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

Answer 1

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.

Answer 2

Essentially/basically no, but technically speaking yes.

No=

The fan blowing the air forward is pushing the boat backwards and basically neutralizes or cancels the amount of force that is being created by the wind hitting the sails. Imagine standing in the bed/box of a pickup truck and pushing on the back of the cab, to are pushing the front half-forward with your hands, and the back half backwards with your feet. You go nowhere.

Yes=

Depending on what type of sail, the air will hit the sail circulate and a very small amount of the air will be deflected backwards off the edges of the sail towards the back of the boat giving you a very small amount of force that would push you forward. It is such a totally inefficient use of the energy being generated it is completely and utterly pointless to do it though.

You may as well just turn the fan around so it is blowing backwards and pushes you forward like a Bayou swamp airboat or how a hovercraft moves forward. Same thing with a wing sail of you put the fan on the front and were to tack into the wind from it, because you have to be 30 degrees offset from the direction the wind is coming from, the amount of forward momentum being created is so small from the actual sail, you may as well just use the fan