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320 weights, but it cannot be balanced against variations in the turning mo- ment, and therefore any ship using it will be subject to vibrations against which the steam turbine is perfectly free. Another thing, the latter drives a smaller screw and the immersion can be made deep enough to lessen the throb of the propeller materially so that vibration from that cause may in large part be eliminated. The vibra- tion is of small moment in a freight ship and it is not of primary import- ance even in a passenger ship; but it may become very serious in battleships if it interferes with sighting the guns while moving at high speed. On this is- sue the turbine should undoubtedly be selected for all heavily armed ships. Limitations of the Turbine. The types of ships for which the turbine is to be preferred to the re- ciprocating engine are war vessels and passenger ships of high speed, but it must be remembered that by far the greater part of the world's com- merce is carried in freighters of very moderate or low speed. For these the reciprocating engine must be used until a more efficient turbine at low speeds can be designed, or a more efficient propeller at high speeds. The three- cylinder triple-expansion engine occu- pies this field admirably. The exact speed at which it should give place to the turbine has not yet been worked out for different classes of ships. The deciding factors are probably displace- ment and shape of hull, together with the arrangement and design of pro- petiers.: =. The last word has not been spoken about either method of driving. ships. It is possible that few more large re- ciprocating engines will be built for ships exceeding 20 knots in speed, and that few turbines will be used in freight- ers and other vessels with a speed of 15 knots or less. Between 15 and 20 knots the field is open to either. Combination of Reciprocating Turbine Engine. A compromise in the nature of a combination of the two, giving to the steam engine the high-pressure part and to the steam turbine the low- pressure -part, may ultimately become the standard method of propulsion ex- cepting for very high or very low speeds. This combination has been tried with very favorable results in several cases. It was first used in the Velox, a torpedo destroyer built for the Par- sons Marine Steam Turbine Co., in 1903, and sold to the British admiralty. Two triple-expansion reciprocating engines of 150 horsepower are coupled to the THe Marine REVIEW central shafts carrying the low-pressure turbines. For cruising the steam en- ters the two steam engines and passes successively through the outside wing turbines and the central low-pressure turbines. For high power, the steam engines are uncoupled. When the low power of the reciprocating engines, 300 horsepower, is compared with the 10,000 developed by the turbines, the installation in the Velox becomes a very poor test of the combination. A torpedo boat built by the Yarrow company in 1903 had a triple-expansion engine on the center shaft for cruising purposes and led the exhaust direct to the condenser. The engine was en- tirely independent of the turbines on the wing shafts, and was therefore not used in combination with them. These turbines were of the Rateau type and this boat made on trial 26.39 knots with the reciprocating engine at 576 revolutions per minute and the tur- bines at 1,300. The first real test of the combination for marine service was in the Otaki, built in 1908,:sfor the New Zealand Shipping Co. She «is. said: tobe. about 10 per cent more economical than her sister ships, the Orari and Opawa, fitted with reciprocating engines. The _ best example of the combination is afforded by the White Star liner Laurentic, built for the British-Canadian service. The ship: is 565) ft. long oby. G7 sft: beam and 14,500 gross tonnage. There are three screws. Each wing shaft is coupled to a four-cylinder triple-ex- pansion engine of 7,800 horsepower and the center shaft is driven by a low- pressure turbine of 4,600 horsepower. There are two condensers used for the turbine, or for the exhaust of the wing engines when the turbine is: not run- ning. A large change valve in the exhaust nozzle of the steam engines enables the engineer to turn the ex- haust quickly from the turbine to the condenser when reversing. 'The total horsepower is about 20,000 and the ship is to make 18 knots... The steam. en- gines in this arrangement seem rather large for such a combination but the designer was probably influenced some- what by the requirements for navigat- ing the St. Lawrence river. ~ The types 'of ships to which this combination seems to be specially adapt- ed are high-speed cruisers and_ battle- ships which usually cruise at low speed for the purpose of saving coal. Several possible arrangements and designs of machinery suggest themselves. Those adopted for the Laurentic would be satisfactory, but the proportion of power on the triple-expansion engines is too great. The best general de- September, 1909 sign would probably be two _ three- cylinder compound engines on _ the wing shafts with a low-pressure turbine on the center shaft capable of taking half the load at full speed. For max- imum power live steam might be turned into all the cylinders with the exhaust going to the turbine. For small speeds the wing engines could be used as compound with the ex- haust going direct to the condenser, and the center screw uncoupled. Transmission by Electricity. A method of transmission by e'ectrici- ty has been suggested by a number of engineers as the best solution of the marine problem, and _ recently a paper by W. P. Durtnall before the British Institute of Marine Engineers has presented the case in concrete form. He proposes high-speed turbines to drive generators from which the cur- rent passes to motors on the main shafts. He would use polyphase alter- nating currents: with synchronous gen- erators and squirrel-cage induction mo- tors for the purpose of keeping down weight and promoting efficiency of transmission at a number of speeds. One example worked out with a view to determining the actual weight of machinery for a 4,000-horsepower ship gave 20.5 pounds per shaft horsepower. This is heavier than for all-turbine propulsion, but inasmuch as there will be greater economy in the use. of steam for all speeds the reduction of weight in the fire room will more than offset the increase in the engine room, and the total weight chargeable to propulsion will be less than that for all-turbine or all-steam-engine installa- tions. The calculation is entirely theoretical and for that reason it is not convincing. Other systems have been suggested; in fact, when the producer and gas engine are added to the list, the prob- able outcome of the keen competition for the field cannot be predicted. The whole subject of marine propulsion is in such a state of flux that anything may survive. It is possible that the steam engine may take a new lease of life for high-speed ships and come back to its own once more. While all sorts of schemes are put forward, one test should invariably be applied; that is, the probability of derangement or breakdown. That probability is in- creased more than in proportion to the number of links between the boiler and the propeller. It is just like multiply- ing a number of efficiencies together-- the ultimate efficiency, as well as the ultimate reliability, is less than that of any single link. Gonsequently, the