I don't understand what the goal is? Is it to hold a regatta for monohull sailors to try out catamarans? Is it to hold a regatta and provide youth without their own boats a boat to sail for the regatta?

If the goal is to put on a regatta with as much local participation as possible then advertise in all the local sailing newsletters, magazines, public sailing centers, West Marine, any place you can advertise. Get out some public service announcements in the local media. Send out press releases to the newspapers. Find out if/how you can distribute a flyer through the schools. Build the hype for the event.