146 ELECTRIC PROPULSION OF SHIPS. 



Mr. Emmet says that I have understated a few of the points of advantage or of at- 

 traction in regard to electric drive. I can merely reply to this by saying that I endeavored to 

 be ultra-conservative throughout the entire paper, so as not to invite undue criticism from 

 gentlemen who oppose electric drive or who are not yet prepared to accept it. This is one 

 reason why I placed maintenance on not worse than a parity with other drives. I am sure that 

 the electrically driven ships in operation have already shown a surprisingly low main- 

 tenance cost. 



I have given considerable thought to the efficiency of a synchronous drive, as compared 

 with an induction drive, and have always concluded that there can be only a very inapprecia- 

 ble difference in efficiency. The suggestion by Mr. Emmet that the adoption of electric aux- 

 iliaries throughout will enable the installation of larger auxiliary turbines which would be 

 conducive to- lower steam consumption for excitation is, of course, correct, but I think that, 

 considiering everything, the ultimate difference in efficiency of the two drives would be 

 inconsequential. The largest attraction of the synchronous drive is in the weight, space and 

 cost saving resulting from unity power factor. As I have shown later in the paper, these 

 advantages can be obtained with the induction drive, using the power factor corrective 

 apparatus. 



The larger air gap and better access of the S3Tichronous motor were mentioned in the 

 last paragraph of my description of the synchronous motor drive. 



By "high-speed Diesels" I do not have in mind high piston speeds. The matter of piston 

 speed must be treated with conservatism in high R. P. M. engines, as well as in low R. P. M. 

 engines. The reciprocating parts being smaller, the cylinder walls and cylinder heads thin- 

 ner (this being conducive to quicker heat transfer and lower temperature strains), and the 

 absence of water cooling on the pistons should decrease rather than increase the maintenance 

 of the engines. Repairs are easier to make because of the smaller parts to be handled. There 

 are engines of 200-horse-power capacity at 250 R. P. M. that have been operating con- 

 tinuously, except for periodic valve grinding, since 1914, and during this time the mainte- 

 nance was practically negligible. Mr. Smith has also questioned the reliability of the higher 

 R. P. M. engines. 



Regarding the application of alternating current for Diesel electric drive, I am quite 

 sure that the flexibility in control advantages alone of the direct current would go a long 

 way to favor the direct current, even though parallel operation of a multiplicity of alternating 

 current units were entirely feasible. It has been shown definitely in the case of land in'^tal- 

 lations that Diesel driven alternators do work satisfactorily in parallel, but it must be re- 

 membered that these engines operate at constant speed instead of at adjustable speed, as 

 would be necessary in the case of ship drives. Where the Diesel engine units are sufficiently 

 large so that one unit would supply one motor, as in the case of a twin-screw drive, alternating 

 current is feasible and will undoubtedly be used under certain conditions. 



Mr. Emmet mentions that the interchangeability of electrical units in a ship installation 

 was not brought out in the paper. I did not deem it desirable to go into this matter in detail, 

 as the principal part of the paper was on merchant ships, whereas such opportunities are only 

 offered on high-powered ships having two or more generators. Reference was, however, 

 made to this feature under the captions of "Reliability," "Economy^' and "Control" in the 

 section covering war vessels. This advantage was also referred to in connection with the 

 Diesel electric drive. For large ships these advantages are important, as is perfectly obvious, 

 for the reason that balanced power can be obtained on all screws, regardless of the number 

 of generator sets in operation. 



