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Note: Closed for being off-topic (whoops!), see here for the re-post on the Engineering site.

If ships at sea can tack in a zigzaggy pattern by converting backwards force from the wind (using the sail) into sideways force, then that sideways force into forward force by bracing against the (stationary) via the rudder... if image is unavailable, find it here: https://physics.stackexchange.com/questions/94541/sailing-against-the-wind-is-this-a-fair-model
source: another stackexchange question
...couldn't the same principle be true with the roles of the wind and water reversed --- as in a river with stationary air above it? Are there any rivers with the right conditions to make this happen? If the air won't work, could zigzagging cables stretched across the river do it?

Palbitt
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  • try https://engineering.stackexchange.com/questions/tagged/mechanical-engineering . – anna v Jul 13 '20 at 05:19
  • If you stand on a boat that's drifting down a river in still air (as reported by a person standing on shore), then you will feel a relative wind that appears to blow directly up stream. It "blows" in exactly the direction that you want to go. A sail boat typically puts out a spinnaker (effectively, a big parachute) when it wants to go in the same direction as the wind. But, a sail boat can never go as fast as the wind in that direction. The best you can hope for is to slow your down-stream progress. You won't be able to reverse it. – Solomon Slow Jul 13 '20 at 12:56
  • @Solomon Slow Sailboats may not be able to sail faster than the wind directly downwind (except for those with windmills replacing the sails), but many boats can go fast enough by heading a few degrees higher, to achieve a velocity component in the downwind direction faster than the wind. The result is that it is actually possible to sail upriver on a perfectly calm day. – D. Halsey Jul 13 '20 at 22:01
  • See https://www.boatdesign.net/threads/artemis-technology-sustainable-future.61730/ – D. Halsey Jul 13 '20 at 22:11
  • Wow. Thanks, @D. Halsey. It's cool that it actually works, and on a famous river like the Amazon, no less. The way they put it, though, makes it sound like free energy (which of course it isn't). Sailing against the wind is so counter-intuitive. – Palbitt Jul 18 '20 at 19:53
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    Voting to reopen. I'm sure this is on topic at Engineering, but it's certainly also on topic here. It's a question of forces, constraints and physical intuition. – N. Virgo Jul 19 '20 at 04:02
  • Suggest transferring this question over to engineering stackexchange. – Mahendra Gunawardena Jul 19 '20 at 14:30

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