Chapter 3

If the apparent slip be zero, the real slip is the speed of the wake, and if the apparent slip be negative, the real slip is less than the speed of the wake. The real slip is greater than the apparent slip, and can never be a negative quantity. From Mr. Froude's model experiments, it appears that this speed of wake for the A class of ship amounts to about 10 per cent. of the speed of the A screw. If this value is correct, then the real slip is (10 + 17.6) per cent., or 27.6 per cent. This is shown in Fig. 6, where O is the point of no slip, being 17.64 from the point of real slip. Slips to the right of O are positive apparent slips, slips to the left are negative apparent slips. The vessel F would certainly have a wake with a speed considerably less than that of A's wake. From the model experiments, the wake for F is about one-half that for the A class, or, roughly, 5 per cent. of the speed of the screw. For the ship F, O is the point of no apparent slip, and the real slip is (5 + 11.4) or 16.4 per cent. For E, the point of real slip is approximately the same as for F. For B and D, the positions on the curve would be about the same. The ship B has a higher speed of wake than D, but the screw D has the greater apparent slip. The influence of the number of blades on the scale for the slip has been neglected. If this efficiency curve were applicable to full sized screws propelling actual ships, and if the determination of the wakes were beyond question, then we should have a proof that our screws were at or near the maximum efficiency. But, as we know, from the total propulsive efficiencies, that the screws have high and not widely different efficiencies on these ships, we may argue the other way, and say that there is good reason to consider that at least the upper part of the curve agrees with experience obtained from actual ships. Now take Fig. 6 and consider the general laws there represented. Take the speed of the wake as 10 per cent. of the speed of the screw, which is probably an average of widely different conditions, including many single as well as twin screw ships. Then this curve shows that considerable negative slips mean inefficient screws; that screws may have very different positive slips without any appreciable difference in their efficiencies; and that very large positive slips and inefficient screws may be companions. For instance, a screw with a large positive slip in smooth water is frequently inefficient at sea against a head wind, which increases the resistance, and necessitates an increase of slip. I venture to say that these statements, taken in a general manner, are not at variance with experience obtained from the performances of screw ships. Before it is possible to satisfactorily decide if this curve applies in a general manner to full sized screws propelling ships, we require the results of trials of various ships where the screws are working about the region of no slip. Model experiments teach that the scale for the slip varies with the design of the screw, and that with a given screw the speed of the wake (which decides the point of no apparent slip) varies with the type of ship and with the position of the screw with respect to the hull. Remembering these disturbances, it is not improbable that it may be possible to account for or explain what at first sight may appear departures from the curve. The diameters of the screws in the table are not compared with the diameters given by the method explained by Mr. Froude in his paper last year, for there are differences in the slips, the proportions of blade area to disk, and, to some extent, in the shapes of the blades, which are not taken into account in that method. Assuming, however, as Mr. Froude does, a constant proportion of blade area to disk, and a uniform pattern of blade, the determination of the diameter for a given set of conditions may, as a rule, be a complete solution of the problem of the design of a screw, but these assumptions do not cover all the necessities of actual practice, which make it extremely desirable to know something about the influence or efficiency of various proportions of blade area to disk, and of the form or distribution of a given area.

During the discussion which followed, Mr. John said that, both as regarded the mercantile marine and the Royal Navy, there were few data to work upon, but few ships having been built with twin screws. Mr. Linnington's proportions of pitch to diameter of 1.2 to 1.34 was not invariably adhered to. He mentioned a couple of small twin screw vessels where the proportion of pitch to diameter came nearly to 1.5, and he remembered a few years ago the propellers in one of these vessels being changed and the pitch increased, the result being a very considerable improvement. He believed they might go with quick running twin screw engines to a larger proportion of pitch to diameter than they could with a single screw. He might instance the change in the Iris. She was first engined with the pitch equal to the diameter, and she gained two knots or thereabout when the diameter was reduced 2 ft. and the pitch increased 2 ft.

Admiral De Horsey said that he tried experiments with the single screw in the Aurora. She had a feathering serew, and when the sails were used to assist, they commonly altered the pitch of the screw according to the strength of the wind. The screw could be altered while it was revolving, and as the wind freshened they coarsened the pitch, and when they wanted to stop the engines they coarsened the pitch so as to bring the screw right fore and aft, so that they never altered the way of the ship in changing from steam to sail alone. The reason why twin screws had been adopted in the navy was that if one was damaged there was the other still available. But it gave them a still further advantage, as it enabled them to have a fore and aft bulkhead, which with a single screw was difficult. The mercantile marine had not as yet looked favorably on twin screws. Their finest and fastest ships were single screws, probably because, in very bad weather, the single screw was better.

Mr. Spyer said that in designing propellers for ships of war, they were obliged to attempt to obtain the highest possible speed, and that was not necessarily coincident with a propeller of maximum efficiency. On the other hand, for mercantile purposes, coal consumption was obviously of paramount importance, and the speed of any particular vessel must be obtained with the smallest possible amount of indicated horse power, and a propeller of maximum efficiency. Regarding the position of the propellers in a small pinnace, the propellers were shifted six or seven inches further out, and with about ten per cent. less indicated horse power she obtained three tenths of a knot more speed.

Mr. Barnaby asked Mr. Linnington whether, in designing twin screws for a vessel of 8,000 i.h.p., he would make each screw, which would have to take 4,000 i.h.p., of the same diameter as a screw for a single ship of 4,000 i.h.p., of the same speed. Unfortunately in high speed vessels, from one point of view, the faster they went for a given power the smaller the diameter of the screw had to be, and the larger the pitch, so that in very high speed twin screw vessels the ratio of pitch to diameter would be found to come out very great indeed. In a twin screw torpedo boat, to be tried shortly, they had a ratio as high as 1.64. In the case of the Inflexible it was found, owing possibly to the position of the screw, that the whole of the plates immediately over the screws were damaged. Mr. Beckett Hill had been using, during the past three or four years, the twin screw steamers the Ludgate Hill, Richmond Hill, and Tower Hill. These were all over 4,000 tons register, and indicated, when at work at full speed, 2,500 h.p. Before he and his friends built these steamers, they built some very large tug boats on the twin screw principle. At the present moment, four of the fastest steamers building for the Atlantic service were to have twin screws. The great obstacle to the extension of the twin screw in the mercantile navy had been the fear that the projection of these screws would make the vessels very difficult to handle, but he had found no such difficulties. He had found it an advantage to put the point of the propeller as near the deadwood as he could, without actually touching it, and in the large steamers, as well as in the tugs, the distance was a few inches. As to the point of safety, he thought it a great advantage to have twin screws, and on two occasions twin screw vessels had met with accidents which, but for the twin screws, would have necessitated their putting back to New York for repairs. The Richmond Hill, on one occasion, met with an accident to her machinery two days after leaving New York; but she was able to come on with the second set of engines, and was only one day late in the passage. No difficulty had been found in the docking and undocking of these vessels, either in London or Liverpool, and while with single screw vessels they had sometimes to employ one or two dock boats to dock and undock them, they never had to do so with the twin screw vessels. These vessels were 400 ft. long, with 48 ft. breadth of beam--a very large size to handle in a river like the Thames. He noticed in the paper a propeller with a diameter of 15 ft. 6 in. to indicate 11,110 h.p., so that a great Atlantic steamer, which should indicate 11,000 or 12,000 h.p., and have a beam of about 65ft., would have her screws very well protected.

Mr. White said that as soon as it was found that with twin screws they lost nothing in efficiency, ship owners generally were contemplating their adoption, an admirable example of which had been set in the vessels of the Hill line. In adopting twin screws, the question whether they should overlap was one that deserved very serious consideration, and it was interesting to know, from experience gained by the vessels of the Hill line, that there was no difficulty in the way of the projection of the screws. With a moderate power, and with vessels of considerable size, the screws were well sheltered: but in the large ships which were contemplated, where there must necessarily be larger screws, this might be different, and become a difficulty.

Mr. Linnington, in reply, said there was no reason to think that the twin screw at sea might not be as satisfactory, in comparison with the single screw, as it appeared in smooth water. As a matter of fact, one of the great advantages of twin screws was that at sea the condition of weather which would bring the single screw out of the water, and make it extremely inefficient, would have no appreciable effect on the twin screws. In vessels of deep draught especially, they were well immersed, and they were really more efficient at sea than in smooth water. In ships of full form, the longitudinal position of the screws was of importance; but in the ships referred to in this table the run was very fine, and the screws were well covered by the hull. He did not think, in such a case, any small difference in longitudinal position would affect the performance. If any alteration were made, it would probably be better to put the screws farther off. When the rudder was hard over, the blades of the screw should be about a foot clear of the rudder.--_Industries_.

* * * * *

RECENT ADVANCES IN SEWING MACHINERY.

[Footnote: A recent lecture before the Society of Arts, London.]

By JOHN W. URQUHART.

The distinct improvements in sewing machinery to which I would invite your attention this evening have reference more particularly to the results of inventive effort within the past ten years. But although marked development in the machines has occurred in so short a time, it may be taken for granted that those advances are but the accumulated results of many years' prior invention and experience of stitching appliances.

The history of the sewing machine, and the decision of the great question, Who invented an apparatus that would unite fabrics by stitches? do not at present concern us. Many sources of information are open to those who would decide that extremely involved problem. But whether the production of the first device of this kind be claimed for England or for America, it is quite certain that no one man invented the perfect machine, and that those fine specimens of sewing apparatus shown here to-night embody the labors of many earnest workers, both in Europe and America.

Most of us are familiar with the arrangements of an ordinary lock stitch machine, and an able paper by Mr. Edwin P. Alexander, embracing not only a good account of its history, but most of the elements of the earlier machines, has already (April 5, 1863), been read before you. This, and sundry descriptions of such apparatus in the engineering papers, confine my remarks to the more recent improvements in three great classes of machines. These are, briefly, plain sewing machines; sewing machines as used in factories, where they are moved by steam power; and special sewing machines, embracing many interesting forms, only recently introduced. We have thus to consider, in the first place, the general efficiency of the machine as a plain stitcher. Secondly, its adaptability to high rates of speed, and the provision that has been made to withstand such velocities for a reasonable time. And, thirdly, the apparatus and means employed to effect the controlling of the motive power when applied to the machines.

To deal with the subject in this way must, I fear, involve a good deal of technical description; and I hope to be pardoned if in attempting to elucidate the more important devices, use must be made of words but seldom heard outside of a machinists' workshop.

It appears scarcely necessary to premise that the sewing machine of twenty years ago has almost faded away, save, perhaps, in general exterior appearance; that the bell crank arms, the heart cams, the weaver's shuttles, the spring "take ups," rectangular needle bars, and gear wheels, have developed into very different devices for performing the various functions of those several parts.

The shuttle is perhaps the most important part of a lock stitch machine. But what is a shuttle? So many devices for performing the functions of the early weaver's shuttle have been introduced of late, that the word shuttle, if it be used at all, must not be accepted as meaning "to shoot." We have vibrating shuttles, which are, strictly speaking, the only surviving representatives of the weaver's shuttle in these new orders of machines; and stationary shuttles, oscillating shuttles, and revolving shuttles, besides the earlier rotating hook, in several new forms, difficult to name. But the general acceptation of the word shuttle, as indicating those devices that pass bodily through the loop of upper thread, is, I venture to think, sufficiently correct.

Many changes have been effected in the form, size, and movements of the shuttle, and we may profitably inquire into the causes that have induced manufacturers to abandon the earlier forms. The long, weaver's kind of shuttle, originally used by Howe and Singer, had many drawbacks. Mr. A.B. Wilson's ingenious device, the lock stitch rotating hook, was not free from corresponding faults. The removal of these in both has led to the adoption of an entirely new class of both shuttles and revolving hooks. It is well known that the lock stitch is formed by the crossing of two threads, one of which lies over, and the other under, the cloth to be sewn. This crossing point, to insure integrity of the stitch, must occur as nearly as possible in the middle of the thickness of the fabric. The crossing must also be effected while a certain strain, called tension, is imposed upon both threads. If the tension of one thread should outweigh that of the other, the locking point becomes displaced. If the tension be insignificant, the stitches will be loose. If the tension should vary, as in the long shuttle, there will occur faulty points in the seam.

In the earlier rotating hook the tension depended upon the friction developed between the spool and the hook. This tension, therefore, varied in proportion to the speed of the latter, and could never be constant. This was quite apart from the frictional resistance offered to the upper thread in passing over the cavity of the hook.

In the shuttle the tension was obtained by threading through holes in the shell, or beneath a tension plate, as in Howe's machine. This tension, so long as the reel ran between spring centers, was never constant. The variation was chiefly due to the angular strain set up when unwinding from the reel. This strain varied according to the point of unwinding. It was light in the middle of the reel and heavy at either extremity. These drawbacks caused immense anxiety to the first makers of sewing machines, and numerous attempts to overcome them led to little improvement. With reference to high rates of speed, the older shuttle, requiring a long and noisy reciprocation, had its disadvantages.

The only effective remedy for these drawbacks was a radical one. It was necessary to substitute depth of reel for length. Hence, several attempts have been made to construct disk or ring shuttles. Many forms of those have been tried. They all depend upon the principle of coiling up the thread in a vertical plane, rather than in horizontal spirals. Some makers placed the disk in a horizontal plane, and caused it to revolve. Nothing could be worse, as will be seen, if we follow the course the enveloping loop must take in encircling such a shuttle. But a complete solution of the difficulty of employing a ring shuttle has been achieved in the oscillating form, invented by Mr. Phil. Diehl, and known as Singer's (Fig. 1). A short examination of it may profitably engage your attention. The shuttle itself is sufficiently well known, but certain features of it, and to which it owes its efficiency, appear to call for some explanation. Its introduction dates back some years, during which time it has undergone certain modifications.

It consists of a thick disk bobbin of thread, _h_, fitting loosely in a case constructed in the form of a bivalve, _a_ and _d_. This case is furnished with a long beak, usually forming a continuation of the periphery. The beak is intended to enter and detain the loops of upper thread, and lead them so that they ultimately envelop the shuttle, a motion of the thread which is chiefly due to the oscillation of the shuttle in a vertical plane. The oscillating movement is to the extent of 180 degs. of the circle, which suffices to cast the loops freely over the shuttle. The center of oscillation is not coincident with the center of the shuttle; but it is nearly so with the periphery of the thread reel, and exactly coincides with the point where the under thread is drawn from the shuttle, _g_. The shuttle thread is thus entirely freed from any tendency to twist, an objection frequently urged against circular or revolving shuttles. It will be observed, also, that the body of the shuttle is extremely narrow. Bulging of the thread loops to one side or the other is thus obviated.

But the long beak in this description of shuttle serves an important purpose other than that of seizing the upper thread loops, otherwise a very short beak would be preferable. It adds so much to the efficiency of the machine that a little further explanation of it appears essential. In the old fashioned machines the thread required to envelop the shuttle was dragged downward through the cloth, while the needle still remained in the fabric. This necessitated the use of large needles with deep side channels, to enable the thread to run freely, and as a consequence the punctures that had to be made in the fabric were unnecessarily large, and could not in any case be entirely filled by the thread, a condition which is now recognized as essential in linen stitching and for waterproof boots.

The long beak in both shuttles and hooks offers an immediate solution of the old difficulty experienced with long shuttles. When the needle begins to rise, the shuttle commences to oscillate, through the loop, the motions so coinciding that the long beak, c, merely detains the loop until the eye of the needle has ascended above the cloth; then, and then only, does the envelopment of the shuttle commence, and the thread required for it flows downward through the puncture. The envelopment is completed before the needle has attained its highest point, and the consequent loose thread is immediately pulled up by a lever, called a positive take-up, before the needle begins to descend for a fresh stitch. In this way little or no movement of the thread is required in the cloth while the puncture made is occupied by the needle. The result is the capability of such apparatus to work with an incredibly fine needle--indeed, so fine as to be no thicker than the incompressed thread itself. This would have been considered quite impossible of accomplishment by our earlier machine makers. The advantage thereby gained in stitching linen goods, and in sewing leather, where every puncture of the needle should be quite filled by the thread, is at once apparent. Indeed, a rubber or leather sack, stitched in this way, will contain water without leakage--a very extreme test.

_Revolving Shuttles_.--The class of shuttles known as revolving or rotating, and which really consist of a combination of the disk shuttle and the earlier rotating hook of Wilson, have been under trial by several makers for many years. If, for example, the oscillating shuttle we have just examined were to complete its circular movement, it would constitute a revolving shuttle, but would not be quite similar to those devices now known as such. The most remarkable device of this kind yet introduced is to be found in Wheeler & Wilson's machine known as No. 10 D, and invented by Mr. Dials last year. It consists, in fact, of a detached hook, and its inventor declines to class it with shuttles at all, styling it a detached hook. It consists of an exterior shell or skeleton of steel, capable of rotation in an annular raceway. Its detachment from the axis forms a striking exception to the general construction of interlocking apparatus in this company's machines. Under the beak of this curious device is found an oblong recess, into which fits loosely a carrier or driver, rotating with a differential or variable motion. The space between the carrier and the sides of the recess is sufficient to permit the free passage of the thread in encircling the shuttle, and the differential movement ingeniously releases the contact between the hook and carrier. The skeleton of this device is only one-sided, and does not really carry its bobbin in the course of its revolution. The bobbin is placed in a cup-like holder, which lies within the shuttle or hook body, and is retained in position by a latch hinged to the bed of the machine. The cup and bobbin are prevented from partaking of the rotatory movement by a steel spur projecting from the cup, and fitting loosely into a notch in the latch. Tension upon the under thread is obtained by passing it under a tension plate upon the bobbin cup. Twisting of the thread is by these means entirely obviated. In this apparatus, the disk-like appearance of the bobbin is partially lost in its considerable breadth, and there is thus a distinct departure from the lines of the ring shuttles before mentioned. The diagrams exhibit the hook in several positions during its revolution, and the position of the threads corresponding thereto.