A transmission system for a counter-rotational propulsion device is easily incorporated into an existing outboard drive of a watercraft to convert the outboard drive from a single propeller drive to a counter-rotational dual propeller system. The transmission system involves a first transmission which selectively couples an inner propulsion shaft with an existing drive shaft of the outboard drive. The inner propulsion shaft in turn drives a rear propeller. A second transmission of the transmission system is provided between the inner propulsion shaft and an outer propulsion shaft. The second transmission reverses the rotational drive direction input by the inner propulsion shaft so as to drive the outer propulsion shaft in an opposite rotational direction. The outer propulsion shaft drives a front propeller which spins in an opposite direction to that of the rear propeller, but asserts a thrust in the same direction as the rear propeller.