136 ELECTRIC PROPULSION OF SHIPS. 



On referring to the plan view of the electric geared machinery, it will be seen that there 

 are two central compartments, each of which contains a complete turbo-generator and the 

 various auxiliary machinery. There are eight boilers divided into two groups, four being 

 arranged in separate compartments on each side of the turbo-generator spaces, the total width 

 of the machinery spaces being approximately 61 feet. The uptakes from the four boilers are 

 led to two funnels, arranged on the center line of vessel and directly above the turbo-genera- 

 tor compartments. 



Multiple shaft arrangements of machinery came into use solely to suit direct-driven 

 steam turbines, and it is rather curious that the electrical firms merely copied a type that was 

 never considered suitable for battleships of 28,000 S. H. P'. to 32,000 S. H. P. by many naval 

 constructors and naval engineers in Washington. 



Referring again to Mr. Thau's paper, electric gearing has its limitations just as well as 

 all other forms of propelling machinery ; among these may be mentioned : 



Backing qualities. Experience has shown that it is not necessary to have more than 60 

 per cent astern power, and it is overlooked by many that it is "stopping qualities" which are 

 required. In other words, two large-diameter slow-turning propellers will bring any large 

 vessel to rest quicker than four fast-turning propellers which at all high speeds of revolution 

 do not take a grip on the water but tend to cavitate in a vortex. 



The electric-geared battleship does not maneuver or go-astern with full "ahead" shaft 

 horse-power. I believe it is now an official order in the Navy that all maneuvering has to be 

 carried out with one turbo-generator transmitting current to the four propellers in parallel. 



This limits the maneuvering capabilities to a large extent, for all propellers must re- 

 volve at the same speed, if in operation. For instance, the two port propellers cannot run at 

 maximum speed "ahead" whilst the starboard propellers revolve "slow astern." A complete 

 explanation for this limitation when manuevering will be found in a paper published in the 

 Journal of the American Society of Naval Engineers, August, 1921, in the article "Pro- 

 pulsion Wiring Circuits, U. S. S. New Mexico," under the sub-title "The Steam Limit De-- 

 vice" on pages 509-511. 



Weights, space and economy are all in favor of mechanical geared turbine machinery, 

 but until further data are available regarding these items in connection with electric gearing 

 in war vessels, it is useless to attempt to discuss these features. 



The battleship North Dakota, built in 1909, has had the original direct-driven twin tur- 

 bines: replaced by twin sets of single-reduction Parsons geared turbines, and the new machin- 

 ery has resulted in a gain of 31 per cent in fuel consumption at all speeds. For further in- 

 formation on this point, reference should be made to the report of the Engineer-in-Chief , 

 U. S. N., for 1919. 



I should also- like to mention that this new machinery effected a saving of at least 50 

 per cent in the weight, went into the same engine-room space with very few changes, and 

 has done very well ever since the vessel went back into commission. Further, this replace- 

 ment could not have been carried out with any of the present forms of electric gearing. 



Referring to the data accompanying the performance of the New Mexico and the so- 

 called sister ships, I think the author cannot have realized that these figures were quoted in a 

 paper read by Mr. Eskel Berg at a meeting of A. I. E. E. during May of this year. Mr. 

 Berg gave the source of his information; the data were submitted to the editor of Marine 

 Engineering and published in an editorial in the issue of May, 1920, entitled "Two Years 

 of Electric Propulsion on New Mexico." 



I shall not comment further on this point but suggest that anyone who is interested in 



