Going down-wind faster than the wind (GDWFTTW) was discussed at length here:
https://www.eng-tips.com/viewthread.cfm?qid=274009 in 2010.
A sailplane uses up drafts to go up. Otherwise, it always loses altitude with time.
GDWFTTW is an difficult concept to grasp, even for most engineers. Further discussion here might be fun for some.
They tack downwind so that the wind is still coming over the bow of the boat so they can go faster than the wind.
6 races into the Americas Cup, and they are tied 3 each out of first to win 7.
I’ll accept Berkshire’s explanation.
I finally understood the car and propeller going faster than the wind:
It is not the wind direction, it is the apparent wind direction that must be considered.
Consider a car with a propeller geared to the wheels and facing into the wind.
With the wind spinning the propeller, the energy extracted may be geared to the wheels to move the car into the wind.
With the wind from the rear, the car is not self starting, the car must be pushed fast enough that the apparent wind speed is from the front.
Then it will work.
That mechanism won’t work for a boat, but tacking will work for a boat.
A sailboat cannot go downwind faster than the wind. It can go faster than the wind in the direction it is tacking, but the velocity in the wind direction must always be less that the wind velocity.
It is kind of silly to suggest that a car going into the wind can be self-starting but one going downwind cannot self-start.
The wind pushes against the prop. disc, which is the area swept by the rotating (or non-rotating) propeller, which acts much like a sail. A rotating prop, however, is a more effective sail than a non-rotating one. This pushes the vehicle forward. The wheels drive the prop. So far, it is just like sailing.
The hard part to visualize is how wind can push on an object that is traveling faster than the wind and in the same direction. The prop is pushing air back into the wind so that after the vehicle has passed, the wind speed, relative to the ground, has slowed-down even though the vehicle is going faster than the wind. This means that energy has been extracted from the wind, even though the vehicle is going faster than the wind. The limit to this technique is that air in the wake of the vehicle must still be moving at some low velocity in the wind direction. To reverse the air direction would require an source of energy other than the wind.
It is important to understand that power flows from the wheels to the propeller, and not the other way.
As the speed increases, there is a null point where the vehicle speed matches the wind speed and net wind speed is zero.
That is why the effect is not self starting and why the vehicle must be push started.
I would like to see the test that “prove” that this works.
This is supposed to work when the vehicle is going fast enough that the effective wind speed is from the front.
If that is the case, the effect should still work when there is no wind and the vehicle is push started.
This really looks like a subtle over-unity scheme.