Originally Posted by Dirtboy
... The fact that it creates apparent wind and then uses that aparent wind to increase speed ......... if you carry that thought through, it could potentially continue to accelerate to amazing top speed....
I just wanted to elaborate on what some others have already commented about, i.e. the prop blades are not acting as sails to propel the vehicle. In other words, there are no sails that use the apparent wind or benefit from its effect, as DB's post above seems to suggest (if I understand him correctly -- apologies DB if I misunderstood what you meant).
This is how I understand it:
The vehicle initially gets moving simply by wind pushing on the surface area of the entire vehicle from astern. This causes the wheels to spin, albeit very slowly at first. The wheels are connected to the propeller via a drivetrain that has a geared advantage. So, the propeller spins in turn.
The initial forward motion is VERY slow because the vehicle gets almost no benefit from the turning propeller at slow speeds. In fact, the thrust from the propeller only gradually improves as the apparent wind approaches zero. That is because the breeze is blowing downwind faster than the propeller is thrusting air aft.
Once apparent windspeed drops to zero, the vehicle begins to accelerate much more quickly. This is because the propeller, driven by the wheels via the drivetrain, is now creating more thrust relative to the eliminated tailwind.
With a sailboat sailing dead-downwind, driven by pressure from the wind on the sails, apparent wind drops to zero when (if) the boat reaches true windspeed. At this point the sailboat is at a disadvantage, because apparent windspeed is zero and since the vessel is sail-driven, there's no more wind power to tap.
Just the opposite with this vehicle. In this particular type of windpowered vehicle, it is actually advantageous to have apparent windspeed drop to zero (this is why you see small scale versions do so well on the treadmill with no fan running). Because, this is the point where the tail wind is no longer subtracted from the propeller's thrust. From that point forward, the propeller is moving "with the current" as it were, so speed over ground increases dramatically. The new apparent wind (now a headwind) and the prop thrust begin to work together to complement eachother.
From what I can gather, the controversy surrounding this vehicle and what it demonstrated has less to do with the concepts and more to do with how poorly they were expressed/explained. Unfortunately, proponents consistently employed oddly chosen terminology, and made the mistake at the outset of implying that there was a connection between this vehicle and the sport we know as sailing. There is not. Yes, it is windpowered, but the similarity ends there.
This vehicle would be more analogous to a wind-powered "motorboat". On the water, you'd build it with a pointy bow and a lot of surface area at the stern, and paddle side wheels. The paddle wheels would be connected to an underwater propeller via a geared transmission. As the boat began to move downwind, the paddles would spin due to resistance in the water and they would in turn drive the propeller and produce thrust. The whole effect would be much less efficient than on land due to the greater resistance of the fluid environment as well as lack of "traction" for the paddles -- and of course a displacement hull would be constrained by its hull speed -- but it would be the same concept.
Anyway, that's the way I see. Neat concept, nicely demonstrated. But I'm confident it poses no threat to SailFan(TM) technology, which is actually applicale to sailboats and has a proven trackrecord.