Originally Posted by anthonycolfelt
I have a better question... Can you lift yourself up by your bootstraps? What if you jump a little first to give yourself some momentum?
In all seriousness, the fan is going to act just like a propeller would in the water. It will give you some thrust if its big enough. You may even get a little extra from the sails, but no more than you would from equivalent thrust from a submerged propeller.
It's newton's third law of physics. Every action has an equal and opposite reaction. If there's thrust pushing the boat forward, its from the fan onto the sail and rig (pushing it equally backward). The fact that the boat moves forward because of the fan's thrust may draw a little extra air over the foil (sail) which will begin to cause some lift. But this isn't any different than if the 'fan' was underwater I would think.
I agree as far as trying to get the greatest efficiency from the power produced from an engine you very likely would be better off putting that propeller in the water rather than the air.
However, it is known you can get higher speed and higher efficiency with a propeller in water driven boat if in addition you use sails. You are in effect "pulling yourself up by your own bootstraps" since the apparent wind acting on the sails is coming from the power of the motor, which nevertheless acts to produce greater speed than from the propeller acting alone.