ELECTRIC PROPULSION OF SHIPS. 119 



2. Maintenance and repairs should not exceed tlaose of other drives, and in some cases 

 should show a saving. 



3. Electric drive is ideal for ship propulsion and will soon be recognized, if it is not 

 already recognized, as a standard type along with tlie reciprocating-engine, geared-turbine and 

 Diesel-engine drives. 



4. Electrical machines have longer life than engines or geared turbines (drives) and do 

 not decrease in efficiency with age. 



5. Electric drive (Diesel electric) is as reliable as any economical drive, generally weighs 

 less than any other drive, is as economical as the best, in most cases costs less than any other 

 drive,* provides more reserve power in case of casualty to prime movers, and affords simplest 

 and most flexible control. 



WAR VESSELS. 



The electric propulsive equipments for war vessels have been described and discussed 

 in many articles in the technical press, and therefore only the principal phases will be dealt 

 with here. 



The fact that the last nineteen capital ships of the U. S. Navy are or will be equipped 

 with the electric drive is sufficient testimony in behalf of what the builders and users of war 

 vessels think of its merits. The prime requisite of reliability in any type of machinery de- 

 signed to propel war vessels was recognized in the electrical machinery at the time of the 

 first installation. Also the calculations showed that the unit fuel consumption over a wide 

 range of operating speeds should be better than anything yet proposed. Service operation 

 of two 30,000-horse-power electrically propelled battleships has indisputably proven that the 

 reliability is all that was claimed, and that the fuel economy, as compared with other ships of 

 the same type using direct-connected turbines with geared cruising turbines, is vastly superior. 



Two other factors in which the electric drive shows a marked improvement over other 

 drives have been emphasized since the first battleship was built, namely, the superior protection 

 from torpedo attack afforded the machinery by virtue of the arrangement of the electric 

 plant and the superior maneuvering qualities of the electric drive. The large horse-power 

 requirements of the present war vessels (60,000 horse-power and 180,000 horse-power) pre- 

 clude the use of reciprocating-engine drive, and this leaves electric drive with a decided ma- 

 neuvering advantage over any other form of turbine drive. 



Of the two types of electric propulsive equipments, only the turbine electric is suitable 

 for propelling war vessels, because of the large capacities required. The nineteen drives in- 

 stalled and building employ essentially the same system in that induction motors are used 

 in all cases. In details, the systems differ in regard to the type of induction motor. Of the 

 three ships in service (the Maryland having been recently commissioned), the New Mexico 

 motors have the double squirrel-cage rotor winding, the Tennessee motors have the form- 

 wound rotor with external starting and maneuvering resistance, and the Maryland motors 

 have combined single squirrel-cage and form-wound rotors. Each of these arrangements 

 has its advocates. However, continued service alone will decide which method is the most 

 suitable, all factors being considered. 



Because of limitations in weight and space factors, electric drive is not well suited to 

 small, high-powered, fast craft such as destroyers and scout cruisers. In the case of ships 

 where conditions are suitable for electric drive, the following discussion of the more im- 

 portant factors will be of interest: 



*It is predicted that future developments will bring the item of cost below that of any other drive. 



