Equal and opposite reactions, Physics is a wonderful thing. The fan will impart a force to the boat where it is mounted equal and opposite to the wind force the fan can create, so technically since the sail canÆt catch all of the "wind" created by the fan, if the fan was big enough to over come the friction with the water, the boat would actually move backwards. When you think about it a fan is nothing more than a propeller.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs