130 ELECTRIC PROPULSION OF SHIPS. 



Referring to the table on page 110 and to the number of men allowed for the Diesel elec- 

 tric, I should say that this class of machinery would require at least as many men as other 

 types. 



I understand from the paragraph on maintenance that the Diesel electric is supposed to 

 be less expensive. This, I think, is very doubtful. My experience with marine machinery is 

 that high-speed reciprocating engines are more expensive in maintenance and are avoided for 

 this reason. Certainly the slow-speed engine is the one most favored for reliability and low 

 maintenance. I do not think many marine engineers will accept the author's appraisal of 

 this item, which at best is a very uncertain one. 



Referring to performance characteristics on page 109, I wish to call especial attention to 

 one feature of electric drive machinery which I think is very much ever-rated and over- 

 advertised — that is, its backing and maneuvering qualities. (It may be noted here that direct- 

 turbine drive installations are not considered in the following or at all in this discussion.) 



About the only time when the full backing power of the ordinary ship is required is 

 when stopping in emergency at full speed or nearly full speed. I will therefore consider only 

 this latter condition, because the ordinary requirements of backing are fully met by all of the 

 types of machinery under discussion. 



Many persons seem to have a misconception as to the backing power of the ordinary 

 steam engine. It is supposed that the engine develops 100 per cent full power when stopping 

 at full speed. This is not the fact; only about half of the full power is developed. (See 

 tests of U. S. S. Delaware, Birmingham, Salem, etc.) 



This is due to the fact that, when wide open in either direction, the m. e. p. and the 

 torque of the engine are approximately the same. Dut to the characteristics of the propeller 

 in reversing, the full torque gives an average r. p. m. in stopping only half of the ahead 

 r. p. m. Consequently, the astern power is only half of the ahead power. In this connection, 

 it would appear better to refer to backing torque instead of backing power. 



The torque of turbine increases as the speed decreases, and it is also possible to allow 

 an increased steam flow for emergency reversal, which, as is well known, is required very 

 seldom indeed; consequently properly designed astern turbines will develop from 75 to 100 

 per cent of the ahead full torque when required. This has been proven to be quite ample ; 

 which is a point which is well to keep in mind before making assertions to the contrary. The 

 above refers to compound reversing geared turbines. 



It is also well to keep in mind that the thrust of the propeller under such conditions de- 

 pends as much on the size of the wheel as on the torque applied to it. Consequently the large, 

 slow wheel of a geared turbine may retard more than a smaller and faster wheel of an elec- 

 tric drive. I mention this because we are nearly always urged to use from 20 to 40 per 

 cent higher propeller speed for electric motor drives than for other types to reduce the size, 

 weight and cost of the motor. 



I believe that the motors of electric drives are good for a reversing torque of one and 

 one-half times the ahead full torque, the pull-out torque being double. In order to obtain 

 the superior performance referred to by the author and others a torque of one and one- 

 quarter to one and one-half times the full torque is imposed on the motor in reversing. This 

 results in one of two efifects : Either the normally designed shafting, propeller, etc., are 

 stressed beyond the safe limit; or the shafting, etc., must be increased in size, weight, and 

 cost to allow for about 50 per cent more strength. 



It is my opinion, from careful observation and study of this subject, that this feature is 



