[LeifCarrotson]

Because the cylindrical Earth experiment neglects the reaction forces of a sailboat's keel, or the land-boat's wheels. The propeller shaft would just rotate, you need that cylindrical Earth to have, essentially, a ballscrew spline.

You can't generate the differential with clever aileron wings, either - has to be in a different medium, otherwise the thing would just tumble.

[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.