Can a sail boat be propelled by air blown at the sails from a fan attached to the boat?

No, that doesn't work. Even though the breeze created by the fan will exert force on the sail, there is an equal and opposite force being exerted on the fan itself, and since the sails and the fan are both connected to the same boat, the forces simply cancel each other out. That's why it is better to use a propeller in the water.