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for marine work. 46 for marine propulsion has been con- fined to small craft, such as submar- ines, and one or two small barges and other craft on the Russian rivers and canals. A small electrically oper- ated and propelled dredger has also been constructed in Germany. The motors in these cases are con- tinuous current machines. The continuous current motor which is so convenient for small ap- plications of power up to 100 horse- power becomes less convenient over this size. Above 1,000 horsepower it is not a very practical machine for power purposes where starting and stopping have to be frequently accom- plished, and it is not specially suitable The multiphase in- duction motor on the other hand has all the properties to be desired in a marine motor except hitherto the facility of speed control. This prob- lem may now be said to be solved. Some years ago the idea of rotat- ing the outer member of the motor, usually called the stator, occurred to the author, and appeared to offer a solution of the starting difficulty, and this suggestion was made at a meet- ing of the Institution of Civil Engi- neers in 1904. The proposal was that the external member' should be mounted so as to be free to revolve concentrically with the rotor or arma- ture of the motor. The armature be- ing connected to the load and the outer member being free to rotate, the outer member starts to revolve in a direction opposite to the working direction of the rotor, and having only its own inertia and friction to overcome, it quickly reaches syn- chronous speed. The motor is now in a position to develop its full load torque with full load current, and all that is necessary is to apply a me- chanical or other brake to the exter- nal member, bringing it to rest grad- ually while the rotor starts and ac- celerates as the outer member or "spinner" comes to rest. The start can thus be made without excessive calls for current and without jerk. This idea soon developed into a proposal to modify the ordinary working speed of the motor by so ar- ranging the external member that it could be driven at any desired speed by gearing interconnecting both members, and an arrangement was worked out and patented for putting this idea into practical shape. It will be seen that by the use of a fast and loose pulley or friction clutch to transmit a part only of the power of the motor the starting can be accom- plished with a very small loss of en- the normal propeller THe Marine REVIEW ergy. This method, although serving the purpose for which it was des- signed and giving a_ useful solution of the difficulty, did not appear to be the final stage. The idea of rotating the outer: member and controlling its speed suggested the idea of control- ing its motion independently of gear- 'ing or other arrangements which in- volved disadvantageous complications, and the simple apparatus now exhib- ited was evolved, which entirely gets over the difficulty of starting, and gives a change of speed in either direction. In this machine the outer member or spinner is made to act as the inner member 'or rotor of a sec- ond motor, the stator of which em- braces the spinner. The machine is thus two motors concentricallly ar- ranged round the common axis. The three speeds which may be described as prime, secondary, and tertiary are attained without loss of efficiency, but if intermediate speeds are desired they are obtained by a_- slipping brake or external resistance in each case equally with a consequent loss of efficiency. This loss of efficiency is very small, and is less than the loss involved where series wound contin- uous current motors are used for speed variations; or intermediate speeds may be obtained by providing a separate supply of current to the external motor and changing its speed by change of periodicity of the sup- ply. The attainment of three speeds without any loss of efficiency in this class of motor is a very important improvement, because it at once ren- ders it available for such work as steel rolling mills, haulages, hoists, calendering and_ printing machines, lathes and boring mills, and last, and perhaps the most important of all, driving vehicles either on or off rails, including vessels on the sea. A proposition to apply electric mo- tors to drive a ship involves primarily a consideration of the conditions with respect to the propeller and _ the prime mover. Naturally, the propell- er with its shaft comes first, as upon its efficiency of transformation of en- ergy depends the size of the prime mover. It is necessary to institute a comparison for any given ship of as used with existing machinery, and see what are the prospects of economy by the substitution of electric transmission for the direct connection between the engine and the screw. At first sight it does not appear that any economy could be made by the introduction' of two -- successive energy transformations in the dynamo and motor respectively to 'take the place of a direct transmission. The maximum possible efficiency of these two transformations with the transmission shaft is not more than about 88 per cent; while the efficiency of the direct transmission shafting is probably not less than 95. We thus have a loss of at least 7 per cent as a primary handicap against the pro- posal. This consideration may _ be taken as disposing of the possibility by change of transmission of any economy in ships of moderate size with slow-running propellers of up- wards of 70 per cent efficiency. If electric driving is to be introduced into such ships it must be on other grounds than propeller economy. The case is different on coming to deal with ships in which the power is large and the propellers are run at a high speed. Such cases are becoming common in vessels driven by steam turbines. A very short incursion into the study of propellers is sufficient to show the trend of the conditions which here emerge. The subject has been treated in some detail in Prof. Biles' book on "The Marine Steam Turbine," 1906, where are given the results of re- searches on propellers and the meth- ods generally adopted in the design of propellers ate- .. fully ex- plained. .The causes of loss of effi- ciency in turbine vessels due to adopt- ing propellers too small for the best efficiency are also discussed in the same book. A ready and convenient method of making comparisons without the de- tailed labor of designing groups of propellers is desirable in the' present connection, and for this purpose the author has developed a method.* It will be seen that for any given ship, the designer who is at liberty to choose a large propeller for any given power has considerable advan- tage, asthe efficiency. may be a high as 70 per cent, and as low as 45 per cent. To say what improve- ment is possible in any case is by no means an easy matter, because the ac- tual efficiency of any given propeller is often unknown; but the difference in efficiency between' the probable bests when the diameter can be changed, may be found approximately, and it is this which has been at- tempted. The introduction of electric drive enables one to include in the consideration all speeds of revolution of the propeller, both low and high. In seeking for higher efficiency in the propeller, the limits imposed by practical considerations must, of *See "Note" in Appendix,