ELECTRIC PROPULSION OF SHIPS. 



141 



Important ships that would be much better equipped with the single screw have gone 

 to the trouble, weight and expense involved in using twin screws to secure the advantages of 

 the divided unit and to obtain sufficient power while avoiding the expense of the larger 

 engines required with the double efficiency losses of the electric drive. 



As to larger powered engines, with the new compound much larger units are possible 

 without approaching the size and space occupied by the simple Diesel engines. Through com- 

 pounding the combustion engine (see author's paper on this subject before the 1921 Annual 

 Meeting of the American Society of Mechanical Engineers), tail shaft powers up to 12,000 

 horse-power are available with the present cylinder and cylinder wall limitations, with only 

 six combustion cylinders in line in each engine, utilizing the arrangement of Fig. 5. If 

 a similar pair of engines is used aft of the gear, then a shaft horse-power of 24,000 can 

 easily be secured, and the weight can be held to a point far below the present weight per shaft 

 horse-power of combustion engines or steam plants for continuous heavy duty service. 



With the simple device of the heavy duty air-gap drive, everything claimed to be ac- 

 complished by the electric drive is accomplished, and much better and with greater economy. 

 The efficiency of the air-gap drives alone is better than 99.5 per cent. They weigh less than 

 2 per cent of the weight of the electric drive, and as to valuable space occupied, they are moved 

 into the small flywheels of the engines themselves, where they are accommodated perfectly, 

 and from which point they make their varied and most important contributions to ship 

 propulsion. 



The following table gives the percentage of clutch excitation for fractional tail shaft 

 speeds below the engine speed, the first half when the oil engine is running at one-half nor- 

 mal speed, the second when running at one-third normal speed : 



No. 50 Electromagnetic Clutch or Air-Gap Drive Applied to a 900 Horse-Power 

 Diesel Engine, 140 R. P. M. Geared to Propeller at 90 R. P. M. 



Engine 



R.P.M. 



Propeller 



R.P.M. 



Per cent 

 engine 

 speed 



H.P. 



Torque 

 Ib.-ft. 



Air-gap drive 



Torque 

 Ib.-ft. 



Per cent of 

 normal excitation 



Full speed .... 

 One-half speed 

 One-half speed 

 One-half speed 

 One-half speed 

 One-half speed 



Full speed 



One-third speed 

 One-third speed 

 One-third speed 

 One-third speed 

 One-third speed 



140 

 70 

 70 

 70 

 70 

 70 



90 

 45 

 33 

 22.5 

 11 

 5.5 



100 

 100 



75 



50 



25 



12.5 



900 

 112.5 

 47.5 

 14.0 

 1.76 

 0.22 



53, 000 



13, 200 



7,300 



3,260 



820 



206 



70, 000 



17, 000 



4,780 



2,120 



530 



133 



100.0 

 100 to 50.0 

 Slipping 50.0 

 Slipping 11.5 

 Slipping 2.0 

 Slipping 0.43 



140 

 46.7 

 46.7 

 46.7 

 46.7 

 46.7 



90 



29.5 



22.5 



15 

 7.1 

 3.7 



100 

 100 



75 



50 



25 



12.5 



900 

 33.3 

 14.0 

 4.15 

 0.52 

 0.07 



53, 000 



5,900 



3,300 



1,470 



366 



91.5 



70, 000 



7,560 



2,120 



945 



236 



59 



100.0 

 100 to 33.0 

 Slipping 33.0 

 Slipping 7.4 

 Slipping 1.3 

 Slipping 0.27 



Full or normal excitation of magnetic drive = 1.4 kw. or 0.2 per cent efficiency = 99.8 per cent. 



