The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and p

The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and prevent it from spinning and it would still move forward. After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed. But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.