The earliest torpedoes featured a vane to offset the torque of a single propeller, which would otherwise cause it to rotate and thus veer off course. It would be replaced by contra-rotating screws that would set the pattern for all subsequent designs. Clearly there are benefits to replacing a rudder with a means of steering by a differential operation of contra-rotating screws, besides fixing motors at the rear to counterbalance nose-down pitch at rest.
Equally one such motor cannot be fixed at the leading edge for two reasons: at low speeds with screws running in opposite directions they 'walk' the craft the same way, while at planing speeds impingement of efflux on the keel would offset the trajectory. With both motors at the rear however the control is complimentary viz. acceleration of one motor over the other both rolls and yaws the craft in the opposite direction.
