Chapter 2

The keel was laid in August, 1885, and the ship was launched July 29, 1886, on which occasion it was christened Greif. On the trial trip it was found that the slender shape of the vessel adapted it for the development of a very high rate of speed under favorable conditions, when it can make at least 22 knots an hour, so that the speed of 19 knots an hour guaranteed by the builders can certainly be reached, even when traveling at a disadvantage. In spite of its great length, the Greif can be easily maneuvered. When moving forward at full speed, it can be made to describe a circle by proper manipulation of the rudder, and by turning one screw forward and the other backward, the ship can be turned in a channel of its own length.

A large and rapid cruiser, also for the German navy, is being built by the corporation "Germania". This vessel is of about the same length as the Greif, has more than double its displacement, and will make 18 knots an hour, an unusual rate of speed for a vessel of its class. It will be launched by the last of the summer or early in the fall.

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TWIN SCREW TORPEDO BOAT.

We give several illustrations of a sea going twin screw torpedo boat lately built for the Italian government by Messrs. Yarrow & Co., of Poplar. The vessel in question is 140 ft. long by 14 ft. wide, and her displacement approaches close on 100 tons. The engines are of the compound surface condensing type ordinarily fitted by this firm in their torpedo boats, excepting where triple compounds are fitted. The general arrangement is shown by the sectional plan. As will be noticed, there are two boilers, one before and the other aft of the engines, and either boiler is arranged to supply either or both the engines. Yarrow's patent water tight ash pans are fitted to each boiler, to prevent the fire being extinguished by a sudden influx of water into the stokehold. There is an independent centrifugal pumping engine arranged to take its suction from any compartment of the boat. There are also steam ejectors and hand pumps to each compartment. These compartments are very numerous, as the space is much subdivided, both from considerations of strength and safety. Bow and stern rudders are fitted, each having independent steam steering gear, but both rudders can be worked in unison, or they can be immediately changed to hand gear when necessary. The accommodation is very good for a vessel of this class. Officers' and petty officers' cabins are aft, while the crew is berthed forward.

The armament consists of two bow tubes built in the boat. There are two turntables, as shown in the illustrations, each fitted with two torpedo tubes. These, it will be noticed, are not arranged parallel to each other, but lie at a small angle, so that if both torpedoes are ejected at once, they will take a somewhat divergent course. Messrs. Yarrow have introduced this plan in order to give a better chance for one of the torpedoes to hit the vessel attacked. There are two quick firing three pounder guns on deck, and there is a powerful search light, the dynamo and engine being placed in the galley compartment.

We believe, says _Engineering_, this torpedo boat, together with a sister vessel, built also for the Italian government, are the fastest vessels of their class yet tried, and it is certain that the British Navy does not yet possess a craft to equal them. It is an extraordinary and lamentable fact that Great Britain, which claims to be the foremost naval power in the world, has always been behind the times in the matter of torpedo boats.

The official trial of this boat was recently made in the Lower Hope in rough weather. The following is a copy of the official record of the six runs on the measured mile:

Boiler | Receiver | |Revolutions | | |Second Pressure.| Pressure.| Vacuum. | per Minute.| Speed.| Means.| Means. -------------+----------+---------+------------+-------+-------+------ |lb. | lb. | in. | | | | 1 | 130 | 32 | 28 | 373 | 22.641| | | | | | | | 24.956| 2 | 130 | 32 | 28 | 372.7 | 27.272| | 24.992 | | | | | | 25.028| 3 | 130 | 32 | 28 | 372 | 22.784| | 25.028 | | | | | | 25.028| 4 | 130 | 32 | 28 | 377 | 27.272| | 25.138 | | | | | | 25.248| 5 | 130 | 32 | 28 | 375 | 23.225| | 25.248 | | | | | | 25.248| 6 | 130 | 32 | 28 | 377 | 27.272| | +------+----------+---------+------------+-------+-------+------- Means.| 130 | 32 | 28 | 274½ | | | 25.101 | | | | | | knots -------------+----------+---------+------------+-------+-------+-------

--_Engineering_.

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SOME RECENT HIGH-SPEED TWIN SCREWS.

[Footnote: A paper recently read before the Institution of Naval Architects, London.]

By E.A. LINNINGTON.

One of the most interesting and valuable features in the development of naval construction in recent years is the great advance which has been made in the speeds of our war ships. This advance has been general, and not confined to any particular vessel or class of vessel. From the first class armored fighting ship of about 10,000 tons displacement down to the comparatively diminutive cruiser of 1,500 tons, the very desirable quality of a high speed has been provided.

These are all twin screw ships, and each of the twins is driven by its own set of engines and line of shafting, so that the propelling machinery of each ship is duplicated throughout. The speeds attained indicate a high efficiency with the twin screws. In all ships, but more especially in high speed ships, success depends largely upon the provision of propellers suited for the work they have to perform, and where a high propulsive efficiency has been secured, there is no doubt the screws are working with a high efficiency. The principal purpose of this paper is to record the particulars of the propellers, and the results of the trials of several of these high speed twin screw ships. The table gives the leading particulars of several classes of ships, the particulars of the screws, and the results obtained on the measured mile trials from a ship of each class, except C. The vessels whose trials are inserted in the table have not been selected as showing the highest speeds for the several classes. Excepting C, they are the ships which have been run on the measured mile at or near the designed load water line. On light draught trials, speeds have been attained from half a knot to a knot higher than those here recorded. No ship of the class C has yet been officially tried on the measured mile, but as several are in a forward state, perhaps the actual data from one of them may shortly be obtained. All these measured mile trials were made under the usual Admiralty conditions, that is to say, the ships' bottoms and the screws were clean, and the force of the wind and state of the sea were not such as to make the trials useless for purposes of comparison. On such trials the i.h.p. is obtained from diagrams taken while the ship is on the mile, and the revolutions are recorded by ruechanical counters for the time occupied in running the mile. Not less than four runs are made during a trial extending over several hours. The i.h.p. in the table is not necessarily the maximum during the trial, for the average while on the mile is sometimes a little below the average for the whole of the trial. The revolutions are the mean for the two sets of engines, and the i.h.p. is the sum of the powers of the two sets. The pitch of the screw is measured. The bolt holes in the blade flanges allow an adjustment of pitch, but in each case the blades were set as nearly as possible at the pitch at which they were cast. The particulars given in the table may be taken to be as reliable and accurate as such things can be obtained, and for each ship there are corresponding data; that is, the powers, speeds, displacements, revolutions, pitches, and other items existed at the same time. There are a few points of detail about these propellers which deserve a passing notice. In Fig. 1 is shown a fore and aft section through the boss. It will be observed that the flanges of the blades are sunk into the boss, and that the bolts are sunk into the flanges. The recess for the bolt heads is covered with a thin plate having the curve of the flange, so that the flanges and the boss form a section of a sphere. This method of construction is a little more expensive than exposed flanges and bolts, which, however, render the boss a huge churn. With the high revolutions at which these screws work, a spherical boss is extremely desirable, but, of course, the details need not be exactly as shown in the illustration. The conical tail is fitted to prevent loss with eddies behind the flat end of the boss, and is particularly valuable with the screws of high speed ships. The light hood shown on the stern bracket is for the purpose of preventing eddies behind the boss of the stern bracket, and to save the resistance of the flat face of the screw boss. The edges of the blades are cast sharp, instead of being rounded at the back, with a small radius, as in the usual practice--the object of the sharp edge being the diminution of the edge resistance. The driving key extends the whole length of the boss, and the tapered shaft fits throughout its length.

These points of detail have been features of all Admiralty screws for some years.

The frictional resistance of screw propellers is always a fruitful source of inefficiency. With a given screw, the loss due to friction may be taken to vary approximately as the square of the speed. This is not to say that the frictional resistance is greater in proportion to the thrust at high than at low speeds. The blades of screws for any speed should be as smooth and clean as possible, but for high speed screws the absolute saving of friction may be considerable with an improvement of the surface. There is no permanent advantage in polishing the blades. No doubt there is some advantage for a little time, and, probably, better results may thereby be secured on trial, but the blades soon become rough, and shell fish and weed appear to grow as rapidly on recently polished blades as on an ordinary surface. These screws are of gun metal. They were fitted to the ships in the condition in which they left the foundry. It appears that within certain limits mere shape of blade does not affect the efficiency of the screw, but, with a given number of blades and a given disk, the possible variations in the form or distribution of a given area are such that different results may be realized. The shapes of the blades of these propellers are shown in Figs. 2, 3, and 4. It will be seen the shapes are not exactly the same for all the screws, but the differences do not call for much remark.

Fig. 2 shows the blades for the A screw. C and D have the same form. Fig. 3 shows in full lines the blades of the B screw, and, though very narrow at the tips, they, like A, are after the Griffith pattern. The blades of E and F are of a similar shape, as shown in Fig. 4, and approach an oval form rather than the Griffith pattern. The particulars of these propellers would be considered incomplete without some reference to their positions with respect to the hulls. When deciding the positions of twin screws, there is room for variation, vertically, longitudinally, and transversely. For these screws, the immersions inserted in the table give the vertical positions. The immersion in A is 9 ft., showing what may be done in a deep draught ship with a small screw. Whatever the value of deep immersion may be in smooth water, there can be no question that it is much enhanced in a seaway. The longitudinal positions are such that the center of the screw is about one-fifth of the diameter forward of the aft side of the rudder post. The positions may, perhaps, differ somewhat from this rule without appreciably affecting the performance, but, if any alteration be made, it would probably be better to put the screws a little farther aft rather than forward. The forward edges of the blades are from 2 ft. to 3 ft. clear of the legs of the bracket which carries the after bearing. The transverse positions are decided, to some extent, by the distance between the center lines of the engines. As regards propulsive efficiency, it would appear that the nearer the screws are to the middle line, the less is the resistance due to the shaft tubes and brackets, and the greater is the gain from the wake in the screw efficiency, but, on the other hand, the greater is the augment of the ship's resistance, due to the action of the screws. Further, the nearer the screws are to the hull, the less are they exposed. But experience is not wanting to show that the vibration may be troublesome when the blades come within a few inches of the hull. The average of the clearances between the tips of the blades and the respective hulls is about one-eighth of the diameter of the screw.

An interesting and noteworthy fact in connection with these propellers is the wide differences in the pitches and revolutions, though the products of the two do not greatly vary. Such differences are extremely rare in the mercantile marine for similar speeds, but in war ships they are inseparable from the conditions of the engine design. As a general rule, with (revolutions × pitch) a constant, an increase of revolutions and the consequent decrease of pitch allow a diminution of disk and of blade area--other modifying conditions, such as the thrust, slip, number, and pattern of blades, being the same. The screws for E and F are interesting, because, with practically the same speeds and slips, there is a considerable difference in the revolutions. It will be observed that F is a vessel of finer form and a little less displacement than E, and, therefore, has less resistance. Although E has the greater resistance and the screw the smaller pitch/diameter, the higher revolutions permit the use of a smaller screw. But from this example the influence of the high revolutions in diminishing the size of screw does not appear so great as some empirical rules would indicate. The screws for A and B are also worthy of attention. Although the ship A has a much greater resistance than B, the screw of the former is much the smaller, both in the blade area and the disk. A's screws, however, in addition to 22 per cent. more revolutions than B, have a much larger slip, and the blades have rather a fuller form at the tips. Compared with the practice in the mercantile marine, the revolutions of these screws are very high, and from the foregoing remarks it may appear that much larger screws would be required for a merchant ship than for a war ship of the same displacement and speed. There would, however, be several items favorable to the use of small screws. For a given displacement the resistance would be less in the mercantile ship, and with the lower revolutions the proportion of blade area to the disk could be increased without impairing the efficiency. Thus in passing from the war vessel to a merchant ship of the same displacement, there are the lower revolutions favorable to a larger screw, but, on the other hand, the smaller resistance, larger proportion of blade area, and the coarser pitch, are favorable to a diminution of the screw. The ship B has a very large screw at 88 revolutions, but the tips are very narrow. If the blade were as dotted for a diameter of 16 ft., the same work could be done with the same revolutions, but with a little coarser pitch and a little more slip.

There is something to be said for large screws with a small proportion of blade area to disk. For instance, two bladed screws have frequently given better results than four bladed screws of smaller diameter, neglecting, of course, the question of vibrations. Twin screws, however, should, as a rule, be made as small as possible in diameter without loss of efficiency. The advantages of small twin screws are the shorter shaft tubes and stern brackets, deeper immersion, and less exposure as compared with large screws. The exposure of the screws is usually considered an objection, but, perhaps, too much has been made of it, for those well qualified to speak on the subject consider that careful handling of the ship would, in most cases, prevent damage to the screws, and that where the exposure is unusually great, effectual protection by portable protectors presents no insuperable difficulty.

HIGH SPEED TWIN SCREWS. --------------------------------------------------------------------- |Ship A.|Ship B.|Ship C.|Ship D.|Ship E.|Ship F. --------------------------------------------------------------------- Length, ft. | 325 | 315 | 300 | 300 | 220 | 250 Breadth, ft. | 68 | 61 | 56 | 46 | 34 | 32½ | | | | | | Draught on trial, | 26 ft | 24 ft | | 15 ft | 12 ft | 13 ft forward. | 2 in | 6 in | .... | 6 in | 10 in | 1 in | | | | | | Draught on trial, | 27 ft | 25 ft | | 19 ft | 15 ft | 14 ft aft. | 3 in | 6 in | .... | 9 in | 2 in | 7 in Displacement, | | | | | | tons. | 9,690 | 7,645 | 5,000 | 3,584 | 1,560 | 1,544 I.M.S., sq. ft. | 1,560 | 1,287 | 1,000 | 744 | 438 | 392 Speed of ship, | | | | | | knots. | 16.92 | 17.21 | 18.75 | 18.18 | 16.91 | 17 I.H.P. |11,610 |10,180 | 8,500 | 6,160 | 3,115 | 3,045 Revolutions per | | | | | | minute. | 107.2 | 88 | 120 | 122.6 | 150.4 | 132.1 | | | | | | Pitch of | 19 ft | 22 ft | 18 ft | 17 ft | 12 ft | 14 ft screw. | 5 in | | 9 in | 6 in | 7½in | 9 in | | | | | | Slip. per cent | 17.6 | 10 | ... | 14.2 | 9.7 | 11.4 | | | | | | Diameter of | 15 ft | 18 ft | 14 ft | 13 ft | 10 ft | 11 ft screw. | 6 in | | 6 in | | 6 in | | | | | | | Diameter of | 4 ft | 4 ft | 3 ft | 3 ft | 2 ft | 2 ft boss. | 4 in | 11 in | 9 in | 5 in | 9 in | 10 in Number of blades | 4 | 4 | 3 | 3 | 3 | 3 Blade area of one | | | | | | screw. | 72 | 87 | 60 | 47 | 24 | 24 Shape of blade. |Fig. 2.|Fig. 3.|Fig. 2.|Fig. 2.|Fig. 4.|Fig. 4 Pitch | | | | | | ---------- | 1.25 | 1.22 | 1.3 | 1.34 | 1.2 | 1.34 Diameter | | | | | | Disk | | | | | | -------- | 2.62 | 2.92 | 2.75 | 2.82 | 3.6 | 3.96 Blade area | | | | | | Immersion of | 9 ft | 5 ft | | 4 ft | 2 ft | 1 ft screw. | | 3 in | .... | 4 in | 9 in | 10 in --------------------------------------------------------------------

The slips of these screws vary from 10 to 17½ per cent., which is certainly not an extensive range, considering the widely different working conditions. Slip, as an indication of the efficiency of the screw, is not only an interesting subject, but it is often one of importance. In these ships, however, there is nothing about the slips which would give rise to any doubts as to the fitness of the screws for their work.

The ancient fallacy that small slip meant a high screw efficiency was supported by the great authority of the late Professor Rankine. Experience proved that considerable slips and efficient screws were companions. The late Mr. Froude offered an explanation of this general rule in a paper read before this Institution in 1878, and gave a curve of efficiency with varying true slip. In Mr. R E. Froude's paper last year there was a form of this curve, with an arbitrary abscissa scale for the slip, devised to illustrate in one diagram the wide conditions covered by his experiments. In the screws now under consideration, the values of the pitch/diameter vary only from 1.2 to 1.34, and for these the abscissa values for the same slips do not differ much. Taking the mean value, and bringing the slips to a common scale, Fig. 5 is obtained, which would approximately represent the relation between the efficiency of any one of these screws and its true slip, if this curve were applicable to full sized screws propelling actual ships. The slips in Fig. 5 being real or true, are not the slips of commerce, which are the apparent slips, such as those given in the table. Let us endeavor to split up these real slips into the apparent slips and another item, the speed of the wake. We then at once meet with the difficulty that the wake in which the screw works has not a uniform motion. Complex, however, as are the motions of the wake, the screw may be assumed to work in a cylinder of water having such a uniform forward velocity as will produce the same effect as the actual wake on the thrust of the screw. It is then readily seen that the real slip is the sum of the apparent slip and the speed of the hypothetical wake. To make this clear, let V be the speed of the ship, Vs the speed of the screw, _i.e._, revolutions × pitch, and V the speed of the wake; then--

Apparent slip = Vs - V. Real slip = Vs - speed of ship with respect to the wake. " = Vs - (V - V) = (Vs - V) + Vw. " = Apparent slip + speed of the wake.