ELECTRIC PROPULSION OF SHIPS. 131 



unnecessary and undesirable for ordinary vessels. I think the shafting, etc., should be de- 

 signed as usual and the motor torque limited so as not to exceed the maximum designed 

 stress. With such an installation, there would be practically no difference in the possible 

 backing torques of different types. In this connection, I may call attention to the fact that 

 the full backing power of steam engines and steam turbines is seldom used, and especially on 

 vessels of high power, for the reason that heavy vibrations are set up. In very high-powered 

 vessels tliese vibrations are imusually violent; so much so that I doubt the wisdom of apply- 

 ing even the full torque to vessels such as scout and battle cruisers. 



In general, the backing power of steam engines and compound geared turbines is ample; 

 it meets the requirements, and no more has been called for. Consequently, I fail to see the 

 advantage of giving more than is needed or used. 



On page 119, also, the author refers to the superior maneuvering qualities for naval ves- 

 sels. Rapidity of reversal is a point often referred to in this connection. This advantage 

 does not appear to be of great importance, since all types can be handled quicker than they 

 usually are. Also, all types can be reversed in 10 to 20 seconds, which is satisfactory, con- 

 sidering that the propellers must act for a considerable period of time to overcome the mo- 

 mentum of the ship. 



In general, I consider the superior backing and maneuvering qualities of electric drive 

 as largely theoretical, and believe there is little if any real practical advantage for ordinary 

 vessels. 



On page 118, the author states that the Diesel electric is superior in fuel consumption 

 weight, etc., to other types. I have not found this to be the fact, and am giving particulars 

 in Table I to show that there is a wide difference between this data and the author's opin- 

 ions. In general, I think the author's claims and conclusions would be more convincing if 

 specific data had been given. Specific and accurate comparisons are the best methods of de- 

 termining which is the best type of machinery for a particular vessel and service, and this 

 method is suggested to the shipowner who has to choose a new type of machinery. Also, 

 if fair and reliable comparisons of this kind are published, there will be much less room for 

 differences of opinion. 



Referring to page 118, the author's views as to the advantages of the Diesel electric over 

 the direct-drive Diesel are not concurred in. I should say that the direct drive is more reli- 

 able due to the lower r. p. m. I cannot see the alleged advantage in maneuvering. The 

 absence of engine reversing gear is more than made up by switchboards, switching gear, 

 etc. I find no advantage in propeller application except the additional complication when 

 twin screws are used. In this connection, the trend seems to be towards single-screw en- 

 gines. I also fail to see the advantages in reserve power and maintenance. In the designs 

 which I have handled, the reserve power of the Diesel electric has been more closely limited 

 than in any other type. 



The author states that the engines are designed and built on the same conservative basis 

 as direct-drive Diesels. His views in regard to conservative designs and revolutions are 

 evidently at variance with those of most marine engineers. It is the general view that low- 

 revolution engines are the more reliable. I do not think engines at 250 to 350 r. p. m. will be 

 accepted as equally as conservative as engines at 75 to 125 r. p. m. In general, the use of 

 high-speed engines is regarded as questionable, and they are considered as not sufficiently 

 conservative. 



If a high-speed engine should prove desirable because of weight, cost, etc., it would seem 



