The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

course, be taken into account, such as the beam and draught of the ves- sel. On the other hand, the revolu- tions at which the propeller may be efficiently used are fixed by the diam- eter and pitch. While, therefore, one may have as- certained what is the possible im- provement in efficiency, one must, be- fore claiming the possibility of such improvement, deal with the problem with the limiting diameter of the pro- peller as a first element in the calcu- lation. It... is...,this. considétation «which makes any saving in propeller effici- ency difficult in high speed passen- ger steamers of shallow draught. The difficulty of improving the efficiency of ships of moderate size with slow- running propellers has been shown, so that the most attractive field. of enterprise, from the propeller point of view, is in large ships of deep draught and high speed. Coming to the consideration of the prime mover, the scope of operations appears to be much wider. If the ship is to be electrically driven, it will probably be found that the most convenient steam _ prime mover will in all cases be a tur- bine. The limit of speed of rotation, when the turbine is to be used. for driving a dynamo or alternator, is of a different order altogether from the limit of sped imposed upon the turbine by its direct connection to the propeller. In some cases it will be easy to adopt an increase of 10 times in the speed of revolution. High speed of the turbine means high effi- ciency, and therefore the turbo-elect- ric generator with its motors com- pares favorably in economy, cost, and weight with the marine steam turbine directly connected to the propeller shaft, and therefore running at a speed lower than that called for by con- siderations of steam economy in the turbine. The reversibility and speed varia- bility of the electric motor driven by the turbo generator, run unidirection- ally at constant speed, dispenses en- tirely with the objectionable heavy and costly go-astern turbines mount- ed on the propeller shaft. The marine steam turbine has al- ready fairly established its claim to economy in cases where the require- ments of the propeller permit of a reasonably high speed of rotation rel- atively to the power used, and there- fore a direct comparison may _ be made of the results to be obtained on a ship of large size driven direct by steam turbines, with what can be THE MarRINE. REVIEW done by the application of electric gear. The . ship. has. in. the, first..case.a total of 17,000 horsepower delivered to the three propellers, and a speed of 2034 knots. The propellers are 8 ft. 1% tm -in: diameter, aad the speed of revolution is 377 per min- ute. This example is worked out in detail in the appendix, and it is there shown that the maximum _ probable eficiency of these propellers is 62 per cent, a figure which certainly does not err against the propeller in ques- tion. An electric equipment would per- mit of the use of propellers of about 14 feet diameter at 140 revolutions per minute, and an efficiency not less than 70 per cent--an improvement more than sufficient to cover the loss in the electric motors. It is further to be noted that the slower speed propellers would be free from the risks of cavitation troubles, which arise when the resistance is in- creased on a propeller already pro- ducing a high value of thrust per unit area, No attempt 1s amade to pit-4 figure on the economy emerging un- der this head, but its importance is evident. Turning now to the generators, it will be found that the steam turbo-al- ternator as used on land in sizes cor- responding to those. now under dis- cussion, can deliver an electric horse- power to the motors of 12.2 pounds of steam per horsepower hour, allow- ance having been made for all inter- mediate losses, and for 4 per cent for driving auxiliaries, no superheat being used, so as to make the com- parison with normal marine practice. This figure is startlingly lower than anything which has been touched at sea. It is for full speed and power, but the comparison. must also be made at low speeds and powers, and more particularly at the normal work- ing speed of the ship. It will be found that theré is cer- tainty of economy in this comparison, again because of the higher speed of revolution of the turbine, which in the electric case runs at the same speed for all powers, while, of course, the direct connected turbine must vary its speed of revolution with the speed of the ship. It is not possible to base any gen- erally conclusive argument on these data, but there is evident encourage- ment to give detailed studv from this standpoint to any particular case which may arise. The same limita- tion applies to weight and cost. In the example referred to an estimate 47 has been made of the weight, with the result that it appears probable that the decrease in the weight of the steam equipment will more than com- pensate for the heavy slow-running motors and propellers so that the weight will not be greatly changed. The cost will probably closely follow the weight. Figs. 1, 2 and 3 illustrate a small 5 horsepower model of a three phase a alternating current "spinner" motor for ship propulsion. This motor is shown in elevation and part longitudinal section in Fig. 3. ; The electric current is 'delivered from an outside source to the pri- mary windings of the stator and spin- ner respectively, passing in each case through a simple reversing switch which determines. the direc- tion of rotation. The stator circuit also supplies a magnet, which, when no current is passing, releases a brake which brings and _ keeps the spinner at rest. When current is pass- ing, the magnet lifts the brake and leaves the spinner free to revolve. The three-speed motor provides a means of obtaining all the speed vari- ations which are required on a ship. The intermediate speeds between the three normal speeds of the motor are obtained by variations in the speed of the generating plant, which are within the limits of practicability and economy. Each propeller shaft is provided with a directly connected motor on which there is co-axially superimposed a second motor for speed regulation. The regulating mo- tor is mechanically connected and magnetically entrained with the first