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Sailboats operate in an environment with two fluids moving along different vectors: the air, typically moving as wind, and the water, typically almost stationary. Two foils, the sail and the keel, use the relative movement of the two fluids to produce lift and move the boat.

In principle, it seems that one could use the shallow and deep water of a river, which move at different speeds, in a similar way to move the boat. Instead of having a sail and a keel, you'd have a sub-keel down deep and a surface-keel in the shallow water. The sub-keel would be in mostly stationary water near the bottom, playing the role of the keel on a normal sailboat, and the surface-keel would be in fast-moving water near the surface, playing the role of a normal sail.

Could this actually work? Has it ever been tried? What would be the difficulties with this design?

Sam
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Yes. It is done with RC glider planes. They call it dynamic soaring, where they fly a circular path that cuts through a wind shear. Some ridgetops have strong shears and speed of over 500 mph have been achieved.

https://www.youtube.com/watch?v=4eFD_Wj6dhk

The problem with boats is the wave drag. You'd need a foiler of some sort. Trying to span the shear with two foils doesn't seem likely. Even then it's going to be a struggle, because hydrofoils rarely achieve better than 16:1 L/D ratio as an actual machine. Gliders can do a lot better than that, giving them an advantage both in foil performance and in the way momentum plays into the game.

Phil Sweet
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In principle you may be right -- it would depend on how efficiently your system would work, and how many times you could put up with your deep keel hitting obstructions and breaking off before you gave up.

I could see the shallow part of the keel generating forward thrust while it generates side thrust, and the deep part of the keel generating opposite side thrust while in generates drag. All in all you'd be generating a pretty strong rolling moment on the boat, so balance would be an issue.

I see this as something that'll work really well in a research lab's flow tank. In the real world, given that the bottoms of rivers aren't reliably smooth, I don't see how you'd get something that works without constantly tearing off your keel.

TimWescott
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