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Me AS Re Noe R BE Mo doe sequently the size of the propellers) is mainly determined by the principal cross-section of the ship, whereas, on the other hand, the size of the turbines is limited only by the speed of rotation, and not by the power developed. The speed of the turbine must be reduced in proportion to the speed of the ship, so that the dimensions of the former are increased, either by the number or the diameter of the moving rings, whilst the power diminishes approximately as the inverse of the cube of the speed. There is, therefore, a lower limit of speed, below which the use of turbines cannot be recommended. The author has already expressed the opinion that this limit is in the neighborhood of 20 knots. The author is aware that certain ships now under construction for transatlantic service, and of a proposed speed of 17 knots, are being fitted with turbine engines, but the future will show how these will turn out. : EFFICIENCY AT LOW SPEED. If the steam turbine is capable of giving good results at the maximum power, it cannot be gainsaid that the results are certainly unsatisfactory at reduced speeds, not so much on account of the reduction of power, as on account of the reduction in the speed of rotation, which involves a lowering of what is termed the "hydraulic efficiency" of the turbine. The curves in Fig. 2 show the general behavior of the steam consumption per horse power of a turbine, compared to that of a reciprocating engine, in terms of the speed of the ship, assuming that this consumption is about the same in both cases at the maximum speed. In these approximate curves, it will be seen that at reduced speed the consumption of steam per horse power for the turbine is much higher than for the reciprocating engine. This drawback does not signify in the case of merchant vessels that keep at about their maximum speed. On the other hand, it becomes a serious one for war- ships that are rarely working at full power. The increase in the coal 'consumption at speeds of, say, 12 to 15 knots, at which they are usually working, would, however, greatly diminish their radius of action. A partial remedy, as used by Mr. Parsons, may be effected by adding a supplementary turbine for cruising purposes, into which the steam first en- ters when proceeding at low speeds. This, however, does not improve the hydraulic efficiency of the turbine, and the steam consumption nevertheless remains high. The author considers that under no circumstances can turbines alone be econom- ically worked at low speeds, and that the only satisfactory solution is the employment of a reciprocating engine of more or less power, according to circumstances, in conjunction with turbines. With this combination, economical results can be obtained at all speeds, and an example of this will be given later, REVERSING AND MANEUVERING. ~ With a reciprocating engine, stopping and reversing are ef- fected in the simplest possible manner, whereas, the very principle of the turbines is essentially opposed to this. Vari- ous inventors have tried to solve this problem by means of special blades to enable the same rings to be used for both directions of motion, but these attempts do not appear likely to come to anything, as one can only obtain reversibility by a considerable sacrifice of efficiency in forward motion. It is, . therefore, necessary to supplement the turbine by special en- gines for going astern, and, as it is obviously impossible to have the latter as powerful as the former, one must be satis- fied with a very much smaller speed astern than ahead. This difficulty in freely going astern makes maneuvering very awk- ward. The engine for going astern may be a reciprocating one, which would also be of use for going ahead, but it can just as well be a steam turbine. From the very start, Mr. Looking| Forward eview MARINE R FIG. 5. GENERAL ARRANGEMENT OF FIRST-CLASS TORPEDO BOAT WITH COMBINED TURBINE AND RECIPROCATING MACHINERY. Parsons used in his vessels special turbines for going astern, and these were attached to the same shafts as the main tur- bines; but this arrangement has the inconvenience of taking up a good deal of space lengthways. In my patent of 18098 I have indicated how these can be fitted so as to be, as it were, hidden inside the main turbines on the low-pressure side, and without taking up any addi- tional space. When they revolve freely, the astern rings offer no appreciable resistance while the main turbine is at work, and, conversely, the latter is idle when the astern turbine is in motion. This is the arrangement we have got in torpedo boat No. 243 and in the Libellule, and it has the advantage of great simplicity. I think that Mr. Parsons has also made use of a similar arrangement in a certain number of his re- cent vessels. According as the astern turbine is more or less developed, so the astern speed is more or less increased. With a single live ring, as on torpedo boat No. 243, and for the same expenditure of steam, the astern speed will be about 4o per- cent of the speed ahead, but with two rings it can be in- creased to 50 per cent. Adding more rings, however, adds very little to the speed, unless the number is so greatly in- creased as to make this engine almost as important as the