ELECTRIC PROPULSION OF SHIPS. 153 



simple apparatus, should not be much in excess of the cost of the air-gap clutch, which is a 

 refined piece of apparatus, flexible coupHngs, gears, larger air compressor plant, refined en- 

 gine governors, and engine reversing gear. Another point to be remembered is that this 

 apparatus requires very careful alignment. 



Fourth, in addition to eliminating the necessity for reversing Diesel engines as viewed 

 merely from a matter of reversing gear, the one-direction rotation, electric-drive Diesel en- 

 gine offers considerable simplicity to the air problem, since it is only necessary to start one 

 engine by means of air in getting under way, as the remainder of the units can be started 

 electrically. Therefore, the compressor equipment, air bottles, etc., together with reversing 

 gear, must be balanced against the cost of the electrical apparatus in the Diesel electric sys- 

 tem. I am, therefore, not prepared to agree with Mr. Sperry in that the "maneuverability 

 with these engines and the air-gap clutches accomplished all that the full electric plant does, 

 at a very great saving of plant and control equipment." I think that if Mr. Sperry's clutch 

 were reversible, however, it would represent an advance in the art of ship drive, particularly 

 as related to Diesel engines. 



Later in his discussion, Mr. Sperry gives five important advantages secured by the 

 application of the air-gap clutch system, on which I would like to comment in numerical 

 order : 



1. As pointed out previously, such a system does not give all the advantages and flexi- 

 bility of a multiple engine unit with as great reserve power as a Diesel electric system would. 



2. As stated previously, the air-gap clutch system does not possess the complete flexi- 

 bility of the electric drive, and it is questionable when all items which the electric drive elim- 

 inates are considered, together with the clutch, flexible coupling and gears, whether the ex- 

 pense would be an item for consideration. Mr. Sperry's allusion to the "cumbersome electric- 

 control equipment for handling the heavy currents in maneuvering" is not well founded, as 

 the only thing that is handled is the field rheostat in the generator excitation circuit, and this 

 handles only a small fraction (not exceeding lj4 per cent of the total power), and at that 

 does not even open the circuit. The notice taken of the double losses in motors and genera- 

 tors brings out, of course, a point which does exist when comparing the air-gap clutch drive 

 with the electric system, as there is possibly 8 or 10 per cent additional loss with the electric 

 drive when using the same number of propellers. If the electric uses fewer screws, this 

 economy difference vanishes. 



4. The safeguards due to the setting of the torque deveiloped can be accommodated just 

 as well by electric drive. 



5. The subject of "criticals" as mentioned here will not exist with the type of units pro- 

 posed for Diesel electric drive, as the generator armature furnishes the only rotational mass 

 connected to the engine, and this has little, if any more, inertia than the fixed half of the air- 

 gap clutch. The air-gap fluxes of the generators and the motor are comparable to the air 

 gap of tlie clutch. 



As Mr. McClelland states, the comparison of the New Mexico and Tennessee with the 

 Idaho and Mississippi is not a comparison of what might be done with a full gear drive, for 

 the reason that the two latter ships have direct-drive turbines with auxiliary cruising turbines. 

 Nevertheless it will be noted that even at the cruising speeds the electric ships show better per- 

 formance. Mr. McClelland's statements that the geared-turbine drive of an up-to-date installa- 

 tion would show slightly better economy at full speed than the electric drive, and that the elec- 

 tric drive would gain at the cruising speeds, is in fact the analysis that is conceded in this 



