Discussion at Spring convention: Electrical ship propulsion

V. Karapetoff: The Diesel engine is essentially a constant-speed prime mover, and I should like to ask Mr. Thau about its application to the tug service, which, to me, is essentially a variable speed application, and also one involving a high slip, either in the electrical equipment or between the propeller and the churning water. With a d-c. drive, the Diesel engine can run at a constant speed, and the d-c. motor on the shaft can be controlled either by a series resistance or by a resistance in the field circuit. But I should like to know how this is done with a-c. drive, or is the a-c. drive not suitable for the tug service?