Hello sir,
I've been reading your question. Maybe this can help you... If you add a fan to your sailboat you SHOULDN'T PLACE IT AT THE BACK OF THE BOAT (blowing into the sails from behind). Newton’s action-reaction law will slow the boat down. You should, however, place the fan IN FRONT of the boat (blowing backwards, sideways into the sails). This sounds counterintuitively, I know. But this will not only propel the boat forwards, the faster moving air will also create a low-pressure-area which will provide lift for your sails (principle of sailing into the wind). This is an interesting movie about the physics of sailing which explains the principle:
Of course, the most energy-efficient way of moving your boat in a windless environment is having a propeller that thrusts air backwards (without sails). But if you absolutely want to use sails the method mentioned here above is the best way to do it, I think. That, and having a truly gigantic fan.
I hope this was helpful.
