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It slows down air relative to the ground, like any wind powered vehicle.

Here the video with higher placed telltales: For the record of 2.8 x windspeed NALSA measured the wind at the height of the propeller hub.

Yes, but it's irrelevant. The apparent wind at the propeller blades can never be zero with this setup The propeller blades are the analogue to the sail.

Look the vector diagrams in post #25. It's the same aerodynamics.

The same is true for the propeller blades of the DDWFTTW cart, so I don't see the reason to discount it. Again, the analogy is not between the boat and the cart. It is between the sail and the propeller blade. It demonstrates that DDWFTTW can be achieved if you allow for moving parts, For...

Correct, but in their analogy the boat is not moving directly downwind, but diagonally downwind. This is equivalent to the motion of the propeller blade, which also has a velocity component perpendicular to the wind.

The analogy they show is correct, but unfortunately they don't mention the role of the boat's keel which is key here. The keel drives the boat across the wind, just like the wheel drive the propeller blades across the wind.

Yes, but the apparent wind is never zero at the propeller blades, just like it is never zero at the sail going diagonally to the wind. You have to differentiate the apparent wind at the vehicle chassis (which is zero at wind-speed) from the apparent wind at the spinning propeller blades (which...

The propeller blades of this cart also have apparent wind, even when the cart doesn't. The propeller blades are not moving straight downwind, but across the wind, like the tacking sail. The wheels & transmission are forcing the blades to move across the wind, just like the keel of the sailboat...