A blocking circuit is provided in combination with an inboard motorboat engine, blower, and ignition circuit which prevents actuation of the ignition circuit until after the blower has operated for a predetermined period of time to ventilate the engine compartment. A time-delay switch is operated by a bloweractivated switch in circuit with the ignition circuit. Upon actuation of the blower, the blower-activated switch closes energizing the time-delay switch. After a predetermined time interval, the time-delay switch closes and completes the ignition circuit whereupon the craft may be started and operated in a conventional manner. An emergency ignition switch is also provided in parallel with the blocking circuit to allow operation of the engine in the event of an emergency. Use of the emergency switch renders it inoperative requiring that it be replaced after such use to prevent indiscriminate subsequent use by an unconscionable operator.