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POT STP ee ET EE Ne re ee a eh em TAE Marine REVIEW 45 Electric Propulsion of Ships, With Note on Screw Propellers The electrical engineer is on the outlook for new worlds to conquer. He has placed his mark on all other branches of engineering, and it does not seem too much to expect that he may also have a share in the further development of marine engineering. No great advance takes place with- out a preparation for change. The applications of electricity have been conspicuous examples of this tule. .Dhe-end. of, the. first. 30..yéars of the history of the practical use of electricity for the transmission of mo- tive power is approaching. Two gen- erations of steam engineering preceded _ its advent, and prepared the way for rapid development. On land steam engineering and electric engineering have exercised a powerful mutual in- fluence, and the development of the steam turbine has been a direct out- come of this interaction. The use of the steam turbine for propelling ships is already strongly influencing marine engineering, and this would in itself justify a re-ex- amination of the problem from a new standpoint. The use of steam in a turbine in- volves essentially a speed of rotation far above what is customary in re- ciprocating engines. Formerly exist- ing methods have involved direct ap- plication of the energy of the prime mover to the propulsion of the ship, and these methods are closely re- lated to the comparatively slow speed of rotation of the reciprocating steam engine. It is not surprising, therefore, that the steam turbine, when applied to marine propulsion, should be materially modified, and that the marine steam turbine is a compromise in which some sacrifice of economy has been made for the sake of avoid- ing a greater loss due to the charac- teristics of the propelling apparatus. Within the limits of practice, the efficiency of the steam turbine is in- creased by increased speed of revo- lution; while the efficiency of the pro- peller is decreased by a corresponding change. Fast running turbines and slow running propellers are respectively highly _ efficient instruments, but hitherto there has been no_ proper method of coupling them together. "Institution of Engineering and Ship Build- ers in Scotland, BY HENRY A. MAVOR, The underlying principle of the pro- posals here made is the use of a high-speed turbine running at a_ uni- form velocity for all speeds of the ship, and driving a dynamo from which electric power is taken to a variable speed motor or motors coup- led direct to the shafts of propellers of maximum efficiency. The dynamo and motor here occu- py an intermediate position, as _ in- deed they do in all cases, between the power and the work, and in ef- fect constitute a gearing which can submit to modifications of its own characteristics, to meet the character- istics on the one hand of the prime mover and on. the other hand of the machinery to be operated. It is precisely in this region that electricity. has found a place in en- gineering on land, and it is now pro- posed to examine the possibility of its use at sea. If such a possibility can be estab- lished, and practical applications made, no doubt the methods of ap- plying the power will be subjected to modification; but this reaction has been on land of very slow growth, and little has yet been done in the way of making special machinery to take full advantage of the properties of electric transmission. -- For this reason it is not necessary to enter into any questions involving material modification of the ship or the methods of operating it. For present purposes, in considering electric motors, attention may be con- fined to two types which have proved themselves capable of the most exer tended adaptation. The use of alternating currents for | electric distribution is developing very slowly in Great Britain. The reasons for this are various, the most important. being that continuous cur- rent had established itself as a com- mercial success before the advent of the multiphase systems. But even in new countries where development the most convenient lines is free, the continuous current still holds the field for direct application of power, and in the application of electricity to transit, continuous current methods are still in the first position, notwithstanding the fact that the power stations are equipped with alternating current plant, the supply to the street cars is by continuous current, the conver- sion being accomplished in sub-sta- tions. A very important fraction of the total electric power generated in this country is 'used in this way, and this has tended to perpetuate the idea that alternating currents are not suitable for direct applications of power. The superiority of the multiphase alternating induction motor in respect of compactness, endurance, and sim- plicity, together with the fact that nearly all large electric generators over about 1,000 horsepower are made for alternating current, is gradually leading to a more general adoption of this class of machine, and attempts have for some years been in prog- ress to overcome its difficulties in practical application. aa The chief of these difficulties is the low torque at start, that is to say, when the motor is at rest it requires artificial means to enable it to de- velop its full turning moment. These means are more complicated and costly than the equivalent apparatus for continuous current motors. The ordinary multiphase induction motor also compares unfavorably with the continuous current motor in re- spect of its availability for changes in speed. The continuous current mo- tor can now be made to give varying speeds through any necessary range without much loss of efficiency, the only consideration being cost. MHith- erto the induction motor has not been able to meet this requirement. This latter disability is less serious than is generally supposed, because' the cases fequiring a smooth _ speed- change are comparatively few, and where they exist they have been in many instances dealt with by mechan- ical means. Where very frequent starting and stopping have to be combined as they often are with gradually varying speed, something new must be at- tempted if the alternating motor is to take the field. The author has been at work on this problem for some years with the result that he claims to have solved the problem, and that he can meet most of the demands which are likely to arise in practice and deal with some new fields of en- terprise which have hitherto, for various reasons, remained closed to the electrical engineer. Hitherto the use of electric motors oh