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sive to keep up than two engines. I do not attach much imp 22 MARINE REVIEW. TRIPLE-SCREW SHIPS.* THE MARINE RUNDSCHAW SAYS THAT THEY ARE ABSOLUTELY ESSENTIAL FOR THE GERMAN NAVY--THEIR ADVANTAGES CITED CLEARLY-- . ADMIRAL MELVILLE'S POSITION. It has recently been reported from the United States that the new 'armored cruisers that are just about to be laid down are to have two screws, and not three like their predecessors. This report is somewhat surprising, for it is opposed to the views of Admiral Melville, the engi- neer-in-chief of the United States navy, who, only last year, in a very elaborate paper on "The Logical Arrangement of the Motive Power of Warships" expressed himself in favor of the three-screw system for large warships. He claims to have given the subject of triple screws much study, and believes that their employment on warships is entirely logical. In support of this, he explains fully their advantages and disadvantages, aud concludes that the former outweigh the latter. Finally, he favors the arrangement of the three engines in such a manner that the central engine shall develop one-half and the wing engines each one-quarter of the total power. Though I do not quite agree with this eminent authority as to this latter distribution, I nevertheless am of the opinion that, for the German navy, the triple-screw system is the only correct one for battleships and large cruisers. While Admiral Melville, from his position, considers the advantages of this system especially from an engineering point of view, I wish to emphasize the nautical side of the question. In order to get a comprehensive idea of this question, the engineering reasons will be briefly mentioned, as this seems to be nec- essary for a complete review. Ships with more than two screws have been in existence for a long time. In chapter IX of Prof. Busley's well-known book, "The New Express Steamers of the Merchant Marine and the Navy," all cases are -mentioned in which three screws had been employed up to August, 1892. Some of the reasons that led to the use of the triple-screw systems therein mentioned will be referred to later; for the present it will suffice to state the principal reason that led to the adoption of three engines for the Kaiserin Augusta. It was "the possibility of reducing the engine power without involving too great a loss in efficiency, by the adoption of several small engines." This seemed to be specially necessary for steamers that do not often use their full power, but have frequently to go at reduced power and varying speeds. It was thought that, when cruising at speeds of 10 to 12 knots, the triple-screw cruisers would have their wing engines disconnected, and the center engine only in operation. When maneu- vering, and when greater speed was required, the reverse arrangement was intended, and only in special cases were all three screws to be used. The expectations that were based on this method of operation have 'not been realized, as the trials of the United States triple-screw cruisers Columbia and Minneapolis and our Kaiserin Augusta have shown. With the exception of a single condition, which, however, has nothing to do with the general efficiency, the use of the central screw alone is scarcely mentioned nowadays. That the triple-screw ships are especially economi- cal for loneer distances, at cruising speeds, when they use the wing engines only, is. however, unquestioned. For ships operating in squad- ron, we must consider almost exclusively the use of all three engines. In this case it has been found that, at low speeds, the coal consumption of triple-screw ships is somewhat greater than that of twin-screw ships, while at higher speeds it is the same. It is, therefore, necessary to try to make the triple-screw system more efficient for the more frequently occurring condition of cruising speed. Another objection to the installa- tion of three eneines is that a somewhat lower speed is obtained than with two engines for the same total power expended. This, however, had been clearly foreseen, and it was not expected, as has sometimes bee stated, that higher speed would be obtained with three screws. ! MACHINERY MORE COMPLEX AND DIFFICULT OF SUPERVISION. ___In addition to the above two disadvantages there is another of great military importance, and that is that the entire installation is more com- plex and more difficult of supervision. The machinery installations of modern warships are of such complexity that an increase in this direction must always be regarded as a great disadvantage. It is evident that the entire pipe system for three engines must be much more complicated, the signaling apparatus more intricate, and the number of auxiliary engines more numerous. In regard to the latter condition, however, a rather favorable circumstance should be mentioned--that some of them -can be quite comfortably installed in the after engine room, whereas, with two large engines side by side, room must be found in other parts -of the ship. A greater objection than all three, however, is that the supervision of all machinery is much more difficult for the engineer in charge with three engines than with two. This also depends largely upon the space that can be given up to the various engines, and may accord- ingly be more or less annoying. For instance, in the battleships of the Kaiser class, these conditions cannot be regarded as anything but very unfavorable, This complexity of the triple-screw system is such a great objection that it should not be increased under any circumstances: but this would happen if the proposition of Admiral Melville to put in an unequal distribution of power for three screws were followed. Further comment on this is probably unnecessary, A further disadvantage in the use of three screws is that the whole engine space requires more room lengthwise, say from two. to three frames. In consequence of this, the heavy after turret must be placed still further aft, which involves a larger amount of broadside armor for that part of the ship which-is between the two large turrets. In the new ships of nearly all nations this armor reaches part of the upper deck and part to the gun deck. The increase in weight is made up by the smaller weight of the triple engine machinery, as previously mentioned: for the latter is always lighter than the installation of two engines, Though of less importance, it may be mentioned that three engines are more expen- ortance to the further objection to three engines, that a larger personnel is required for attendance. A more serious objection may be urged against three screws on con- *Translated for the Journal of the American Societ A. H. Raynal, associate member of the society. y of Naval Engineers by [June'6,. structive grounds, due to the difficulty in securing a good construction of the stern. Twin 'screws are placed at either side of the stern post, from which the rudder can be easily and securely suspended. If a third screw is added, it must be arranged like the screw of a single screw ship. Its shaft must pass through the stern post, and the screw itself must be placed immediately aft of the stern post, while a special post must be arranged for the rudder aft of the screw. For this post, or for the rudder, a special support is necessary, and it must be of strong, and, therefore, of heavy construction. As is well known, such a support is not a very - desirable addition, especially when it forms the lowest part of the ship. It has, t*erefore, been arranged in the more recent triple-screw ships to dispense with this, and to merely suspend the rudder from a post that is specially constructed for the purpose. To better protect it from injury, it can be built so that its lowest point will be above the bottom of the ship's hull proper. I do not believe that the effect of the rudder is thereby diminished. If it were, the rudder might be made correspond- ingly longer; and, further, it will be shown that the excellent efficiency of the rudder with the triple-screw ship depends entirely upon the action of the center screw. It is admitted that there is great difficulty in obtain- ing a good and secure suspension of a rudder that is supported only from above. The stern frame, as is well' known, is supported by the central portion of the ship, and is, therefore, greatly exposed to the vibra- tions of the engines. The difficulties are not insurmountable, as I have been assured by naval constructors. As a proof of this, the alterations. to the rudder of the Victoria Luise may be mentioned. She lost her rudder support, and it was not replaced; the post, and the other parts from which the rudder was suspended, were merely strengthened, and, on the trials following this alteration, the arrangement of the rudder was found to be perfectly satisfactory. The difficulty in well and securely arranging the rudder, when three screws are used, cannot, therefore, be regarded as of such great importance as to justify serious objections to the triple-screw system. SOME OF THE ADVANTAGES OF THE TRIPLE-SCREW SYSTEM. To offset the above disadvantages of the triple-screw system there are numerous important advantages. First of all, it will be apparent that with three engines at disposal the assurance of working the entire machinery plant is greater than with only two engines. This advantage may be of special significance in battle. If from any cause, either internal or external, an engine is disabled, there still remains two-thirds of: the entire power, while in case of the disablement of one engine where there are but two screws, one-half of the entire engine power is lost. This will be referred to later. Many advantages result from the smaller dimen- sions of the machinery by reason of the division of the power into three parts. As an illustration, let us assume a special case. Let the question be whether a battleship of about 12,000 tons, with engines of 15,000 °H.P., shall have two or three screws. These are about the proportions that obtain with our recent battleships. In one case there are required two. engines of 7,500 H.P. each, and in the other three of 5,000 H.P. each: It is evident that the three smaller engines can be built simpler and lighter than the two larger ones; and it may also be mentioned that the smaller engines are cheaper. All their details of wrought or cast material will be smaller, can be manufactured more easily, and are more likely to be free from defects. While the consequences from a failure of one engine are of less importance, it follows also that such failures are less likely when smaller engines are used. With the steam pressure now used, it is possible to build an engine of 5,000 H.P. with three cylinders, while one of 7,500 would necessitate four cylinders. While I do not wish to assert that four cylinder engines will never satisfy the high requirements de- manded of them from a military point of view, nor have the handling qualities required in the engines of a battleship, it is certainly true that three-cylinder engines satisfy the requirements much better in this regard. The advantage of the triple-screw system in this respect is, therefore, not to be underestimated. The fact that smaller engines are more easily managed and supervised requires no argument. From a purely engineering standpoint there are further advantages of smaller engines, for which I would refer to Admiral Melville's paper, and now proceed to a consideration of the constructive advantages. To obtain. high speeds it has been found necessary to design ships with very fine lines aft, because the water displaced by the vessel will thus find easy egress, and have better access to the screws, thus securing better efficiency. But in a vessel that is fine aft, it is much easier to arrange for three smaller engines, of which the central one is aft of the others, than for two large ones side by side. The latter have to be placed much farther forward, and while this, as already mentioned, has some advantages, it is, on the other hand, very unfavorable for the propeller shafts. These have to be much longer, and will, therefore, be much more subject to torsion and to bending strains when the vessel is pitching. With engines in a fine afterbody, the foundations are very much lighter, because the frames are placed much nearer together and can, therefore, support them to better advantage. A further advantage of smaller engines is their lesser height. This affects, on the one hand, the draught of the vessel, and on the other, the armor for the engines. The lesser draught that can be obtained with three smaller engines was the first reason for installing them. This reason still exists, and is of special importance for navies that have to consider shallow waters, which is especially the case with the navy of the United States. Besides, it must not be forgotten that each inch of additional draught calls for greater engine power to obtain a given speed. As regards armor protection, the situation is best illustrated by a comparison between our Hertha class and the English cruisers of the Arrogant class. These two types are of approximately the same size, and have equal engine power. While the three engines of our ships have been placed below.the uninterrupted protective deck, the engines of the Eng- lish ships project above it, and had to be protected by a special armor shield. Therefore, the protective armor for an installation of three engines is simpler, lighter and cheaper than it is for one of two engines. Whether in this special case it would have been possible to install below the protective deck two engines of 7,500 H.P. each, on the comparatively light draught that is required for our navy, I very much doubt. NAUTICAL ASPECT OF THE QUESTION. We have now reached the nautical aspect of the question. A great many circumstances might arise under war conditions which would