KiloB:
... you can never go faster than the wind speed directly downwind
You might want to rethink that position as the North American Land Sailing Association (NALSA.org) just oversaw testing of the vehicle this last weekend and collected data from 18 recording sensors showing it traveling more than 3.5x the speed of the wind while moving dead downwind. Expect a NALSA ratified record to be issued very soon for a speed above 3x the speed of the wind.
Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- it is sometimes pushed by hand up to a few mph to save time during testing (it starts very slow otherwise)
-- there's no "null point' at windspeed.
-- It will maintain it's speed well above wind speed indefinitely
-- By design, this particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause it to go upwind rather than down
It's very counterintuitive, but it's real.