the least resistance at the speed it is driven through the water. There can be no doubt since the screw-propeller can be made subservient in its operation to propel bodies of any form (instead of being antagonistic, as when it is now applied for propelling ships), first by removing the water at the forward end or bow, and placing it at the stern or after end where it is required, a speed commensurate to the power employed to propel it will be obtained, provided the wedge form at the bow and stern of the body that is to be propelled be made at such an angle as would give the best result according to the power that is exerted to propel it. It may appear objectionable on account of the reduction in the displacement to elongate the angle of incidence at the bow and stern, which would be of no material consequence, since a very slight lengthening in the midship of the body to be propelled will make up for it.

With regard to what form of ship offers the least resistance in being propelled through the water, I am not aware of any reliable information where the body that was to be propelled acted in harmony with the instrument that propelled it, except in sailing vessels, in which experience has proved how the propelling power, the wind and sails, and the ship's form, can be made to harmonise with each other to obtain the best result. Since the introduction of the screw-propeller, the object which chiefly

occupied the attention of engineers and naval architects was to obtain the greatest speed which the power exerted by the engine could give, and very little attention was paid to anything else except what they termed the loss of speed by the slip of the screw- -a greater fallacy than which could not have been conceived.

There can be no doubt that, unless the supply as well as the discharge of water necessitated by the screw can be carried on without being detrimental to the speed of the ship, partly by drawing the water from the stern and discharging it from the screw in such a way that it does not draw the water from the stern or keep back the water that is flowing forward after the ship, a great amount of the power that is given out by the engine is consumed to overcome these obstructions. We will suppose for a moment if there was a screw-propeller in operation worked with power equal to propel the ship at 10 knots, though not in any way connected with the vessel, but worked at the stern of a sailing ship going at 10 knots, the resistance to the sailing ship in that case would be very considerable. I believe Mr. Froude has made the experiment, and found the resistance to the ship, when the screw was at work, was just double what it was without the screw; this loss that is occasioned by the action of the screw is much reduced on my system, as the screw neither draws the water from the stern of the ship nor stops the forward flow of the water that follows it.

Fig. 1 represents the stern of H.M.S. Bruiser, fitted with a casing over the screw, having a funnel-mouth to it of 1 foot greater diameter than that of the screw, and a small lip below that admitted altogether about 40 per cent. more water than that due to the screw's disc. The speed of the Bruiser, before the casing was applied, was 8.016 knots, and with the casing 8-280 knots, all other circumstances being the same. I have no doubt that if there had been a different screw, and the mouth of the tunnel supplying it had been placed forward so as not to take the water from the stern-such as I have found to give the best result in models—a much better result in speed would have been obtained. The casing stopped the vibration from the screw, and prevented the racing of the engines in a heavy sea. The screw-propeller, when worked in a casing, is a rotary pump, and subject to the same laws―viz., doubling the speed of the water forced through it-requires four times the power which will give a corresponding thrust to the screw shaft; and in order to prove several points which do not appear to be well understood in screw propulsion, I tried a number of experiments on a small launch at Keyham. Fig. 2 represents it with the apparatus to indicate the pressure of water as it left the screw. Under different circumstances the pressure varied considerably at different places of the screw's disc as shown; but when the screw was disconnected, and the launch towed at a speed of 6 knots, the screw revolving at the rate of 4.2 knots, the mean pressure over the screw's disc was 0.76 pounds per square inch; when the screw was working, and