152 ELECTRIC PROPULSION OF SHIPS. 



I cannot agree with Mr. Sperry in his comparison of the electric clutch system with 

 the Diesel electric system using motors and generators, as obviously the latter is considerably 

 more flexible than the former and possesses distinct advantages that are not common to the 

 two systems. The clutch, for instance, does not eliminate the reversing gear of the engine 

 and does not eliminate the necessity for varying the engine speed in order to obtain speed 

 control of the screw. Even assuming that the screw can be reduced by engine throttling to 

 half speed, and controlled from there to zero by slipping the clutch, I am sure that if this 

 were continued for any length of time it would be necessary to use special means for dissi- 

 pating the heat resulting from the slip energy in the clutch, and this introduces a complica- 

 tion of some kind. The energy resulting from the slipping of the clutch below its syn- 

 chronous speed is comparable to that which exists in the case of an induction motor, and 

 with this type of machinery it has always been found necessary to dissipate excessive slip 

 energy externally; for this reason, when adjustable speed induction motors are required, it 

 is necessary to furnish the wound secondary type so that the slip resistance can be incorpo- 

 rated external to the motors. It is true that the slip energy at low propeller speeds is con- 

 siderably less than at high propeller speeds, for the reason that the propeller power varies 

 approximately as the cube of the speed, but the powers are hardly sufficiently small below 

 half speed to enable slip energy to be absorbed in the clutch, without providing some means 

 for dissipating the heat. 



Mr. Sperry gives four items of performance contributed by the Diesel electric system 

 of propulsion. In the first place, he endeavors to show that the propeller speed is compro- 

 mised by the application of a motor. This, however, is not the case, as with direct-current 

 motors we can accommodate any desired propeller speed. Furthermore, the generators can 

 accommodate any engine speed, so that the selection of propeller speed is, therefore, not in- 

 fluenced. There are no limitations in D. C. motors and generators which influence their 

 speeds for ship drive. 



Second, the amount of reserve power in the case of the air-gap clutch drive, based on an 

 equal number of engine units, is not as great as with the Diesel electric drive using genera- 

 tors and motors, as the engine speed in the case of the air-gap clutch drive must be reduced 

 in proportion to the reduction in the speed of the ship, therefore reducing the available horse- 

 power of the remaining engines. 



Third, in varying the speed of an air-gap clutch drive, it is necessary to vary the speeds 

 of all engines simultaneously, and the engines must govern at any speed ; otherwise there will 

 be an interchange of power between the engines and a waste of energy. With the Diesel 

 electric using motors and generators, the generators run at constant speed for any condi- 

 tion of propeller speed and need have only the crudest type of governors, because with series 

 operation it is not even necessary to have theengines running at the same speed. The motor 

 speed is controlled by varying the generator voltage, which is effected through the operation 

 of a simple field rheostat, so that in going from full speed ahead to full speed astern, it is 

 merely necessary to move the lever from one extreme to the other extreme. Certainly nothing 

 more flexible than this could be desired, and these advantages in flexibility do not obtain with 

 the air-gap clutch Diesel drive. The operation at slip speeds of the air-gap clutch has been 

 discussed previously. When the clutch is slipped, the losses vary directly with the speed re- 

 duction and power. This means poorer efficiency at low speeds than is the case with straight 

 electric. The matter of cost is something which the author does not propose to know any- 

 thing about, but it seems that the cost of generators, motors and control, which are all 



