> you can propel a boat using a fan that is mounted on the boat itself You mean like an air boat [1]? Using a sail just makes it less efficient when self power

> you can propel a boat using a fan that is mounted on the boat itself You mean like an air boat [1]? Using a sail just makes it less efficient when self powered. It wouldn't work like that in space because there is no working fluid (air) against which to push - which is what the fan is doing, whether it's using a sail or not. Conservation of momentum would eliminate any propulsion if the propellant is recollected. [1] https://en.wikipedia.org/wiki/Airboat https://en.wikipedia.org/wiki/Airboat