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134 Tre Marine REVIEW June, 1909 | Marine Steam Turbines HERE is a _ widespread idea that the turbine can be ef- ficiently applied to a very wide range of merchant work. Ac- tually this is not so, and its superiority for anything but a very limited class of work has yet to be proved. This is, of course, quite contrary to the popularly accepted idea, so that it will be necessary to justify this statement. In order to systema- tize the discussion let us recapitulate some of the advantages claimed for the turbine system when applied to merchant vessels. They are pririci- 'pally:--(1) Increase in economy; (2) saving in weight; (3) saving in space; (4) saving in oil and attend- ance; (5) absence of vibration. © Considering (1) economy, it will be readily recognized that. this is a point of vital importance in the merchant service, and it is doubt- ful whether any other consideration can properfy compensate for any deficiency in this direction. That the turbine itself is generally superior to the. piston engine as_ regards economy is universally conceded. On the other hand, what concerns the ship owner is not the efficiency of the turbine alone, but the combined efficiency of turbine and _ propeller. Thus the important consideration to the ship owner is not the consump- tion of steam per hour per shaft horsepower developed by the tur- bine, but the consumption per effect- ive horsepower. The effective horse- power is that portion of the total horsepower which is usefully em- ployed in propelling the ship and which corresponds with the actual propeller thrust. Steam Consumption Per Shaft Horse- power. Leaving out the question of pro- peller efficiency for the moment, let us consider a few figures relating to steam consumption per horse- power per hour. The results of numerous trials of turbine-driven ves- sels indicate that this consumption varies very little from 14 lbs. to 15 Ibs. for all purposes. These figures apply to all classes of marine steam turbines with the exception of those fitted to torpedo craft, in which it is rather higher. With the piston en- gine, on the other hand, the steam consumption per shaft horsepower varies very widely with the type of vessel. Assuming the mechanical ef- ficiency of the engine to be from 93 to 95 per cent, the figure for high speed cross-Channel steamers would be fram 185. lbs. te 20.5. tbe. In high speed ocean liners it would be about 1525 Ibs. to. 16.5 Jbs., and. for the slower speed liners of, say, less than 22 knots, the figure for' their highly efficient piston engines would | be from 14.75 Ibs. to 15.75 Ibs., while in the ordinary cargo tramp it amounts to from 15 Ibs. to 15.75 Ibs. As regards the reciprocating engines of naval vessels, the corresponding consumption for battleships and the heavier classes of cruisers would be from 19 Ibs. to 20 lbs., and about 21 lbs, 40° 22 tbs. tor: the. lighter cruisers, scouts and torpedo craft. Thus it would appear that the tur- bine considered alone is decidedly superior in economy to the recipro- cating engines of naval vessels and high speed cross-Channe! steamers; the advantage is only slight in the case of racing liners, while there is little to choose between the two types when applied to the slower speed "intermediate" liners. Misleading Comparisons. Now the only published compari- sons between the two systems relate to third-class cruisers and cross- Channel steamers, and show a. re- markable saving in favor of the tur- bine. On the other hand--as_ the above figures indicate--the piston en- gines fitted to third-class cruisers are extremely uneconomical, and the same remark applies to those in high- speed cross-Channel steamers, so that the vessels compared are of the types most unfavorable to the reciprocating engine. It would seem, then, that any general deductions from the re- sults of such comparative trials are very misleading and manifestly unfair to the older system. Thus it must be conceded that the results of a com- parison between turbines with a consumption of steam per shaft horse- power per hour of 14.5 Ibs., and the reciprocating engines of third-class cruisers, with a consumption of 21 Ibs., cannot be expected to apply in the case of, say, an ocean liner of the intermediate type, in which case the consumption with turbines may be 14.25 Ibs., and that of correspond- ing piston engines 15 Jbs, : Propeller Efficiency. Now, let us go a _ step further, and take propeller efficiency into our calculations. It is obvious that the comparison of steam consumptions per shaft horsepower is of little prac. tical value, as it takes no account whatever of propeller efficiency. What is required to make a true estimate of the relative economy of the two systems is the steam consumption. per effective horsepower per hour for every type of vessel. A _ better word, perhaps, for effective horse- power would be thrust horsepower, as it is the horsepower corresponding with the actual propeller thrust. Thus it would seem that a more rational method of comparing marine steam turbine performance would be by means of the actual thrust produced rather than by the sha't horsepower, as the latter figure only represents an intermediate stage in the trans- formation, . Thus it 18. of little ae. sistance to know that a set of tur- bines is developing 20,000 shaft horse- power while we are absolutely in the dark as to whether the thrust horse- power--that portion usefully employed in propelling the ship--is 8,000 or 10,000. There can be no doubt whatever that the high-speed propellers with fine pitch ratios that are required for turbine-driven vessels are much less efficient than the slower running screws used with piston engines. In instance it has been found that the shaft horsepower as meas- ured by the torsionmeter is consid- erably in excess of the indicated horsepower necessary to produce the same speed with reciprocating engines. Thus the writer remembers the case of a turbine-driven vessel which was expected to attain the required speed with 6,500 horsepower, this figure be- ing estimated from the performance of similar vessels fitted with piston engines. The actual speed obtained fell somewhat short of that anticipat- ed, and a torsionmeter was then fitted. The shaft horsepower, as measured by the torsionmeter, was found to be 8,500 instead of the 6,500 expected. every Again, whereas the shaft horse- power of the Tribal class of 33-knot destroyers was estimated from recip- rocating engined vessels to be 15,000 the actual shaft horsepower necessary was over 20,000. In the 26-knot Coastal torpedo boats the horse- power is nominally 4,000, while ac- tually over 5,000 is required. In the absence of any apparatus for measur- actual propeller thrust it is by comparisons, or by calculating ing such