Application of electric propulsion to double-ended ferry-boats

The double-ended ferry-boat propelled by means of a bow and stern propeller has become the recognized standard type, due to its maneuvering possibilities and general handiness in congested harbors. In all cases in which the prime mover is directly coupled to the two propeller shafts which must necessarily turn at the same revolutions per minute, the over-all propulsive efficiency is lowered due to the performance of the bow propeller. The electric drive system permits of applying power when and where required and to any degree desired. Tests on the double-ended ferry-boat W. R. HEARST show a material gain in propulsive efficiency when driving the bow propeller electrically at a speed which gives neutral thrust. Later tests indicate, however, that there is no substantial difference in the propulsive efficiency, whether the bow propeller is driven electrically at neutral thrust, or is electrically disconnected and driven by the water. Sufficient tests have not been made, however, to show that this is true in all cases. The reciprocating steam-engine or Diesel-engine type of drive, in which both shafts are direct connected, require approximately 19 per cent more horse power at the propeller shafts than the electric system, due to the difference in propulsive efficiency. The calculated fuel consumption of a typical reciprocating steam-engine drive with the direct-connected system shows that it requires approximately 40 per cent more fuel than the steam turbine electric system, due to the difference in propulsive and thermal efficiencies. The electrical transmission losses are less than the propulsive efficiency losses of the direct systems. In addition to the more efficient method of power application, electric drive also has many inherent advantages, such as rapid maneuvering qualities and ease of control. The Ward Leonard system, similar to that used on the Chicago fire boats which were put in operation in 190%, permits of the use of pilot-house control, eliminating the personal factor which is always present with the engine-room telegraph. The operating records of ferry-boats in service prove electric drive to be reliable. The respective field of application of turbine electric drive or Diesel electric, drive for double-ended ferry boats depends upon the relationship of first cost to the operating changes and needs of the service.