Funny thing is that this isn’t technically true. If the fan is strong enough, the current will hit the sail and reflect backward and to the side, creating some very inefficient thrust. Any fan strong enough to achieve this would do better to remove the sail entirely and just point the fan backwards, but it would technically work.
If the fan blew across an airfoil aligned perpendicular to the axis of the boat, it can definitely generate thrust. The top of the airfoil will develop a lower pressure than the bottom, and the difference in pressure causes force that moves the boat. It’s just way more efficient to use a fan to push the boat.
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Funny thing is that this isn’t technically true. If the fan is strong enough, the current will hit the sail and reflect backward and to the side, creating some very inefficient thrust. Any fan strong enough to achieve this would do better to remove the sail entirely and just point the fan backwards, but it would technically work.
And by the same principle, you could shine a powerful laser towards a very reflective sail and actually move, with the laser as a part of the boat.
I too always found this very funny. Confuses the hell out of people who just understood why the OP would not work.
I believe you’ve invented an air boat.
See the following reference material: https://www.imdb.com/title/tt0074564/
Full disclosure, I’m an idiot, so I could be misunderstanding something, but im pretty sure that this is based on that principle.
If the fan blew across an airfoil aligned perpendicular to the axis of the boat, it can definitely generate thrust. The top of the airfoil will develop a lower pressure than the bottom, and the difference in pressure causes force that moves the boat. It’s just way more efficient to use a fan to push the boat.