I take no credit, I essentially ripped off Spork's ideas. But this is sort of a hybrid yoyo/cart idea.
Let's say the diameter of the wheel is twice the diameter of the axle to which the rope is attached.
Wind speed = W
Cart speed = V
As seen from the ground:
The wind pushes a sail, which is attached to a rope on the wheel of the cart, which pulls the sail backwards. From the frame of reference of the observer on the ground, the boat will move forward with speed V, but since the sail is being pushed back, it moves with speed V/2.
Since the maximum speed of the sail w/ reference to the ground is the windspeed (W), this is V/2 in the previous paragraph. Solve for V=2W, and the cart moves twice as fast as the windpeed.
As seen from the Boat:
Windspeed is W
Boat speed is V.
Apparent wind on the boat is (nonrelatvistically
) W-V. Apparent wind drops to zero when the boat is travelling downwind as fast as the wind.
But the sail is moving backwards with speed V/2 thanks to the rope/pulley system, so the apparent wind on the SAIL is W-(V/2). The apparent wind on the sail doesn't hit zero until the boat is moving at V=2W, or twice as fast as the wind.
The catch is that the total force on the boat is now half what it would be with just a stationary sail. By making the inner wheel larger, we can increase the max speed of the boat, but we're decreasing the total force of the wind, and thus the acceleration. As we reach the limit Vmax=infinity, Acceleration will go to zero.
I feel I'm preaching to the choir now that spork has managed to get everyone on his side, but i just wanted to add my thoughts.
Upon further thought, the "moving sail" concept is also very similar to the boats in a box and the propeller contraption.
Disbelievers, see the truth.
dammit, too late, you already called this one the conveyor belt method.
I live in the city, and I mostly kite Sherman or Chrissy, but poke my head around the other spots too.