COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS.

Defects of the Screw in Head Winds.-In the case of ocean vessels, it is found that paddle vessels fitted with the ordinary radial wheels, and screw vessels fitted with the ordinary screw, are about equally efficient in calms and in fair or beam winds with light and medium immersions. If the vessels are loaded deeply, however, as vessels starting on a long voyage and carrying much coal must almost necessarily be, then the screw has an advantage, since the screw acts in its best manner when deeply immersed, and the paddles in their worst. When a screw and paddle vessel, however, of the same model and power are set to encounter head winds, the paddle vessel it is found has in all cases an advantage, not in speed, but in economy of fuel. For whereas in a paddle vessel, when her progress is resisted, the speed of the engine diminishes nearly in the proportion of the diminished speed of ship, it happens that in a screw vessel this is not so,—at least to an equal extent,- but the engines work with nearly the same rate of speed as if no increase of resistance had been encountered by the ship. It follows from this circumstance, that whereas in paddle vessels the consumption of steam, and therefore of fuel, per hour is materially diminished when head winds occur, in screw vessels a similar diminution in the consumption of steam and fuel does not take place.

The speed of the two vessels will nevertheless be the same, unless the strength of the head wind be so great as to bring the vessels nearly to a state of rest, and on that supposition the screw vessel will have the advantage. Such cases occur very rarely in practice; and in the case of the ordinary resistances imposed by head winds, the speed of the screw and paddle vessel will be the same, but the screw vessel will consume most coals.

The cause of this is, that when the screw is so proportioned in its length as to be most suitable for propelling vessels in calms, it is too short to be suitable for propelling vessels which encounter a very heavy resistance. It follows, therefore, that if it is prevented from pursuing its spiral course in the water, it will displace the water to a certain extent laterally, in the manner it does if the engine be set on when the vessel is at anchor; and a part of the engine power is thus wasted in producing a useless disturbance of the water, which in paddle vessels is not expended at all.

Screw and Paddle Vessel tied stern to stern.—If a screw and paddle vessel of the same mould and power be tied stern to stern, the screw vessel will preponderate and tow the paddle vessel astern against the whole force of her engines. And seeing that the vessels are of the same mould and power, so that neither can derive an advantage from a variation in that condition, the natural inference would be that the preponderance of the screw vessel shews that the screw must be the most powerful propeller.

That inference, however, would not be a correct one. All steam vessels when set into motion, will force themselves forward with an amount of thrust which, setting aside the loss from friction and other causes, will just balance the pressure on the pistons. In a paddle vessel, as has already been explained, it is easy to tell the tractive force exerted at the centre of pressure of the paddle wheels, when, the pressure urging the pistons, the dimensions of the wheels, and the speed of the vessel are known; and that force, whatever be its amount, must always continue the same with any constant pressure on the pistons. In a screw vessel the same law applies; so that with any given pressure on the pistons, and discarding the consideration of friction, it will follow that whatever be the thrust exerted by a paddle or a screw vessel, it must remain uniform whether the vessel is in motion or at rest, and whether moving at a high or a low velocity through the water. Now to achieve an equal speed during calms in two vessels of the same model, there must be the same amount of propelling thrust in each; and this thrust, whatever be its amount, cannot afterwards vary if a uniform pressure of steam be maintained. The thrusts, therefore, caused by their respective propelling instru

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ments, when a screw and paddle vessel are tied stern to stern, must be the same as at other times; and as at other times those thrusts are equal, so must they be when the vessels are set in the antagonism supposed.

The screw vessel preponderates, not by virtue of a larger thrust exerted by the screw in pressing forward the shaft and with it the vessel, but by the gravitation against the stern of the wave of water which the screw raises by its rapid rotation. This wave will be only raised very high when the progress of the vessel through the water is nearly arrested, at which time the centrifugal action of the screw is very great; and the vessel under such circumstances is forced forward partly by the thrust of the screw, and partly by the hydrostatic pressure of the protuberance of water which the centrifugal action of the screw raises up at the stern.

The screw vessel will not preponderate if a screw and paddle vessel be tied bow to bow and the engines of each be then reversed. In some screw vessels the amount of thrust actually exerted by the screw under all its varying circumstances, has been ascertained by the application of a dynamometer to the end of the shaft. By this instrument which is formed by a combination of levers like a weighing machine for carts — a thrust or pressure of several tons can be measured by the application of a small weight; and it has been found, by repeated experiment with the dynamometer, that the thrust of the screw in a screw vessel when towing a paddle vessel against the whole force of her engines, is just the same as it is when the two vessels are maintaining an equal speed in calms. The preponderance of the screw vessel must, therefore, be imputable to some other agency than to a superior thrust of the screw, which is found by experiment not to exist.

In the case of paddle vessels the dynamometer has not been applied to the vessels themselves, as in the case of screw vessels, but it has been employed on shore to ascertain the amount of tractive force that a paddle vessel can exert on a rope.

Performances of Screw and Paddle Vessels at sea.— Numerous experiments have been made to determine the comparative performances of screw and paddle vessels at sea; of which the best known are probably those made on the screw steamer Rattler and the paddle steamer Alecto, two vessels of the same model, size, and power, each vessel being of about 800 tons burden and 200 horses power. Subsequently another set of experiments with the same object was made with the Niger screw steamer and the Basilisk paddle steamer, both vessels being of about 1000 tons burden and 400 horses power. The general results which were obtained in the course of these experiments are those which have been already recited.

The Rattler is 176 feet 6 inches long, 32 feet 8 inches broad, 888 tons burden, 200 horses power, and has an area of immersed midship section of 380 square feet at a draught of water of 11 feet 5 inches. The Alecto is of the same dimensions in every respect, except that she is only of 800 tons burden, the difference in this particular being wholly owing to the Rattler having been drawn out about 15 feet at the stern, to leave abundant room for the application of the screw. The Rattler was fitted with a dynamometer, which enabled the actual propelling thrust of the screw shaft to be measured; and the amount of this thrust, multiplied by the distance through which the vessel passed in a given time, would determine the amount of power actually utilised in propelling the ship. Both vessels were fitted with indicators applied to the cylinders, so as to determine the amount of power exerted by the engines.

Twelve trials in all were made; but we need not refer to those in which similar or identical results were only repeated. The first trial was made under steam only, the weather was calm and the water smooth. At 54 minutes past 4 in the morning both vessels left the Nore, and at 30 minutes past 2 the Rattler stopped her engines in Yarmouth Roads, where in 20 minutes afterwards she was joined by the Alecto. The mean speed achieved by the Rattler during this trial was 9.2 knots per hour; the mean speed of the Alecto was 8.8 knots per hour. The slip of the screw was 10-2 per cent. The actual power exerted by the engines, as shown by the indicator, was in the case of the Rattler 334-6 horses, and in the case of the Alecto 281-2 horses; being a difference of 534 horses in favour of the Rattler. The forward thrust upon the screw shaft was 3 tons, 17 cwt. 3 qrs. and 14 lbs. The horse power of the shaft-or power

Comparative Advantages of the rays of light would do in a parabolic reflector. But this particular configuration is not important, seeing that the same convergence which is given to the particles of the water, with a screw of uniform pitch bent back into the form of a parabola, will be given with a

Fig. 517.

different Kinds of Screws.

387 leading edge of the screw without shock, as its advance is only equal to the advance of the vessel, and before the screw leaves the water it is projected directly astern. At the same time, the curved flange at the rim of the screw prevents the dispersion of the water in a radial direction, and it consequently assumes the form of a column or cylinder of water, projected backward from the ship.

Beattie's Screw.-Beattie's screw, fig. 519, is an arrangement of the screw propeller whereby it is projected beyond the rudder, and the main object of the arrangement is to take away the vibratory motion at the stern, an intention which it accomplishes in practice. There is an oval eye in the rudder, to permit the screw shaft to pass through it, as shown in fig. 520.

Fg. 519.

THE EARL OF DUNDONALD'S PROPELLER.

screw bent back into the form of a triangle if the pitch be suitably varied between the centre and the circumference. The pitch may be varied in two ways. A screw may have a pitch increasing in the direction of the length, as would happen in the case of a spiral stair, if every successive step in the ascent was thicker than the one below it; or it may increase from the centre to the circumference, as would happen in the case of a spiral stair, if every step were thinner at the centre of the tower than at its outer wall. When the pitch of a screw increases in the direction of its length, the leading edge of the screw enters the water without shock or impact, as the advance of the leading edge per revolution will not be greater than the advance of the vessel. When the pitch of a screw increases in the direction of its diameter, the central part of the screw will advance with only the same velocity as the water, so that it cannot communicate any centrifugal velocity to the water; and the whole slip, as well as the whole propelling pressure, will occur at the outer part of the screw blades.

There is a slight advantage derived from these forms of screws, but it is so slight as hardly to balance the increased trouble of manufacture, and, consequently, they are not generally or widely adopted. Then there is the corrugated screw, the arms of which are corrugated, so as it were to gear with the water during its revolution, and thereby prevent it from acquiring a centrifugal velocity. There is Griffith's screw, which has a large ball at its centre, which, by the suction it creates at its hinder part, in passing through the water, produces a converging force, which partly counteracts the divergent action of the arms. Finally, there is Holm's screw, which has now been applied to a good number of vessels with success.

that, in fact, the following edge stands in the plane of the shaft, or in the vertical longitudinal plane of the vessel. Then the ends of the arms are bent over into a curved flange, the edge of which points astern, and the point where this curved flange joins the following edge of the screw is formed, not into an angle, but into a portion of a sphere, so that this corner resembles the bowl of a spoon. When the screw is put into revolution, the water is encountered by the

When the diameter of the cylinder of water projected backwards by a screw, and the force urging it into motion are known, the velocity it will acquire may be approximately determined.

We may take for illustration the case of the Minx, already referred to, which will show how such a computation is to be conducted. The speed of this vessel, in one of the experiments made with her, was 8-445 knots; the number of revolutions of the screw per minute, 231-32; and the pressure on each square foot of area of the screw's disc, 214 lbs. If a knot be taken to be 6075.6 ft., then the distance advanced by the vessel, when the speed is 8.445 knots, will be 37 ft. per revolution, and this advance will be made in about 26 of a second of time. Now the distance which a body will fall by gravity, in 26 of a second, is 1087 ft. ; and a weight of 214 lbs. put into motion by gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity of 1087 ft. during the time one revolution of the screw is being performed. The weight to be moved, however, is 3.7 cubic feet of water, that being the new water seized by the screw each revolution for every square foot of surface in the screw's disc; and 37 cubic ft. of water weigh 231.5 lbs., so that the urging force of 214 lbs. is somewhat less than the force of gravity, and the velocity of motion communicated to the water will be somewhat under 1.087 ft. per revolution, or we may say it will be in round numbers 1 ft. per revolution. This, added to the progress of the vessel, will make the distance advanced by the screw through the water 4.7 ft. per revolution, leaving the difference between this and the pitch, namely 1.13 ft., to be accounted for on the supposition that the screw blades had broken laterally through the water to that extent. It would be proper to apply some correction to this computation, which would represent the increased resistance due to the immersion of the screw in the water; for a column of water cannot be moved in the direction of its axis beneath the surface, without giving motion to the superincumbent water, and the inertia of this superincumbent water must, therefore, be taken into the account. In the experiment upon the Minx, the depth of this superincumbent column was but small. The total amount of the slip was 36-53 per cent.; and there will not be much error in setting down about one half of this as due to the recession of the water in the direction of the vessel's track, and the other half as due to the lateral penetration of the screw blades.

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"For a normal development of 327-34 horses' power, we find that with an effective mean pressure of 12 pounds per square inch on the steam piston, and a piston velocity of 6 ft. per second, the diameter of each of the two equal steam cylinders employed must be 3.07 feet, and the piston stroke 2:40 ft.

"If we bring the data thus obtained for the progressive screw in juxtaposition with those for an ordinary screw, we get the following comparative dimensions for their respective motors, to be applied to the same ship under the same circumstances.

Ordinary Progressive Screw. Screw.

6:00 8.95

525-495 4.213

Stroke of each pisten, in feet

2.350

Mean effective steam pressure in pounds per square inch

12.000

327.34 3.07 2:40 12.00

14:06

75.06

We are not aware of any case in which so large an advantage, as is here stated, has been obtained from screws with an increasing pitch in practice.

Importance of a fine Run.—It is most important to make the stern of screw vessels very fine, with the view of diminishing the slip, and increasing the speed. The Rifleman, a vessel of 486 tons, had originally engines of 200 horses power, which propelled her at a speed of 8 knots an hour. The Teazer, a vessel of 296 tons, had originally engines of 100 horses power, which propelled her at a speed of 61⁄2 knots an hour. The engines of the Teazer were subsequently transferred to the Rifleman, and new engines of 40 horse power were put into the Teazer. Both vessels were simultaneously sharpened at the stern, and the result was, that the 100 horse engines drove the Rifleman, when sharpened, as fast as she had previously been driven by the 200 horse engines; and the 40 horse engines drove the Teazer, when sharpened, a knot an hour faster than she had previously been driven by the 100 horse engines. The immersion of both vessels was kept unchanged in each case; and the 100 horse engines of the Teazer, when transferred to the Rifleman, drove that vessel, after she had been sharpened, 2 knots an hour faster than they had previously driven a vessel not much more than half the size. These are important facts for every one to be acquainted with who is interested in the success of screw vessels, and who seeks to obtain the maximum of efficiency with the minimum of expense.*

PROPORTIONS OF SCREWS.

In fixing upon the proportions of a screw proper to propel any given vessel, we should first compute the probable resistance of the vessel, and we would then be able to find the relative resistances of the screw and hull, and in every case it is advisable to make the screw as large in diameter as possible. The larger the screw is, the greater will be the efficiency of the engine in propelling the vessel; the larger will be the ratio of the pitch to the diameter, which produces a maximum effect; and the smaller will be the length of the screw or the fraction of a convolution to produce a maximum effect.

Experiments with the Pelican.-The French screw steamer Pelican was fitted successively with two screws of four blades, but the diameter of the first screw was 98.42 in., and the diameter of the second 54 in. If the efficiency of the first screw be represented by 1, that of the second screw will be represented by 823, or, in other words, if the first screw would give a speed of 10 knots, the second would give little more than 8. The most advantageous ratio of pitch to diameter was found to be 2-2 in the case of the large screw, and 1:384 in the case of the small. The fraction of a convolution which was found to be most advantageous was 281 in the case of the large screw, and 450 in the case of the small screw.

Screws of four blades were found to have less slip than screws with two, but not to be more efficient; the increased slip in those of two

• See Treatise on the Screw Propeller, by John Bourne, C.E.

blades being balanced by the increased friction in those of four. Screws of two blades, to secure a maximum efficiency, must have a finer pitch than screws of four.

The proportions found to be most suitable in the case of the Pelican are applicable to the screws of other vessels which have the same relative resistance of screw and hull. Taking the relative resistance to be the area of immersed midship section, divided by the square of the screw's diameter, it will in the case of the Rattler be 188 or 3.8. From the experiments made by MM. Bourgois and Moll on the screw steamer Pelican, they have deduced the proportions of screws proper for all other classes of vessels, whether the screws are of two, four, or six blades.

Proper Proportions of Screws with Two Blades.-We shall first enumerate those experiments which bear upon screws with two blades. When the relative resistance is 5.5 the ratio of pitch to diameter should be 1 006, and the fraction of the pitch or proportion of one entire convolution should be 0.454. When the relative resistance is 5, the ratio of pitch to diameter should be 1·069, and fraction of pitch 0-428; relative resistance 4:5, pitch 1.135, fraction 0-402; relative resistance 4, pitch 1.205, fraction 0·378; relative resistance 3.5, pitch 1.279, fraction 0·355; relative resistance 3, pitch 1.357, fraction 0-334; relative resistance 2.5, pitch 1.450, fraction 0.313; relative resistance 2, pitch 1.560, fraction 0-294; relative resistance 15, pitch 1.682, fraction 0-275. The relative resistance of 4 is that which is usual in an auxiliary line of battle ship, 3 5 in an auxiliary frigate, 3 in a high speed line-of-battle ship, 2.5 in a high speed frigate, 2 in a high speed corvette, and 1.5 in a high speed despatch boat.

Proper Proportions of Screws with Four Blades.-The corresponding proportions of screws of four blades are as follows:

The ratios of the pitches to the diameter being for each of the relative resistances enumerated above, 1.342, 1·425, 1·513, 1·607, 1·705, 1.810, 1933, 2.080, 2.243, the respective fractions of pitch or fractions of a whole convolution will be 0.455, 0·428, 0·402, 0·378, 0·355, 0·334, 0.313, 0.294, and 0.275, The corre

as

Proper Proportions of Screws with Six Blades. sponding proportions proper for screws of six blades are follows: Beginning with the relative resistance of 5.5 as before, the proper ratio of pitch to diameter for that and each of the successive resistances in the case of screws with six blades, will be 1.677, 1771, 1.891, 1.2009, 2.131, 2.262, 2·416, 2·600, 2·804; and the respective fractions of pitch will be 0·794, 0·794, 0·703, 0·661, 0·621, 0·585, 0·548, 0·515, and 0.481. These are the proportions which will give a maximum performance in every case.'

LIFTING SCREWS.

To enable vessels to act either under steam or under sail, it has been convenient in some cases to construct the screws in such a way that they may be lifted out of the water, and in war vessels this mode of construction is almost invariably adopted. In the case of ordinary merchant vessels screws with three blades fixed immovably upon the shaft are now commonly preferred. But in the case of war vessels the screws employed have almost invariably only two blades, and this configuration is necessary to enable them to be lifted up through a trunk, provided for that purpose at the stern. In vessels fitted with lifting screws the screw is fixed, not upon the end of the screw shaft, but upon a separate short shaft, which is supported by bearings formed in a frame resembling a window sash, which may be raised upwards in guides on the stern and rudder posts; and the end of this short shaft is provided with a square or hexagonal socket, into which a corresponding projecting part of the screw shaft fits, so that when the screw shaft is put into revolution the screw will be turned round. The general nature of the arrangements necessary for carrying into effect this object will be understood by a reference to the accompanying figures. Figs. 521 and 522 represent the arrangements adopted for raising the screw of the Amphion. An upright key is fitted into a small cog wheel situated beneath the deck, and this key is turned round by a cross handle resembling the handle of an auger. The revolution of the small wheel causes the revolu

In my Treatise on the Screw Propeller I have gone into these various questions more fully than would comport with the nature of this publication.-J. B.

tion of two other small wheels in connection with it, which last wheels are fixed on the top of two large vertical screws, and these screws being put into revolution raise up the frame which carries the screw. In fig. 523 is represented the catch for retaining the screw in the vertical position while it is being raised out of the water.

Fig. 524 represents the arrangement for lifting the screw of the Ajax. Here, instead of the long screws, pieces of cast screw are introduced at intervals upon two upright spindles, and these portions of screws work in the sides of the sliding frame, which is properly toothed to enable them to do so. It will be obvious that before the teeth of the sliding frame leave one pair of short screws they will be engaged with the next pair, and thus the same effect will be produced as if long screws had been employed. The arrangement represented in fig. 524 is nearly the same as that employed in the Wasp. In the Wasp the screws are turned by means of ratchets, like the arrangement of ratchet brace used in engine factories, and a very ef Fig. 521.

Fig. 524.

389

fectual motive force is thus obtained with but little apparatus for that purpose. One man may work the handle of each ratchet backwards and forwards, and there will be no difficulty in making the two men, when thus employed, keep time so as to raise each side of the frame with the same speed.

Fig. 525 represents the arrangement employed for lifting the screw of the Dauntless. Here there are two long screws employed as in the Amphion, but they are turned by an apparatus resembling a winch, placed upon the deck. Both the screws and the frame are of brass, and the bearings of the short shaft which carries the screw are cased with brass to obviate corrosion by the sea water. A plate will be observed in these several figures interposed between the stern post and the end of the screw shaft, to receive the strain when the engines are reversed.

In fig. 526 is represented an arrangement employed by Messrs. Seaward for raising the screw, and it is very different from the rest,

ELEVATION OF APPARATUS FOR LIFTING SCREW OF AJAX.

Scale 1-eighth inch to 1 foot.

Fig. 523.

CATCH FOR SCREW OF AMPHION.

The water

as it operates upon the principle of hydraulic pressure. from a small pump, which is worked by any convenient means, is conducted beneath two plungers, upon the top of which the frame rests which carries the screw. These two plungers, however, instead of being made in the usual manner of the plungers of pumps, have two other smaller plungers within them, so that the combination resembles a spy glass, as one tube may be drawn out of the other in the same manner. Now, it will obviously happen, when the pressure of the water is applied, that the larger tubes will rise first, carrying the smaller tubes within them, and the ascent will continue until the limit of the ascent of the larger plungers is reached, when the smaller plungers will begin to ascend, and they will, in their turn, continue to rise until they come against the stop provided to restrain them from going too far, or until the necessary elevation of the screw is reached. The transverse section is made through the line A B. This mode of lifting the screw is not now much employed, but the usual mode is to have short screws fixed in the screw frame, which engage the teeth of racks attached to the stern and rudder posts, and ratchet palls are introduced to prevent the screw frame from falling down, should any of the tackle give way in the act of raising.