[andrewflnr]

I think I get it. I would have thought plain, free-spinning wheels could serve the role of the keel (I guess they do in a normal sail car), but you need to transmit that keel action from the wheels on the ground to the sort of virtual cylindrical Earth the propeller lives in.

[Doxin]

In a normal sail car you'd have to tack, but otherwise the same forces apply. The clever thing about the propellor is you get a sort of cork-screw-tacking for free allowing your motion to be in-line with the wind instead of having to be at an angle.