no 



ELECTRIC PROPULSION OF SHIPS. 

 Table of Probable Operating Personnel. 



* Based on oil-burning ships, 

 by three coal passers. 



For coal ships, the operating personnel would be augmented 



acteristics in either direction of rotation, all types of electric drive inherently afford, or can 

 be made to afford, full torque and power for reversal. For reasons of economy, space and 

 cost, the reversing element of a geared turbine is designed to give only 40 to 60 per cent 

 power in the reverse direction. It has been claimed that full reverse power is not essential 

 to a merchant ship, and this is true under ordinary conditions. In emergency, however, it 

 is desirable to stop the ship very quickly, and in this connection it might be pointed out 

 that the electric ship can be stopped in considerably less time than a geared-turbine ship. 

 This is not altogether the result of less backing power but is due to a great extent to the 

 inefficiency of the reversing element of the geared turbine causing an enormous draft on the 

 boilers which greatly reduces the steam pressure. 



In reversing, the energy put into the screws by the action of the water and the stored 

 energy in the rotating parts attached to the propeller shaft must be dissipated in some man- 

 ner. In the case of the geared turbine this energy is consumed in doing work on the steam. 

 In the case of A. C. electric drives, the energy of reversal is dissipated in the motor and 

 generator windings, the rotors of the motors, or in external resistors connected to the rotor 

 circuits, depending upon the tj^^ of motor used. In D. C. electric drives the reversal energy 

 must be absorbed elsewhere in the system. The amount of energy to be dissipated or ab- 

 sorbed in the case of a given ship depends entirely upon the time taken to stop the screws. 



