This was tested (and confirmed) on an episode of Mythbusters: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail Their boat

This was tested (and confirmed) on an episode of Mythbusters: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail Their boat went about 10% the speed of just pointing the fan backwards.