ELECTRIC PROPULSION OF SHIPS. 147 



The advantage resulting from the change in ratio in motor speeds for a given frequency 

 is also mentioned in the paper under the caption of "Economy" in the section covering war 

 vessels. This item was not discussed in detail, as it has been given considerable airing in 

 recent teclinical publications. Mr. Emmet brings out the comparison that the advantage re- 

 sulting from the change in pole ratios of the motors is more pronounced on the Maryland than 

 on the Tennessee. This is true for speeds between 13 and 15 knots, but for lower speeds 

 the larger number of poles show better economy. This characteristic can only be explained 

 by the combination of turbine and motor and generator efficiency performance. I think the 

 performance of the Tennessee is more or less incidental, as such a condition would not be 

 anticipated. The turbines installed on the Tennessee are particularly economical at a very 

 wide range of speed. 



Three of tlie discussions, at least, dwelt at considerable length on weights and sub- 

 mitted some data which are at variance with the author's analysis of weights, space, cost, etc., 

 for tlie different principal types of propulsion. Any data of this nature must of necessity be 

 considered upon its hypothesis, as there are several ways of comparing such items relating 

 to propulsive machinery. There is, however, only one correct way, and that is to include 

 every single item which by virtue of its fimction is directly related to the propulsive equip- 

 ment, such as engines proper, boilers, condensers, propulsive equipment auxiliaries, shaft- 

 ing, foundations, seatings, water in machinery, reserve water for machinery, stacks, piping, 

 ladders, platforms, thrust bearings, shaft alley bearings, propellers, generators, motors, con- 

 trol, cable, exciters, fuel, lubricating oil, etc. A comparison of the bare machinery weights 

 means nothing. 



The analysis of the Diesel electric propulsive machinery in the paper is based upon a 

 single-screw drive in which the propeller R. P. M. is uninfluenced by the motor speed limita- 

 tions, because with Diesel electric drive, using direct current, the propeller speed can very 

 conveniently be made to suit the preference of the propeller designer, as there is absolutely 

 no fixed relation between the generator speed and the motor speed. 



Because of the well-known fact that practically all of the direct-drive Diesel ships today 

 are twin screw, this arrangement has been considered in the case of such ships. 



It is probably unfortunate that a table of comparative weights showing all items con- 

 sidered was not incorporated in the original paper. The author's statements in comparing 

 the different factors of the various drives were of a general nature, and this system was used 

 purposely to avoid contention on details. To substantiate the statements in the paper, par- 

 ticularly in connection with weight, the table on page 148 which forms the basis of the 

 author's analysis, is included in this reply for reference. 



A brief discussion of this table will reply to several points raised by Mr. Bailey and Mr. 

 Smith. It will be noted that at least every essential item on which there is any difference 

 in the various types of drives has been included. Unfortunately, I have no reliable data on 

 foundations which must be made a part of the ship's structure for supporting the propulsive 

 machinery. However, I believe tliat these items should just about balance in the various 

 types of drives, and what little difference might exist would certainly be of little influence 

 in the ultimate results, or might even favor the Diesel electric. 



The Diesel engines considered in the table for the Diesel electric drive operate at 180 

 R. P. M., and complete with their accessories, generators, exciters, the motor and control, 

 weigh 350 pounds per shaft horse-power. Of this figure, the electrical equipment con- 

 sumes 104 pounds, leaving 246 pounds for the engines. Certainly no one will question the 



