150 ELECTRIC PROPULSION OF SHIPS. 



later developments in electric-drive Diesels the space factor v^rill show greater favor in behalf 

 of the Diesel electric. 



In regard to relative fuel consumption, Mr. Bailey has used the same figure for the 

 single-screw Diesel as for the twin-screw Diesel, and according to analyses of propeller per- 

 formance that have come to my attention, I believe that the fuel consumption per ton-mile 

 should be very nearly the same for the Diesel and the Diesel electric, for the reason that the 

 twin-screw arrangement using the higher speed propellers and additional struts will require 

 more effective horse-power to drive a given tonnage through the water. Furthermore, the 

 fuel consumption figure for turbine electric is given as 0.95 as compared with the geared tur- 

 bine of 1.0. Mr. Bailey, as stated, uses single- reduction geared-turbine drive in this analysis. 

 I think tlie figure would have shown the reverse had double-reduction gearing been used. It 

 is also probable that the figure of 0.95 in comparison represents guaranteed economy rather 

 than actual performance. The above paragraph will also reply to similar remarks by Mr. 

 Smith. 



I believe that with machinery in proper working condition, the fuel consumption per 

 shaft horse-power, or rather per ton-mile, should run in the order of 1 for the geared tur- 

 bine, 1.04 to 1.05 for the turbine electric, and approximately 0.5 to 0.55 for Diesel electric 

 and direct-connected Diesels (the latter being twin screw). 



Such discussions as Mr. Bailey's are very constructive and will undoubtedly be very 

 beneficial in the final solution of the selection of propulsion machinery. Just at present, it 

 is expected to find considerable difference in the analyses made on Diesel and Diesel electric 

 drive, owing to the wide variety of engines available, and since the engine forms a large por- 

 tion of the weight of the Diesel electric drive, a considerable variation in this item would 

 show a large: difference in the total weight. 



Mr. Smith and Mr. Stevens questioned the omission of the junior engineers. The rea- 

 son why the junior engineers have been omitted from the list of personnel for the Diesel 

 electric drive is because one man is sufficient to operate the control, and this man would 

 always be one of the assistant engineers. In the case of the twin-screw Diesel, it is neces- 

 sary tO' station an engineer at each engine, and this requires two men, whereas in the case 

 of the electric drive one man is sufficient, regardless of the number of screws, as the control- 

 lers are easier to operate and require less manipulation than the ordinary surface electric car. 

 This practice will vary with different operating concerns. 



It will be noted that Mr. Smith takes considerable exception to the value of the backing 

 qualities of an electric-drive ship. If a ship and its shafting, etc., will withstand only a defi- 

 nite amount of backing torque, surely it is inadvisable to apply an excessive amoiont. In this 

 connection, the electric drive can be regulated to give any desired amount of backing torque 

 from the minimum to the maximum, as it is merely a function of the control. The principal 

 idea in incorporating in the paper a discussion on backing qualities of electrical machinery 

 was simply to show that this type of drive is as good as any type of drive, and can be made 

 better than some types. In regard to battleships, however, it has always been my under- 

 standing that quick stopping is essential and that the Navy Department lay some stress on 

 this point. Whether or not the ships are so operated as to not utilize the full backing pow- 

 ers of any type of drive, it is of interest to note that, on the trials, tests are made to deter- 

 mine the quickness with which a battleship can be stopped. In this connection it is interesting 

 to note that the Tennessee can be stopped from full speed in less than three minutes. I 

 think it is plausible to conceive a condition where this difference in stopping time would 

 avoid a wreck. 



