A Catechism of the Steam Engine
by John Bourne
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554. Q.—How is the position of the centre of pressure to be determined?

A.—With the foregoing assumption, which accords sufficiently with experiment to justify its acceptation, the position of the centre of pressure may be found by the following rule:—from the radius of the wheel substract the radius of the rolling circle; to the remainder add the depth of the paddle board, and divide the fourth power of the sum by four times the depth; from the cube root of the quotient subtract the difference between the radii of the wheel and rolling circle, and the remainder will be the distance of the centre of pressure from the upper edge of the paddle.

555. Q.—How do you find the diameter of the rolling circle?

A.—The diameter of the rolling circle is very easily found, for we have only to divide 5,280 times the number of miles per hour, by 60 times the number of strokes per minute, to get an expression for the circumference of the rolling circle, or the following rule may be adopted:—divide 88 times the speed of the vessel in statute miles per hour, by 3.1416 times the number of strokes per minute; the quotient will be the diameter in feet of the rolling circle. The diameter of the circle in which the centre of pressure moves or the effective diameter of the wheel being known, and also the diameter of the rolling circle, we at once find the excess of the velocity of the wheel over the vessel.

556. Q.—Will you illustrate these rules by an example?

A.—A steam vessel of moderately good shape, and with engines of 200 horses power, realises, with 22 strokes per minute, a speed of 10.62 miles per hour. To find the diameter of the rolling circle, we have 88 times 10.62, equal to 934.66, and 22 times 3.1416, equal to 69.1152; then 934.66 divided by 69.1152 is equal to 13.52 feet, which is the diameter of the rolling circle. The diameter of the wheel is 19 ft. 4 in., so that the diameter of the rolling circle is about 2/3ds of the diameter of the wheel, and this is a frequent proportion. The depth of the paddle board is 2 feet, and the difference between the diameters of the wheel and rolling circle will be 5.8133, which will make the difference of their radii 2.9067; and adding to this the depth of the paddle board, we have 4.9067, the fourth power of which is 579.64, which, divided by four times the depth of the paddle board, gives us 72.455, the cube root of which is 4.1689, which, diminished by the difference of the radii of the wheel and rolling circle, leaves 1.2622 feet for the distance of the centre of pressure from the upper edge of the paddle board in the case of light immersions. The radius of the wheel being 9.6667, the distance from the centre of the wheel to the upper edge of the float is 7.6667, and adding to this 1.2622, we get 8.9299 feet as the radius, or 17.8598 feet as the diameter of the circle in which the centre of pressure revolves. With 22 strokes per minute, the velocity of the centre of pressure will be 20.573 feet per second, and with 10.62 miles per hour for the speed of the vessel, the velocity of the rolling circle will be 15.576 feet per second. The effective velocity will be the difference between these quantities, or 4.997 feet per second. Now the height from which a body must fall by gravity, to acquire a velocity of 4.997 feet per second, is about .62 feet; and twice this height, or 1.24 feet, multiplied by 62-1/2, which is the number of Lbs. weight in a cubic foot of water, gives 77-1/2 Lbs. as the pressure on each square foot of the vertical paddle boards. As each board is of 20 square feet of area, and there is a vertical board on each side of the ship, the total pressure on the vertical paddle boards will be 2900 Lbs.

557. Q.—What pressure is this equivalent to on each square inch of the pistons?

A.—A vessel of 200 horses power will have two cylinders, each 50 inches diameter, and 5 feet stroke, or thereabout. The area of a piston of 50 inches diameter is 1963.5 square inches, so that the area of the two pistons is 3927 square inches, and the piston will move through 10 feet every revolution; and with 22 strokes per minute this will be 220 feet per minute, or 3.66 feet per second. Now, if the effective velocity of the centre of pressure and the velocity of the pistons had been the same, then a pressure of 2900 Lbs. upon the vertical paddles would have been balanced by an equal pressure on the pistons, which would have been in this case about .75 Lbs. per square inch; but as the effective velocity of the centre of pressure is 4.997 feet per second, while that of the pistons is only 3.66 feet per second, the pressure must be increased in the proportion of 4.997 to 3.66 to establish an equilibrium of pressure, or, in other words, it must be 1.02 Lbs. per square inch. It follows from this investigation, that, in radial wheels, the greater part of the engine power is distributed among the oblique floats.

558. Q.—How comes this to be the case?

A.—To understand how it happens that more power is expended upon the oblique than upon the vertical floats, it is necessary to remember that the only resistance upon the vertical paddle is that due to the difference of velocity of the wheel and the ship; but if the wheel be supposed to be immersed to its axle, so that the entering float strikes the water horizontally, it is clear that the resistance on such float is that due to the whole velocity of rotation; and that the resistance to the entering float will be the same whether the vessel is in motion or not. The resistance opposed to the rotation of any float increases from the position of the vertical float-where the resistance is that due to the difference of velocity of the wheel and vessel—until it reaches the plane of the axis, supposing the wheel to be immersed so far, where the resistance is that due to the whole velocity of rotation; and although in any oblique float the total resistance cannot be considered operative in a horizontal direction, yet the total resistance increases so rapidly on each side of the vertical float, that the portion of it which is operative in the horizontal direction, is in all ordinary cases of immersion very considerable. In the feathering wheel, where there is little of this oblique action, the resistance will be in the proportion of the square of the horizontal velocities of the several floats, which may be represented by the horizontal distances between them; and in the feathering wheel, the vertical float having the greatest horizontal velocity will have the greatest propelling effect.

559. Q.—Should the floats in feathering wheels enter and leave the water vertically?

A.—The floats should be so governed by the central crank or eccentric, that the entering and emerging floats have a direction intermediate between a radius and a vertical line.

560. Q.—Can you give any practical rules for proportioning paddle wheels?

A.—A common rule for the pitch of the floats is to allow one float for every foot of diameter of the wheel; but in the case of fast vessels a pitch of 2-1/2 feet, or even less, appears preferable, as a close pitch occasions less vibration. If the floats be put too close, however, the water will not escape freely from between them, and if set too far apart the stroke of the entering paddle will occasion an inconvenient amount of vibratory motion, and there will also be some loss of power. To find the proper area of a single float:—divide the number of actual horses power of both engines by the diameter of the wheel in feet; the quotient is the area of one paddle board in square feet proper for sea going vessels, and the area multiplied by 0.6 will give the length of the float in feet. In very sharp vessels, which offer less resistance in passing through the water, the area of paddle board is usually one-fourth less than the above proportion, and the proper length of the float may in such case be found by multiplying the area by 0.7. In sea going vessels about four floats are usually immersed, and in river steamers only one or two floats. There is more slip in the latter case, but there is also more engine power exerted in the propulsion of the ship, from the greater speed of engine thus rendered possible.

561. Q.—Then is it beneficial to use small floats?

A.—Quite the contrary. If to permit a greater speed of the engine the floats be diminished in area instead of being raised out of the water, no appreciable accession to the speed of the vessel will be obtained; whereas there will be an increased speed of vessel if the accelerated speed of the engine be caused by diminishing the diameter of the wheels. In vessels intended to be fast, therefore, it is expedient to make the wheels small, so as to enable the engine to work with a high velocity; and it is expedient to make such wheels of the feathering kind, to obviate loss of power from oblique action. In no wheel must the rolling circle fall below the water line, else the entering and emerging floats will carry masses of water before them. The slip is usually equal to about one-fourth of the velocity of the centre of pressure in well proportioned wheels; but it is desirable to have the slip as small as is possible consistently with the observance of other necessary conditions. The speed of the engine and also the speed of the vessel being fixed, the diameter of the rolling circle becomes at once ascertainable, and adding to this the slip, we have the diameter of the wheel.


562. Q.—Will you describe more in detail than you have yet done, the configuration and mode of action of the screw propeller?

A.—The ordinary form of screw propeller is represented in figs. 46 and 47; fig. 46 being a perspective view, and fig. 47 an end view, or view such as is seen when looking upon the end of the shaft. The screw here represented is one with two arms or blades. Some screws have three arms, some four and some six; but the screw with two arms is the most usual, and screws with more than three arms are not now much employed in this country. The screw on being put into revolution by the engine, preserves a spiral path in the water, in which it draws itself forward in the same way as a screw nail does when turned round in a piece of wood, whereas the paddle wheel more resembles the action of a cog wheel working in a rack.

563. Q.—But the screw of a steam vessel has no resemblance to a screw nail?

A.—It has in fact a very close resemblance if you suppose only a very short piece of the screw nail to be employed, and if you suppose, moreover, the thread of the screw to be cut nearly into the centre to prevent the wood from stripping. The original screw propellers were made with several convolutions of screw, but it was found advantageous to shorten them, until they are now only made one-sixth of a convolution in length.

564. Q.—And the pitch you have already explained to be the distance in the line of the shaft from one convolution to the next, supposing the screw to consist of two or more convolutions?

A.—Yes, that is what is meant by the pitch. If a thread be wound upon a cylinder with an equal distance between the convolutions, it will trace a screw of a uniform pitch; and if the thread be wound upon the cylinder with an increasing distance between each convolution, it will trace a screw of an increasing pitch. But two or more threads may be wound upon the cylinder at the same time, instead of a single thread. If two threads be wound upon it they will trace a double-threaded screw; if three threads be wound upon it they will trace a treble-threaded screw; and so of any other number. Now if the thread be supposed to be raised up into a very deep and thin spiral feather, and the cylinder be supposed to become very small, like the newel of a spiral stair, then a screw will be obtained of the kind proper for propelling vessels, except that only a very short piece of such screw must be employed. Whatever be the number of threads wound upon a cylinder, if the cylinder be cut across all the threads will be cut. A slice cut out of the cylinder will therefore contain a piece of each thread. But the threads, in the case of a screw propeller, answer to the arms, so that in every screw propeller the number of threads entering into the composition of the screw will be the same as the number of arms. An ordinary screw with two blades is a short piece of a screw of two threads.

565. Q.—In what part of the ship is the screw usually placed?

A.—In that part of the run of the ship called the dead wood, which is a thin and unused part of the vessel just in advance of the rudder. The usual arrangement is shown in fig. 48, which represents the application to a vessel of a species of screw which has the arms bent backwards, to counteract the centrifugal motion given to the water when there is a considerable amount of slip.

566. Q.—How is the slip in a screw vessel determined?

A.—By comparing the actual speed of the vessel with the speed due to the pitch and number of revolutions of the screw, or, what is the same thing, the speed which the vessel would attain if the screw worked in a solid nut. The difference between the actual speed and this hypothetical speed, is the slip.

567. Q.—In well formed screw propellers what is the amount of slip found to be?

A.—If the screw be properly proportioned to the resistance that the vessel has to overcome, the slip will not be more than 10 per cent., but in some cases it amounts to 30 per cent., or even more than this. In other cases, however, the slip is nothing at all, and even less than nothing; or, in other words the vessel passes through the water with a greater velocity than if the screw were working in a solid nut.

568. Q.—Then it must be by the aid of the wind or some other extraneous force?

A.—No; by the action of the screw alone.

569. Q.—But how is such a result possible?

A.—It appears to be mainly owing to the centrifugal action of the screw, which interposes a film or wedge of water between the screw itself and the water on which the screw reacts. This negative slip, as it is called, chiefly occurs when the pitch of the screw is less than its diameter, and when, consequently, the velocity of rotation is greater than if a coarser pitch had been employed. There is, moreover, in all vessels passing through the water with any considerable velocity, a current of water following the vessel, in which current, in the case of a screw vessel, the screw will revolve; and in certain cases the phenomenon of negative slip may be imputable in part to the existence of this current.

570. Q.—Is the screw propeller as effectual an instrument of propulsion as the radial or feathering paddle?

A.—In all cases of deep immersion it appears to be quite as effectual as the radial paddle, indeed, more so; but it is scarcely as effectual as the feathering paddle, with any amount of immersion, and scarcely as effectual as the common paddle in the case of light immersions.


571. Q.—Whether do you consider paddle or screw vessels to be on the whole the most advantageous?

A.—That is a large question, and can only receive a qualified answer. In some cases the use of paddles is indispensable, as, for example, in the case of river vessels of a limited draught of water, where it would not be possible to get sufficient depth below the water surface to enable a screw of a proper diameter to be got in.

572. Q.—But how does the matter stand in the case of ocean vessels?

A.—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.

573. Q.—But perhaps under such circumstances the speed of the screw vessel will be the greater of the two?

A.—No; the speed of the two vessels will 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.

574. Q.—What is the cause of this peculiarity?

A.—The cause 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.

575. Q.—If a screw and paddle vessel of the same mould and power be tied stern to stern, will not the screw vessel preponderate and tow the paddle vessel astern against the whole force of her engines?

A.—Yes, that will be so.

576. Q.—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, does not the preponderance of the screw vessel show that the screw must be the most powerful propeller?

A.—-No, it does not.

577. Q.—Seeing that the vessels are the same in all respects except as regards the propellers, and that one of them exhibits a superiority, does not this circumstance show that one propeller must be more powerful than the other?

A.—That does not follow necessarily, nor is it the fact in this particular case. All steam vessels when set into motion, will force themselves forward with an amount of thrust which, setting aside the loss from friction and from 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 afterward vary if a uniform pressure of steam be maintained. The thrusts, therefore, caused by their respective propelling instruments, 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.

578. Q.—How comes it then that the screw vessel preponderates?

A.—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 only be 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.

579. Q.—Can you state any facts in corroboration of this view?

A.—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.

580. Q.—Has the dynamometer been applied to paddle vessels?

A.—It 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.

581. Q.—Have any experiments been made to determine the comparative performances of screw and paddle vessels at sea?

A.—Yes, numerous experiments; of which the best known are probably those made on the screw steamer Rattler and the paddle steamer Alecto, each vessel 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.

582. Q.—Will you recapitulate some of the main incidents of these trials?

A.—I may first state some of the chief dimensions of the vessels. The Rattler is 176 feet 6 inches long, 32 feet 8-1/2 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 5-1/2 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 utilized 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.

583. Q.—How many trials of the vessels were made on this occasion?

A.—Twelve trials in all; but I 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 30-1/2 minutes past 2 the Rattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutes afterward 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 53.4 horses in favor 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 actually utilized—ascertained by multiplying the thrust in pounds by the space passed through by the vessel in feet per minute, and dividing by 33,000, was 247.8 horses power. This makes the ratio of the shaft to the engine power as 1 to 1.3, or, in other words, it shows that the amount of engine power utilized in propulsion was 77 per cent. In a subsequent trial made with the vessels running before the wind, but with no sails set and the masts struck, the speed realized by the Rattler was 10 knots per hour. The slip of the screw was 11.2 per cent. The actual power exerted by the engines of the Rattler was 368.8 horses. The actual power exerted by the engines of the Alecto was 291.7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4 cwt., 1 qr., 1 lb. The horse power of the shaft was 290.2 horses, and the ratio of the shaft to the engine power was 1 to 1.2. Here, therefore, the amount of the engine power utilized was 84 per cent.

584. Q.—If in any screw vessel the power of the engine be diminished by shutting off the steam or otherwise, you will then have a larger screw relatively with the power of the engine than before?


585. Q.—Was any experiment made to ascertain the effect of this modification?

A.—There was; but the result was not found to be better than before. The experiment was made by shutting off the steam from the engines of the Rattler until the number of strokes was reduced to 17 in the minute. The actual power was then 126.7 horses; thrust upon the shaft 2 tons, 2 cwt., 3 qrs., 14 lbs; horse power of shaft 88.4 horses; ratio of shaft to engine power 1 to 1.4; slip of the screw 18.7 per cent. In this experiment the power utilized was 71 per cent.

586. Q.—Was any experiment made to determine the relative performances in head winds?

A.—The trial in which this relation was best determined lasted for seven hours, and was made against a strong head wind and heavy head sea. The speed of the Rattler by patent log was 4.2 knots; and at the conclusion of the trial the Alecto had the advantage by about half a mile. Owing to an accidental injury to the indicator, the power exerted by the engines of the Rattler in this trial could not be ascertained; but judging from the power exerted in other experiments with the same number of revolutions, it appears probable that the power actually exerted by the Rattler was about 300 horses. The number of strokes per minute made by the engines of the Rattler was 22, whereas in the Alecto the number of strokes per minute was only 12; so that while the engines of the Alecto were reduced, by the resistance occasioned by a strong head wind, to nearly half their usual speed, the engines of the Rattler were only lessened about one twelfth of their usual speed. The mean thrust upon the screw shaft during this experiment, was 4 tons, 7 cwt., 0 qr., 16 lbs. The horse power of the shaft was 125.9 horses, and the slip of the screw was 56 per cent. Taking the power actually exerted by the Rattler at 300 horses, the power utilized in this experiment is only 42 per cent.

587. Q.—What are the dimensions of the screw in the Rattler?

A.—Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet. The foregoing experiments show that with a larger screw a better average performance would be obtained. The best result arrived at, was when the vessel was somewhat assisted by the wind, which is equivalent to a reduction of the resistance of the hull, or to a smaller hull, which is only another expression for a larger proportionate screw.

588. Q.—When you speak of a larger screw, what increase of dimension do you mean to express?

A.—An increase of the diameter. The amount of reacting power of the screw upon the water is hot measured by the number of square feet of surface of the arms, but by the area of the disc or circle in which the screw revolves. The diameter of the screw of the Rattler being 10 feet, the area of its disc is 78.5 square feet; and with the amount of thrust already mentioned as existing in the first experiment, viz. 8722 lbs., the reacting pressure on each square foot of the screw's disc will be 108-1/2 lbs. The immersed midship section being 380 square feet, this is equivalent to 23 lbs. per square foot of immersed midship section at a speed of 9.2 knots per hour.

589. Q.—In smaller vessels of similar form, will the resistance per square foot of midship section be more than this?

A.—It will be considerably more. In the Pelican, a vessel of 109-3/4 square feet of midship section, I estimate the resistance per square foot of midship section at 30 lbs., when the speed of the vessel is 9.7 knots per hour. In the Minx with an immersed midship section of 82 square feet, the resistance per square foot of immersed midship section was found by the dynamometer to be 41 lbs. at a speed of 8-1/2 knots; and in the Dwarf, a vessel with 60 square feet of midship section, I estimate the resistance per square foot of midship section at 46 lbs. at a speed of 9 knots per hour, which is just double the resistance per square foot of the Rattler. The diameter of the screw of the Minx is 4-1/2 feet, so that the area of its disc is 15.9 square feet, and the area of immersed midship section is about 5 times greater than that of the screw's disc. The diameter of the screw of the Dwarf is 5 feet 8 inches, so that the area of its disc is 25.22 square feet, and the area of immersed midship section is 2.4 times greater than that of the screw's disc. The pressure per square foot of the screw's disc is 214 lbs. in the case of the Minx, and 109-1/2 lbs. in the case of the Dwarf.

590. Q.—From the greater proportionate resistance of small vessels, will not they require larger proportionate screws than large vessels?

A.—They will.

591. Q.—Is there any ready means of predicting what the amount of thrust of a screw will be?

A.—When we know the amount of pressure on the pistons, and the velocity of their motion relatively with the velocity of advance made by the screw, supposing it to work in a solid nut, it is easy to tell what the thrust of the screw would be if it were cleared of the effects of friction and other irregular sources of disturbance. The thrust, in fact, would be at once found by the principle of virtual velocities; and if we take this theoretical thrust and diminish it by one fourth to compensate for friction and lateral slip, we shall have a near approximation to the amount of thrust that will be actually exerted.[1]

[1] See Treatise on the Screw Propeller, by J. Bourne, C. E.


592. Q.—What species of screw do you consider the best?

A.—In cases in which a large diameter of screw can be employed, the ordinary screw or helix with two blades seems to be as effective as any other, and it is the most easily constructed. If, however, the screw is restricted in diameter, or if the vessel is required to tow, or will have to encounter habitually strong head winds, it will be preferable to employ a screw with an increasing pitch, and also of such other configuration that it will recover from the water some portion of the power that has been expended in slip.

593. Q.—How can this be done?

A.—There are screws which are intended to accomplish, this object already in actual use. When there is much slip a centrifugal velocity is given to the water, and the screw, indeed, if the engine be set on when the vessel is at rest, acts very much as a centrifugal fan would do if placed in the same situation. The water projected outward by the centrifugal force escapes in the line of least resistance, which is to the surface; and if there be a high column of water over the screw, or, in other words, if the screw is deeply immersed, then the centrifugal action is resisted to a greater extent, and there will be less slip produced. The easiest expedient, therefore, for obviating loss by slip is to sink the screw deeply in the water; but as there are obvious limits to the application of this remedy, the next best device is to recover and render available for propulsion some part of the power which has been expended in giving motion to the water. One device for doing this consists in placing the screw well forward in the dead wood, so that it shall be overhung by the stern of the ship. The water forced upward by the centrifugal action of the screw will, by impinging on the overhanging stern, press the vessel forward in the water, just in the same way as is done by the wind when acting on an oblique sail. I believe, the two revolving vanes without any twist or obliquity on them at all, would propel a vessel if set well forward in the dead wood or beneath the bottom, merely by the ascent of the water up the inclined plane of the vessel's run; and, at all events, a screw so placed would, in my judgment, aid materially in propelling the vessel when her progress was resisted by head winds.

594. Q.—But you said there are some kinds of screws which profess to accomplish this?

A.—There are screws which profess to counteract the centrifugal velocity given to the water by imparting to it an equal centripetal force, the consequence of which will be, that the water projected backward by the screw, instead of taking the form of the frustum of a cone, with its small end next the screw, will take the form of a cylinder. One of these forms of screw is that patented by the Earl of Dundonald in 1843, and which is represented in fig. 49. Another is the form of screw already represented in fig. 48, and which was patented by Mr. Hodgson in 1844. Mr. Hodgson bends the arms of his propellers backward, not into the form of a triangle, but into the form of a parabola, to the end that the impact of the screw on the particles of the water may cause them to converge to a focus, as 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 screw bent back into the form of a triangle, if the pitch be suitably varied between the centre and the circumference.

595. Q.—Then the pitch may be varied in two ways?

A.—Yes: 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 lower 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.

596. Q.—Is there any advantage derived from these forms of screws?

A.—There is a slight advantage, but it is so slight as hardly to balance the increased trouble of manufacture, and, consequently, they are not generally or widely adopted.

597. Q.—What other kinds of screw are there proposing to themselves the same or similar objects?

A.—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. Then 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.

598. Q.—Will you describe the configuration and action of Holm's screw?

A.—First, then, the screw increases in the direction of its length, and this increase is very rapid at the following edge, so 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 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.

599. Q.—What is the nature of Beattle's screw?

A.—Beattie's screw 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.

600. Q.—When the diameter of the cylinder of water projected backward by a screw, and the force urging it into motion are known, may not the velocity it will acquire be approximately determined?

A.—That will not be very difficult; and I will 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 feet, then the distance advanced by the vessel, when the speed is 8.445 knots, will be 3.7 feet 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 1.087 feet; and a weight of 214 lbs. put into motion by gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity of 1.087 feet 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 3.7 cubic feet 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 feet per revolution, or we may say it will be in round numbers 1 foot per revolution. This, added to the progress of the vessel, will make the distance advanced by the screw through the water 4.7 feet per revolution, leaving the difference between this and the pitch, namely 1.13 feet, 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.

601. Q.—Is it not important to make the stern of screw vessels very fine, with the view of diminishing the slip, and increasing the speed?

A.—It is most important. 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 6-1/2 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.[1]

[1] See Treatise on the Screw Propeller, by John Bourne, C. E.


602. Q.—In fixing upon the proportions of a screw proper to propel any given vessel, how would you proceed?

A.—I would first compute the probable resistance of the vessel, and I would 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.

603. Q.—Will you illustrate this doctrine by a practical example?

A.—The French screw steamer Pelican was fitted successively with two screws of four blades, but the diameter of the first screw was 98.42 inches, and the diameter of the second 54 inches. If the efficiency of the first screw by 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.

604. Q—Were screws of four blades found to be more efficient than screws with two?

A—They were found to have less slip, but not to be more efficient, the increased slip in those of two 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.

605. Q.—Are the proportions found to be most suitable in the case of the Pelican applicable to the screws of other vessels?

A.—Only to those 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 380/100 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.

606. Q.—Will you specify the nature of their deductions?

A.—I will first enumerate those 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 1.5, 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.

607. Q.—What are the corresponding proportions of screws of four blades?

A.—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, 1.933, 2.080, and 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.

608. Q.—And what are the corresponding proportions proper for screws of six blades?

A.—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, 1.771, 1.891, 1.2009, 2.131, 2.262, 2.416, 2.600, 2.804; and the respective fractions of pitch will be 0.794, 0.749, 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.[1]

[1] In my Treatise on the Screw Propeller I have gone into these various questions more fully than would consort with the limits of this publication.


609. Q.—Do you consider that the screw propeller is best adapted for vessels of full power, or for vessels with auxiliary power?

A.—It is, in my opinion, best adapted for vessels with auxiliary power, and it is a worse propeller than paddle wheels for vessels which have habitually to encounter strong head winds. Screw vessels are but ill calculated—at least as constructed heretofore—to encounter head winds, and the legitimate sphere of the screw is in propelling vessels with auxiliary power.

610. Q.—Does the screw act well in conjunction with sails?

A.—I cannot say it acts better than paddles, except in so far as it is less in the way and is less affected by the listing or heeling over of the ship. A small steam power, however, acts very advantageously in aid of sails, for not only does the operation of the sails in reducing the resistance of the hull virtually increase the screw's diameter, but the screw, by reducing the resistance which has to be overcome by the sails and by increasing the speed of the vessel, enables the sails to act with greater efficiency, as the wind will not rebound from them with as great a velocity as it would otherwise do, and a larger proportion of the power of the wind will also be used up. In the case of beam winds, moreover, the action of the screw, by the larger advance it gives to the vessel will enable the sails to intercept a larger column of wind in a given time. It appears, therefore, that the sails add to the efficiency of the screw, and that the screw also adds to the efficiency of the sails.

611. Q.—What is the comparative cost of transporting merchandise in paddle steamers of full power, in screw steamers of auxiliary power, and in sailing ships?

A.—That will depend very much upon the locality where the comparison is made. In the case of vessels performing distant ocean voyages, in which they may reckon upon the aid of uniform and constant winds, such as the trade winds or the monsoon, sailing ships of large size will be able to carry more cheaply than any other species of vessel. But where the winds are irregular and there is not much sea room, or for such circumstances as exist in the Channel or Mediterranean trades, screw vessels with auxiliary power will constitute the cheapest instrument of conveyance.

612. Q.—Are there any facts recorded illustrative of the accuracy of this conclusion?

A.—A full paddle vessel of 1000 tons burden and 350 horses power, will carry about 400 tons of cargo, besides coal for a voyage of 500 miles, and the expense of such a voyage, including wear and tear, depreciation, &c., will be about 190l. The duration of the voyage will be about 45-1/2 hours. A screw vessel of 400 tons burden and 100 horses power, will carry the same amount of cargo, besides her coals, on the same voyage, and the expense of the voyage, including wear and tear, depreciation, &c., will be not much more than 60l. An auxiliary screw vessel, therefore, can carry merchandise at one third of the cost of a full-powered paddle vessel. By similar comparisons made between the expense of conveying merchandise in auxiliary screw steamers and sailing ships on coasting voyages, it appears that the cost in screw steamers is about one third less than in the sailing ships; the greater expedition of the screw steamers much more than compensating for the expense which the maintenance of the machinery involves.


613. Q.—Would not a screw combined with paddles act in a similarly advantageous way as a screw or paddles when aided by the wind?

A.—If in any given paddle vessel a supplementary screw be added to increase her power and speed, the screw will act in a more beneficial manner than if it had the whole vessel to propel itself, and for a like reason the paddles will act in a more beneficial manner. There will be less slip both upon the paddles and upon the screw than if either had been employed alone; but the same object would be attained by giving the vessel larger paddles or a larger screw.

614. Q.—Have any vessels been constructed with combined screw and paddles?

A.—Not any that I know of, except the great vessel built under the direction of Mr. Brunel. The Bee many years since was fitted with both screw and paddles, but this was for the purpose of ascertaining the relative efficiency of the two modes of propulsion, and not for the purpose of using both together.

615. Q.—What would be the best means of accelerating the speed of a paddle vessel by the introduction of a supplementary screw?

A.—If the vessel requires new boilers, the best course of procedure would be to work a single engine giving motion to the screw with high pressure steam, and to let the waste steam from the high pressure engine work the paddle engines. In this way the power might be doubled without any increased expenditure of fuel per hour, and there would be a diminished expenditure per voyage in the proportion of the increased speed.

616. Q.—What would the increased speed be by doubling the power?

A.—The increase would be in the proportion of the cube root of 1 to the cube root of 2, or it would be 1.25 times greater. If, therefore, the existing speed were 10 miles, it would be increased to 12-1/2 miles by doubling the power, and the vessel would ply with about a fourth less coals by increasing the power in the manner suggested.

617. Q.—Is not high pressure steam dangerous in steam vessels?

A.—Not necessarily so, and it has now been introduced into a good number of steam vessels with satisfactory results. In the case of locomotive engines, where it is used so widely, very few accidents have occurred; and in steam vessels the only additional source of danger is the salting of the boiler. This may be prevented either by the use of fresh water in the boiler, or by practising a larger amount of blowing off, to insure which it should be impossible to diminish the amount of water sent into the boiler by the feed pump, and the excess should be discharged overboard through a valve near the water level of the boiler, which valve is governed by a float that will rise or fall with the fluctuating level of the water. If the float be a copper ball, a little water should be introduced into it before it is soldered or brazed up, which will insure an equality of pressure within and without the ball, and a leakage of water into it will then be less likely to take place. A stone float, however, is cheaper, and if properly balanced will be equally effective. All steam vessels should have a large excess of boiling feed water constantly flowing into the boiler, and a large quantity of water constantly blowing off through the surface valves, which being governed by floats will open and let the superfluous water escape whenever the water level rises too high. In this way the boiler will be kept from salting, and priming will be much less likely to occur. The great problem of steam navigation is the economy of fuel, since the quantity of fuel consumed by a vessel will very much determine whether she is profitable or otherwise. Notwithstanding the momentous nature of this condition, however, the consumption of fuel in steam vessels is a point to which very little attention has been paid, and no efficient means have yet been adopted in steam vessels to insure that measure of economy which is known to be attainable, and which has been attained already in other departments of engineering in which the benefits of such economy are of less weighty import. It needs nothing more than the establishment of an efficient system of registration in steam vessels, to insure a large and rapid economy in the consumption of fuel, as this quality would then become the test of an engineer's proficiency, and would determine the measure of his fame. In the case of the Cornish engines, a saving of more than half the fuel was speedily effected by the introduction of the simple expedient of registration. In agricultural engines a like economy has speedily followed from a like arrangement; yet in both of these cases the benefits of a large saving are less eminent than they would be in the case of steam navigation; and it is to be hoped that this expedient of improvement will now be speedily adopted.



* * * * *


618. Q.—Will you describe the structure of an oscillating engine as made by Messrs. Penn?

A.—To do this it will be expedient to take an engine of a given power, and then the sizes may be given as well as an account of the configuration of the parts: we may take for an example a pair of engines of 21-1/2 inches diameter of cylinder, and 22 inches stroke, rated by Messrs. Penn at 12 horses power each. The cylinders of this oscillating engine are placed beneath the cranks, and, as in all Messrs. Penn's smaller engines, the piston rod is connected to the crank pin by means of a brass cap, provided with a socket, by means of which it is cuttered to the piston rod. There is but one air pump, which is situated within the condenser between the cylinders, and it is wrought by means of a crank in the intermediate shaft—this crank being cut out of a solid piece of metal as in the formation of the cranked axles of locomotive engines. The steam enters the cylinder through the outer trunnions, or the trunnions adjacent to the ship's sides, and enters the condenser through the two midship trunnions—a short three ported valve being placed on the front of the cylinder to regulate the flow of steam to and from the cylinder in the proper manner. The weight of this valve on one side of the cylinder is balanced by a weight hung upon the other side of the cylinder; but in the most recent engines this weight is discarded, and two valves are used, which balance one another. The framing consists of an upper and lower frame of cast iron, bound together by eight malleable iron columns: upon the lower frame the pillow blocks rest which carry the cylinder trunnions, and the condenser and the bottom frame are cast in the same piece. The upper frame supports the paddle shaft pillow blocks; and pieces are bolted on in continuation of the upper frame to carry the paddle wheels, which are overhung from the journal.

619. Q.—What are the dimensions and arrangement of the framing?

A.—The web, or base plate of the lower frame is 3/4 of an Inch thick, and a cooming is earned all round the cylinder, leaving an opening of sufficient size to permit the necessary oscillation. The cross section of the upper frame is that of a hollow beam 6 inches deep, and about 3-1/2 inches wide, with holes at the sides to take out the core; and the thickness of the metal is 13/16ths of an inch. Both the upper and the lower frame is cast in a single piece, with the exception of the continuations of the upper frame, which support the paddle wheels. An oval ring 3 inches wide is formed in the upper frame, of sufficient size to permit the working of the air pump crank; and from this ring feathers run to the ends of the cross portions of the frame which supports the intermediate shaft journals. The columns are 1-1/2 inches in diameter; they are provided with collars at the lower ends, which rest upon bosses in the lower frame, and with collars at the upper ends for supporting the upper frame; but the upper collars of two of the corner columns are screwed on, so as to enable the columns to be drawn up when it is required to get the cylinders out. The cross section of the bottom frame is also of the form of a hollow beam, 7 inches deep, except in the region of the condenser, where it is, of course, of a different form. The depth of the boss for the reception of the columns is a little more than 7 inches deep on the lower frame, and a little more than 6 inches deep on the upper frame; and the holes through them are so cored out, that the columns only bear at the upper and lower edges of the hole, instead of all through it—a formation by which the fitting of the columns is facilitated.

620. Q.—What are the dimensions of the condenser?

A.—The condenser, which is cast upon the lower frame, consists of an oval vessel 22-1/2 inches wide, by 2 feet 4-1/4 inches long, and 1 foot 10-1/2 inches deep; it stands 9 inches above the upper face of the bottom frame, the rest projecting beneath it; and it is enlarged at the sides by being carried beneath the trunnions.

621. Q.—What are the dimensions of the air pump?

A.—The air pump, which is set in the centre of the condenser, is 15-1/4 inches in diameter, and has a stroke of 11 inches. The foot valve is situated in the bottom of the air pump, and its seat consists of a disc of brass, in which there is a rectangular flap valve, opening upwards, but rounded on one side to the circle of the pump, and so balanced as to enable the valve to open with facility. The balance weight, which is formed of brass cast in the same piece as the valve itself, operates as a stop, by coming into contact with the disc which constitutes the bottom of the pump; the disc being recessed opposite to the stop to enable the valve to open sufficiently. This disc is bolted to the barrel of the pump by means of an internal flange, and before it can be removed the pump must be lifted out of its place. The air pump barrel is of brass to which is bolted a cast iron mouth piece, with a port for carrying the water to the hot well; within the hot well the delivery valve, which consists of a common flap valve, is situated. The mouth piece and the air pump barrel are made tight to the condenser, and to one another, by means of metallic joints carefully scraped to a true surface, so that a little white or red lead interposed makes an air tight joint. The air pump bucket is of brass, and the valve of the bucket is of the common pot lid or spindle kind. The injection water enters through a single cock in front of the condenser—the jet striking against the barrel of the air pump. The air pump rod is maintained in its vertical position by means of guides, the lower ends of which are bolted to the mouth of the pump, and the upper to the oval in the top frame, within which the air pump crank works; and the motion is communicated from this crank to the pump rod by means of a short connected rod. The lower frame is not set immediately below the top frame, but 2-1/2 inches behind it, and the air pump and condenser are 2-1/2 inches nearer one edge of the lower frame than the other.

622. Q.—What are the dimensions of the cylinder?

A.—The thickness of the metal of the cylinder is 9/16ths of an inch; the depth of the belt of the cylinder is 9-1/2 inches, and its greatest projection from the cylinder is 2-1/2 inches. The distance from the lower edge of the belt to the bottom of the cylinder is 11-1/2 inches, and from the upper edge of the belt to the top flange of the cylinder is 9 inches. The trunnions are 7-1/4 inches diameter in the bearings, and 3-1/2 inches in width; and the flanges to which the glands are attached for screwing in the trunnion packings are 1-1/2 inch thick, and have 7/8ths of an inch of projection. The width of the packing space round the trunnions is 5/8ths of an inch, and the diameter of the pipe passing through the trunnion 4-5/8ths, which leaves 11/16ths for the thickness of the metal of the bearing. Above and below each trunnion a feather runs from the edge of the belt or bracket between 3 and 4 inches along the cylinder, for the sake of additional support; and in large engines the feather is continued through the interior of the belt, and cruciform feathers are added for the sake of greater stiffness. The projection of the outer face of the trunnion flange from the side of the cylinder is 6-1/2 inches; the thickness of the flange round the mouth of the cylinder is 3/4 of an inch, and its projection 1-3/8 inch; the height of the cylinder stuffing box above the cylinder cover is 4-1/8 inches, and its external diameter 4-3/8 inches—the diameter of the piston rod being 2-1/8 inches. The thickness of the stuffing box flange is 1-1/8 inch.

623. Q.—Will you describe the nature of the communication between the cylinder and condenser?

A.—The pipe leading to the condenser from the cylinder is made somewhat bell mouthed where it joins the condenser, and the gland for compressing the packing is made of a larger internal diameter in every part except at the point at which the pipe emerges from it, where it accurately fits the pipe so as to enable the gland to squeeze the packing. By this construction the gland may be drawn back without being jammed upon the enlarged part of the pipe; and the enlargement of the pipe toward the condenser prevents the air pump barrel from offering any impediment to the free egress of the steam. The gland is made altogether in four pieces: the ring which presses the packing is made distinct from the flange to which the bolts are attached which force the gland against the packing, and both ring and flange are made in two pieces, to enable them to be got over the pipe. The ring is half checked in the direction of its depth, and is introduced without any other support to keep the halves together, than what is afforded by the interior of the stuffing box; and the flange is half checked in the direction of its thickness, so that the bolts which press down the ring by passing through this half-checked part, also keep the segments of the flange together. The bottom of the trunnion packing space is contracted to the diameter of the eduction pipe, so as to prevent the packing from being squeezed into the jacket; but the eduction pipe does not fit quite tight into this contracted part, but, while in close contact on the lower side, has about 1/32nd of an inch of space between the top of the pipe and the cylinder, so as to permit the trunnions to wear to that extent without throwing a strain upon the pipe. The eduction pipe is attached to the condenser by a flange joint, and the bolt holes are all made somewhat oblong in the perpendicular direction, so as to permit the pipe to be slightly lowered, should such an operation be rendered necessary by the wear of the trunnion bearings; but in practice the wear of the trunnion bearings is found to be so small as to be almost inappreciable.

624. Q.—Will you describe the valve and valve casing?

A.—The length of the valve casing is 16-1/2 inches, and its projection from the cylinder is 3-1/2 inches at the top, 4-1/4 inches at the centre, and 2-1/2 inches at the bottom, so that the back of the valve casing is not made flat, but is formed in a curve. The width of the valve casing is 9 inches, but there is a portion of the depth of the belt 1-1/2 inch wider, to permit the steam to enter from the belt into the casing. The valve casing is attached to the cylinder by a metallic joint; the width of the flange of this joint is 1-1/4 inch, the thickness of the flange on the casing 1/2 inch, and the thickness of the flange on the cylinder 5/8ths of an inch. The projection from the cylinder of the passage for carrying the steam upwards, and downwards, from the valve to the top and bottom of the cylinder, is 2-1/4 inches, and its width externally 8-5/8 inches. The valve is of the ordinary three ported description, and both cylinder and valve faces are of cast iron.

625. Q.—What description of piston is used?

A.—The piston is packed with hemp, but the junk ring is made of malleable iron, as cast iron junk rings have been found liable to break: there are four plugs screwed into the cylinder cover, which, when removed, permit a box key to be introduced, to screw down the piston packing. The screws in the junk ring are each provided with a small ratchet, cut in a washer fixed upon the head, to prevent the screw from turning back; and the number of clicks given by these ratchets, in tightening up the bolts, enables the engineer to know when they have all been tightened equally. In more recent engines, and especially in those of large size, Messrs. Penn employ for the piston packing a single metallic ring with tongue piece and indented plate behind the joint; and this ring is packed behind with hemp squeezed by the junk ring as in ordinary hemp-packed pistons.

626. Q.—Will you describe the construction of the cap for connecting the piston rod with the crank pin?

A.—The cap for attaching the piston rod to the crank pin, is formed altogether of brass, which brass serves to form the bearing of the crank pin. The external diameter of the socket by which this cap is attached to the piston rod is 3-5/16 inches. The diameter of the crank pin is 3 inches, and the length of the crank pin bearing 3-7/8 inches. The thickness of the brass around the crank pin bearing is 1 inch, and the upper portion of the brass is secured to the lower portion, by means of lugs, which are of such a depth that the perpendicular section through the centre of the bearing has a square outline measuring 7 inches in the horizontal direction, 3-7/8 inches from the centre of the pin to the level of the top of the lugs, and 2-1/2 inches from the centre of the pin to the level of the bottom of the lugs. The width of the lugs is 2 inches, and the bolts passing through them are 1-1/4 inch in diameter. The bolts are tapped into the lower portion of the cap, and are fitted very accurately by scraping where they pass through the upper portion, so as to act as steady pins in preventing the cover of the crank pin bearing from being worked sideways by the alternate thrust on each side. The distance between the centres of the bolts is 5 inches, and in the centre of the cover, where the lugs, continued in the form of a web, meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, and provided with an internal pipe, is cast upon the cover, to contain oil for the lubrication of the crank pin bearing. The depth of the cutter for attaching the cap to the piston rod is 1-1/4 inch and its thickness is 3/8ths of an inch.

627. Q.—Will you describe the means by which the air pump rod is connected with the crank which works the air pump?

A.—A similar cap to that of the piston rod attaches the air pump crank to the connecting rod by which the air pump rod is moved, but in this instance the diameter of the bearing is 5 inches, and the length of the bearing is about 3 inches. The air pump connecting rod and cross head are shown in perspective in fig. 50. The thickness of the brass encircling the bearing of the shaft is three fourths of an inch upon the edge, and 1-1/8 inch in the centre, the back being slightly rounded; the width of the lugs is 1-5/8 inch, and the depth of the lugs is 2 inches upon the upper brass, and 2 inches upon the lower brass, making a total depth of 4 inches. The diameter of the bolts passing through the lugs is 1 inch, and the bolts are tapped into the lower brass, and accurately fitted into the upper one, so as to act as steady pins, as in the previous instance. The lower eye of the connecting rod is forked, so as to admit the eye of the air pump rod; and the pin which connects the two together is prolonged into a cross head, as shown in fig. 50. The ends of this cross head move in guides. The forked end of the connecting rod is fixed upon the cross head by means of a feather, so that the cross head partakes of the motion of the connecting rod, and a cap, similar to that attached to the piston rod, is attached to the air pump rod, for connecting it with the cross head. The diameter of the air pump rod is 1-1/2 inch, the external diameter of the socket encircling the rod is 2-1/8 inches, and the depth of the socket 4-1/2 inches from the centre of the cross head. The depth of the cutter for attaching the socket to the rod is 1 inch, and its thickness 5/16 inch. The breadth of the lugs is 1-3/8 inch, the depth 1-1/4 inch, making a total depth of 2-1/2 inches; and the diameter of the bolts seven eighths of an inch. The diameter of the cross head at the centre is 2 inches, the thickness of each jaw around the bearing 1 inch, and the breadth of each 9/16 inch.

628. Q.—What are the dimensions of the crank shaft and cranks?

A.—The diameter of the intermediate shaft journal is 4-3/16 inches, and of the paddle shaft journal 4-3/8 inches; the length of the journal in each case is 5 inches. The diameter of the large eye of the crank is 7 inches, and the diameter of the hole through it is 4-3/8 inches; the diameter of the small eye of the crank is 5-1/4 inches, the diameter of the hole through it being 3 inches. The depth of the large eye is 4-1/4 inches, and of the small eye 3-3/4 inches; the breadth of the web is 4 inches at the shaft end, and 3 inches at the pin end, and the thickness of the web is 2-5/8 inches. The width of the notch forming the crank in the intermediate shaft for working the air pump is 3-1/2 inches, and the width of each of the arms of this crank is 3-15/16 inches. Both the outer and inner corners of the crank are chamfered away, until the square part of the crank meets the round of the shaft. The method of securing the cranks pins into the crank eyes of the intermediate shaft consists in the application of a nut to the end of each pin, where it passes through the eye, the projecting end of the pin being formed with a thread upon which the nut is screwed.

629. Q.—Will you describe the eccentric and eccentric rod?

A.—The eccentric and eccentric rod are shown in fig. 51. The eccentric is put on the crank shaft in two halves, joined in the diameter of largest eccentricity by means of a single bolt passing through lugs on the central eye, and the back balance is made in a separate piece five eighths of an inch thick, and is attached by means of two bolts, which also help to bind the halves of the eccentric together. The eccentric strap is half an inch thick, and 1-1/4 inch broad, and the flanges of the eccentric, within which the strap works, are each three eighths of an inch thick. The eccentric rod is attached to the eccentric hoop by means of two bolts passing through lugs upon the rod, and tapped into a square boss upon the hoop; and pieces of iron, of a greater or less thickness, are interposed between the surfaces in setting the valve, to make the eccentric rod of the right length. The eccentric rod is kept in gear by the push of a small horizontal rod, attached to a vertical blade spring, and it is thrown out of gear by means of the ordinary disengaging apparatus, which acts in opposition to the spring, as, in cases where the eccentric rod is not vertical, it acts in opposition to the gravity of the rod.

630. Q.—Will you explain in detail the construction of the valve gearing, or such parts of it as are peculiar to the oscillating engine?

A.—The eccentric rod is attached by a pin, 1 inch in diameter, to an open curved link or sector with a tail projecting upward and passing through an eye to guide the link in a vertical motion. The link is formed of iron case-hardened, and is 2-3/4 inches deep at the middle, and 2-3/8 inches deep at the ends, and 1 inch broad. The opening in the link, which extends nearly its entire length, is 1-5/16 inch broad; and into this opening a brass block 2 inches long is truly fitted, there being a hole through the block 3/4 inch diameter, for the reception of the pin of the valve shaft lever. The valve shaft is 1-3/4 inch diameter at the end next the link or segment, and diminishes regularly to the other end, but its cross section assumes the form of an octagon in its passage round the cylinder, measuring mid-way 1-1/4 inch deep, by about 3/4 inch thick, and the greatest depth of the finger for moving the valve is about 1 inch. The depth of the lever for moving the valve shaft is 2 inches at the broad, and 1-1/4 inch at the narrow end. The internal breadth of the mortice in which the valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch, which leaves three eighths of an inch as the thickness of metal round the hole; and the breadth, measuring in the direction of the hole, is 1-1/2 inch. The valve rod is three fourths of an inch in diameter, and the mortice is connected to the valve rod by a socket 1 inch long, and 1-1/8 inch diameter, through which a small cutter passes. A continuation of the rod, eleven sixteenths of an inch diameter, passes upward from the mortice, and works through an eye, which serves the purpose of a guide. In addition to the guide afforded to the segment by the ascending tail, it is guided at the ends upon the columns of the framing by means of thin semicircular brasses, 4 inches deep, passing round the columns, and attached to the segment by two 3/8 inch bolts at each end, passing through projecting feathers upon the brasses and segment, three eighths of an inch in thickness. The curvature of the segment is such as to correspond with the arc swept from the centre of the trunnion to the centre of the valve lever pin when the valve is at half stroke as a radius; and the operation of the segment is to prevent the valve from being affected by the oscillation of the cylinder, but the same action, would be obtained by the employment of a smaller eccentric with more lead. In some engines the segment is not formed in a single piece, but of two curved blades, with blocks interposed at the ends, which may be filed down a little, to enable the sides of the slot to be brought nearer, as the metal wears away.

631. Q.—What kind of plummer blocks are used for the paddle shaft bearings?

A.—The paddle shaft plummer blocks are altogether of brass, and are formed in much the same manner as the cap of the piston rod, only that the sole is flat, as in ordinary plummer blocks, and is fitted between projecting lugs of the framing, to prevent side motion. In the bearings fitted on this plan, however, the upper brass will generally acquire a good deal of play after some amount of wear. The bolts are worked slack in the holes, though accurately fitted at first; and it appears expedient, therefore, either to make the bolts very large, and the sockets through which they pass very deep, or to let one brass fit into the other.

632. Q.—How are the trunnion plummer blocks made?

A.—The trunnion plummer blocks are formed in the same manner as the crank shaft plummer blocks; the nuts are kept from turning back by means of a pinching screw passing through a stationary washer. It is not expedient to cast the trunnion plummer blocks upon the lower frame, as is sometimes done; for the cylinders, being pressed from the steam trunnions by the steam, and drawn in the direction of the condenser by the vacuum, have a continual tendency to approach one another; and as they wear slightly toward midships, there would be no power of readjustment unless the plummer blocks were movable. The flanges of the trunnions should always fit tight against the plummer block sides, but there should be a little play sideways at the necks of the trunnions, so that the cylinder may be enabled to expand when heated, without throwing an undue strain upon the trunnion supports.

633. Q.—What kind of paddle wheel is supplied with these oscillating engines?

A.—The wheels are of the feathering kind, 9 feet 8 inches in diameter, measuring to the edges of the floats; and there are 10 floats upon each wheel, measuring 4 feet 6 inches long each, and 18-1/2 inches broad. There are two sets of arms to the wheel, which converge to a cast iron centre, formed like a short pipe with large flanges, to which the arms are affixed. The diameter of the shaft, where the centre is put on, is 4-1/2 inches, the external diameter of the pipe is 8 inches, and the diameter of the flanges is 20 inches, and their thickness 1-1/4 inch. The flanges are 12 inches asunder at the outer edge, and they partake of the converging direction of the arms. The arms are 2-1/4 inches broad and half an inch thick; the heads are made conical, and each is secured into a recess upon the side of the flange by means of three bolts. The ring which connects together the arms, runs round at a distance of 3 feet 6 inches from the centre, and the projecting ends of the arms are bent backward the length of the lever which moves the floats, and are made very wide and strong at the point where they cross the ring, to which they are attached by four rivets. The feathering action of the floats is accomplished by means of a pin fixed to the interior of the paddle box, set 3 inches in advance of the centre of the shaft, and in the same horizontal line. This pin is encircled by a cast iron collar, to which rods are attached 1-3/8 inch diameter in the centre, proceeding to the levers, 7 inches long, fixed on the back of the floats in the line of the outer arms. One of these rods, however, is formed of nearly the same dimensions as one of the arms of the wheel, and is called the driving arm, as it causes the cast iron collar to turn round with the revolution of the wheel, and this collar, by means of its attachments to the floats, accomplishes the feathering action. The eccentricity in this wheel is not sufficient to keep the floats in the vertical position, but in the position between the vertical and the radial. The diameter of the pins upon which the floats turn is 1-3/8 inch, and between the pins and paddle ring two stud rods are set between each of the projecting ends of the arms, so as to prevent the two sets of arms from being forced nearer or further apart; and thus prevent the ends of the arms from hindering the action of the floats, by being accidentally jammed upon the sides of the joints. Stays, crossing one another, proceed from the inner flange of the centre to the outer ring of the wheel, and from the outer flange of the centre to the inner ring of the wheel, with the view of obtaining greater stiffness. The floats are formed of plate iron, and the whole of the joints and joint pins are steeled, or formed of steel. For sea-going vessels the most approved practice is to make the joint pins of brass, and also to bush the eyes of the joints with brass; and the surface should be large to diminish wear.

634. Q.—Can you give the dimensions of any other oscillating engines?

A.—In Messrs. Penn's 50 horse power oscillating engine, the diameter of the cylinder is 3 feet 4 inches, and the length of the stroke 3 feet. The thickness of the metal of the cylinder is 1 inch, and the thickness of the cylinder bottom is 1-3/4 inch, crossed with feathers, to give it additional stiffness. The diameter of the trunnion bearings is 1 foot 2 inches, and the breadth of the trunnion bearings 5-1/2 inches. Messrs. Penn, in their larger engines, generally make the area of the steam trunnion less than that of the eduction trunnion, in the proportion of 32 to 37; and the diameter of the eduction trunnion is regulated by the internal diameter of the eduction pipe, which is about 1/5th of the diameter of the cylinder. But a somewhat larger proportion than this appears to be expedient: Messrs. Rennie make the area of their eduction pipes, in oscillating engines, 1/22d of the area of the cylinder. In the oscillating engines of the Oberon, by Messrs. Rennie, the cylinder is 61 inches diameter, and 1-1/2 inch thick above and below the belt, but in the wake of the belt it is 1-1/4 inch thick, which is also the thickness of metal of the belt itself. The internal depth of the belt is 2 feet 6 inches, and its internal breadth is 4 inches. The piston rod is 6-3/4 inches in diameter, and the total depth of the cylinder stuffing box is 2 feet 4 inches, of which 18 inches consists of a brass bush—this depth of bearing being employed to prevent the stuffing box or cylinder from wearing oval.

635. Q.—Can you give any other examples?

A.—The diameter of cylinder of the oscillating engines of the steamers Pottinger, Ripon, and Indus, by Miller & Ravenhill, is 76 inches, and the length of the stroke 7 feet. The thickness of the metal of the cylinder is 1-11/16 inch; diameter of the piston rod 8-3/4 inches; total depth of cylinder stuffing box 3 feet; depth of bush in stuffing box 4 inches; the rest of the depth, with the exception of the space for packing, being occupied with a very deep gland, bushed with brass. The internal diameter of the steam pipe is 13 inches; diameter of steam trunnion journal 25 inches; diameter of eduction trunnion journal 25 inches; thickness of metal of trunnions 2-1/4 inches; length of trunnion bearings 11 inches; projection of cylinder jacket, 8 inches; depth of packing space in trunnions, 10 inches; width of packing space in trunnions, or space round the pipes, 1-1/2 inch; diameter of crank pin 10-1/4 inches; length of bearing of crank pin 15-1/2, inches. There are six boilers on the tubular plan in each of these vessels; the length of each boiler is 10 feet 6 inches, and the breadth 8 feet; and each boiler contains 62 tubes 3 inches in diameter, and 6 feet 6 inches long, and two furnaces 6 feet 4-1/2 inches long, and 3 feet 1-1/2 inch broad.

636. Q.—Is it the invariable practice to make the piston rod cap of brass in the way you have described?

A.—In all oscillating engines of any considerable size, the cover of the connecting brass, which attaches the crank pin to the connecting rod, is formed of malleable iron; and the socket also, which is cuttered to the end of the piston rod, is of malleable iron, and is formed with a T head, through which bolts pass up through the brass, to keep the cover of the brass in its place.

637. Q.—Is the piston of an oscillating engine made deeper than in common engines?

A.—It is expedient, in oscillating engines, to form the piston with a projecting rim round the edge above and below, and a corresponding recess in the cylinder cover and cylinder bottom, whereby the breadth of bearing of the solid part of the metal will be increased, and in many engines this is now done.

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