Well I am utterly gobsmacked. My take on it is that once they reach windspeed, the propeller is effectively turning in zero apparent wind. At that point, any existing inertia in the spinning blade will provide some forward force due to the pitch of them ...
The inertia of the rotating parts is irrelevant. The vehicle is not allowed to use internally stored energy for propulsion (NALSA rules for sail craft).
Check out the observer reports on their website:http://www.nalsa.org/DownWind.html
NALSA observers wrote:
S2: We found no evidence of any energy storage devices. There has been some discussion on www discussion groups that the rotating propeller constitutes a form of stored energy that might be converted to propulsion by slowing it down or changing its pitch. Since the propeller is connected to the wheels with a constant ratio connection slowing the propeller to harvest some of its rotational energy also slows the craft. The fact that the craft is required to accelerate during the measurement means, for a fixed gear ratio craft, that rotational energy is being added to the propeller to speed it up during the measurement period rather than the other way around.
From:http://www.nalsa.org/BlackBirdDDWSR/Obs ... 032010.pdf
As long there is true wind, the car can stay DDW above windspeed continuously, just like a ice-boat that can keep a downwind VMG greater than windspeed continuously. See "Sail to Propeller" video above.