It is known with motorsailers that you can improve fuel efficiency by using the apparent wind on the sails created by the motor propelling the boat forward. The use of the sails also increases the speed you can achieve.
Using a propeller in the water is more efficient than using a fan in the air. However, some boats such as the fan-boats in the Florida everglades have to use fans because propellers in the water would get fouled by floating plants and branches in the water. Still, it is also true if you had a fan *at the stern* directed *rearward* and sails forward you could get better fuel efficiency and greater speed, just as with motorsailers. In this scenario though the fan is not blowing over the sails.
I'm asking a little different question. If instead you positioned the fan in front of the sails so it did blow rearward over the sails would you in fact get greater propulsion than what you would get from just using the apparent wind where the fan did not blow over the sails?