A sail is used to create drag. If the air from the fan hit the sail and dispersed perfectly perpendicular to it, then it wouldn’t move the craft at all. The mo

A sail is used to create drag. If the air from the fan hit the sail and dispersed perfectly perpendicular to it, then it wouldn’t move the craft at all. The more realistic outcome would be that the sail did not disperse the air with perfect efficiency, and the craft would just move backwards. What you’re describing would require redirecting the flow of air backwards, like a curved exhaust. Which is really just a less efficient version of turning the fan around.