is as high as 25 per cent; a well-designed wheel on a well-formed ship should not exceed 15 per cent under ordinary circumstances.

If K is the speed of the ship in knots, S the percentage of slip, and R the revolutions per minute,

The diameter, however, must be such as will suit the structure of the ship, so that a modification may be necessary on this account, and the revolutions altered to suit it.

The diameter will also depend on the amount of "dip " or immersion of float.

When a ship is working always in smooth water the immersion of the top edge should not exceed the breadth of the float; and for general service at sea an immersion of the breadth of the float is sufficient. If the ship is intended to carry cargo, the immersion when light need not be more than 2 or 3 inches, and should not be more than the breadth of float when at the deepest draught; indeed, the efficiency of the wheel falls off rapidly with the immersion of the wheel.

C is a multiplier, varying from 0.3 to 0.35; D is the diameter of the wheel to the float centres, in feet.

where N number of floats in one wheel. For vessels plying always in smooth water K = 1200. For sea-going steamers K = 1400. For tugs and such craft as require to stop and start frequently in a tide-way K = 1600.

It will be quite accurate enough if the last four figures of the cube (DX R) be taken as ciphers.

For illustrated description of the feathering paddle-wheel see Seaton's Marine Engineering, or Ŝeaton and Rounthwaite's Pocket-book. The diameter of a feathering-wheel is about one half that of a radial wheel for equal efficiency.

(Thurston.

Efficiency of Paddle-wheels.-Computations by Prof. Thurston of the efficiency of propulsion by paddle-wheels give for light river steamers with ratio of velocity of the vessel, v, to velocity of the paddle-float at centre of pressure, V, or 3/20 radius of the wheel, and

v

3

=

4'

with a dip a slip of 25 per cent, an efficiency of .714; the same slip and ratio of

, and a dip =

=

and for ocean steamers with radius, an efficiency of .685.

JET-PROPULSION.

Numerous experiments have been made in driving a vessel by the reaction of a jet of water pumped through an orifice in the stern, but they have all resulted in commercial failure. Two jet-propulsion steamers, the "Waterwitch," 1100 tons, and the "Squirt," a small torpedo-boat, were built by the British Government. The former was tried in 1867, and gave an efficiency of apparatus of only 18 per cent. The latter gave a speed of 12 knots, as against 17 knots attained by a sister-ship having a screw and equal steam-power. The mathematical theory of the efficiency of the jet was discussed by Rankine in The Engineer, Jan. 11, 1867, and he showed that the greater the quantity of water operated on by a jet-propeller, the greater

is the efficiency. In defiance both of the theory and of the results of earlier experiments, and also of the opinions of many naval engineers, more than $200,000 were spent in 1888-90 in New York upon two experimental boats, the "Prima Vista " and the "Evolution," in which the jet was made of very small size, in the latter case only 5-inch diameter, and with a pressure of 2500 lbs. per square inch. As had been predicted, the vessel was a total failure. (See article by the author in Mechanics, March, 1891.)

The theory of the jet-propeller is similar to that of the screw-propeller. If A = the area of the jet in square feet, Vits velocity with reference to the orifice, in feet per second, v the velocity of the ship in reference to the earth, then the thrust of the jet (see Screw-propeller, ante) is 2AV (V – v). The work done on the vessel is 2AV(V- vjv, and the work wasted on the rearward projection of the jet is 2 × 2ÁV(V – v)2. The efficiency is 2AV(V-v)v This expression equals unity when

2AV (V – v)v + AV (V − v)2

=

2v

V v, that is, when the velocity of the jet with reference to the earth, or Ꮴ v, = 0; but then the thrust of the propeller is also 0. The greater the value of Vas compared with v, the less the efficiency. For V= 20v, as was proposed in the "Evolution," the efficiency of the jet would be less than 10 per cent, and this would be further reduced by the friction of the pumping mechanism and of the water in pipes.

The whole theory of propulsion may be summed up in Rankine's words: "That propeller is the best, other things being equal, which drives astern the largest body of water at the lowest velocity."

It is practically impossible to devise any system of hydraulic or jet propulsion which can compare favorably, under these conditions, with the screw or the paddle-wheel.

Reaction of a Jet.-If a jet of water issues horizontally from a vessel, the reaction on the side of the vessel opposite the orifice is equal to the weight of a column of water the section of which is the area of the orifice, and the height is twice the head.

The propelling force in jet-propulsion is the reaction of the stream issuing from the orifice, and it is the same whether the jet is discharged under water, in the open air, or against a solid wall. For proof, see account of trials by C. J. Everett, Jr., given by Prof. J. Burkitt Webb, Trans. A. S. M. E., xii. 904.

RECENT PRACTICE IN MARINE ENGINES.

(From a paper by A. Blechynden on Marine Engineering during the past Decade, Proc. Inst. M. E., July, 1891.)

Since 1881 the three-stage-expansion engine has become the rule, and the boiler-pressure has been increased to 160 lbs. and even as high as 200 lbs.per square inch. Four-stage-expansion engines of various forms have also been adopted.

Forced Draught has become the rule in all vessels for naval service, and is comparatively common in both passenger and cargo vessels. By this means it is possible considerably to augment the power obtained from a given boiler; and so long as it is kept within certain limits it need result in no injury to the boiler, but when pushed too far the increase is sometimes purchased at considerable cost.

In regard to the economy of forced draught, an examination of the appended table (page 1018) will show that while the mean consumption of coal in those steamers working under natural draught is 1.573 lbs. per indicated horse-power per hour, it is only 1.336 lbs. in those fitted with forced draught. This is equivalent to an economy of 15%. Part of this economy, however, may be due to the other heat-saving appliances with which the latter steamers are fitted.

Boilers.-As a material for boilers, iron is now a thing of the past, though it seems probable that it will continue yet awhile to be the material for tubes. Steel plates can be procured at 132 square feet superficial area and 11⁄2 inches thick. For purely boiler work a punching-machine has become obsolete in marine-engine work.

The increased pressures of steam have also caused attention to be directed to the furnace, and have led to the adoption of various artifices in the shape of corrugated, ribbed, and spiral flues, with the object of giving increased strength against collapse without abnormally increasing the thickness of the plate. A thick furnace-plate is viewed by many engineers with great

suspicion; and the advisers of the Board of Trade have fixed the limit of thickness for furnace-plates at 5% inch; but whether this limitation will stand in the light of prolonged experience remains to be seen. It is a fact generally accepted that the conditions of the surfaces of a plate are far greater factors in its resistance to the transmission of heat than either the material or the thickness. With a plate free from lamination, thickness being a mere secondary element, it would appear that a furnace-plate night be increased from 1⁄2 inch to 3/4 inch thickness without increasing its resistance more than 114%. So convinced have some engineers become of the soundness of this view that they have adopted flues 3/4 inch thick.

Piston-valves.-Since higher steam-pressures have become common, piston-valves have become the rule for the high-pressure cylinder, and are not unusual for the intermediate. When well designed they have the great advantage of being almost free from friction, so far as the valve itself is concerned. In the earlier piston-valves it was customary to fit spring rings, which were a frequent source of trouble and absorbed a large amount of power in friction; but in recent practice it has become usual to fit springless adjustable sleeves.

For low-pressure cylinders piston-valves are not in favor; if fitted with spring rings their friction is about as great as and occasionally greater than that of a well-balanced slide-valve; while if fitted with springless rings there is always some leakage, which is irrecoverable. But the large port-clearances inseparable from the use of piston-valves are most objectionable; and with triple engines this is especially so, because with the customary late cut-off it becomes difficult to compress sufficiently for insuring economy and smoothness of working when in "full gear," without some special device.

Steam-pipes.-The failures of copper steam-pipes on large vessels have drawn serious attention both to the material and the modes of construction of the pipes. As the brazed joint is liable to be imperfect, it is proposed to substitute solid drawn tubes, but as these are not made of large sizes two or more tubes may be needed to take the place of one brazed tube. Reinforcing the ordinary brazed tubes by serving them with steel or copper wire, or by hooping them at intervals with steel or iron bands, has been tried and found to answer perfectly.

Auxiliary Supply of Fresh Water-Evaporators. To make up the losses of water due to escape of steam from safety-valves, leakage at glands, joints, etc., either a reserve supply of fresh water is carried in tanks, or the supplementary feed is distilled from sea-water by special apparatus provided for the purpose. In practice the distillation is effected by passing steam, say from the first receiver, through a nest of tubes inside a still or evaporator, of which the steam-space is connected either with the second receiver or with the condenser. The temperature of the steam inside the tubes being higher than that of the steam either in the second receiver or in the condenser, the result is that the water inside the still is evaporated, and passes with the rest of the steam into the condenser, where it is condensed and serves to make up the loss. This plan localizes the trouble of the deposit, and frees it from its dangerous character, because an evaporator cannot become overheated like a boiler, even though it be neglected until it salts up solid; and if the same precautions are taken in working the evaporator which used to be adopted with low-pressure boilers when they were fed with salt water, no serious trouble should result.

Weir's Feed-water Heater. The principle of a method of heating feed-water introduced by Mr. James Weir and widely adopted in the marine service is founded on the fact that, if the feed-water as it is drawn from the hot-well be raised in temperature by the heat of a portion of steam introduced into it from one of the steam-receivers, the decrease of the coal necessary to generate steam from the water of the higher temperature bears a greater ratio to the coal required without feed-heating than the power which would be developed in the cylinder by that portion of steam would bear to the whole power developed when passing all the steam through all the cylinders. Suppose a triple-expansion engine were working under the following conditions without feed-heating: boiler-pressure 150 lbs.; I.H.P. in high-pressure cylinder 398, in intermediate and low-pressure cylinders together 790, total 1188. The temperature of hot-well 100° F. Then with feedheating the same engine might work as follows: the feed might be heated to 220° F., and the percentage of steam from the first receiver required to heat it would be 10.9%; the I.H.P. in the h.p. cylinder would be as before 398, and in the three cylinders it would be 1103, or 93% of the power developed without

feed-heating. Meanwhile the heat to be added to each pound of the feed-water at 220° F. for converting it into steam would be 1005 units against 1125 units with feed at 100° F., equivalent to an expenditure of only 89.4% of the heat required without feed-heating. Hence the expenditure of heat in relation to power would be 89.4 93.0 96.4%, equivalent to a heat economy of 3.6%. If the steam for heating can be taken from the low-pressure receiver, the economy is about doubled. Passenger Steamers fitted with Twin Screws.

Comparative Results of Working of Marine Engines, 1872, 1881, and 1891.

34%, 57, 92

60

170

11,656