JGWeissman comments on The Correct Contrarian Cluster - Less Wrong
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The greater wind at higher elevation would only be an advantage to the hot air balloon.
No. If there is no wind at all, then if the boat moved forward in any direction, it would face an apparent direct headwind opposing its motion. But with some amount of wind, the boat can travel at some angle from dead down wind, so that the apparent wind will not be directly ahead, so that the perpendicular lift will have a forward component.
Yes.
Well, the total mass of all the air that was deflected by the sails as the boat accelerated past wind speed will be greater than the mass of the boat, but that does not really explain what is going on. The point is that the boat is able to continue to derive enough thrust from the apparent wind to overcome drag forces. And this continues to work even when a component of the boat's velocity is downwind.
It is not exactly the opposite direction, and even a small deviation can be significant.