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JGWeissman comments on The Correct Contrarian Cluster - Less Wrong
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I don't believe it.
Contrary to what the article says, sailboats can't travel downwind faster than the wind (except briefly, when the wind changes). If this were possible, I would have experienced it.
When the vehicle is moving as fast as the wind, in order to go faster, the energy output from the propeller must be more than the energy input through the wheels. The energy output of the propeller comes entirely from the energy input through the wheels, so this is impossible.
Right?
I'm feeling uncertain, because dozens of people reviewed the article and all agreed that the thing works.
Yes, they can. A boat sailing 45 degrees off of dead down wind, making downwind progress at the speed of wind (its total speed being square root of 2 times the speed of wind), will feel an apparent wind 45 degrees off its bow, from which it can generate more thrust and go even faster, until the apparent wind is much closer to directly ahead. Modern racing sailboats do this all the time.
A boat sailing 45 degrees off of dead downwind has its sail out very far to leeward, so that the apparent wind will slow it down, not speed it up. You're thinking of a boat sailing 45 degrees off of upwind. [EDIT: My mistake. When you reach the same speed in the downward direction as the wind, the apparent wind is coming entirely from a direction perpendicular to the wind, and so your sail will be trimmed in perpendicular to the wind and be receiving lift in the same direction as the wind.]
You might be able to move with a component in the downwind direction faster than the wind due to lift - but I wouldn't bet on it. I can't swear that it doesn't happen, because I try never to be in this situation. It's the easiest way to flip a boat, or to get "windlocked" (when the wind is too strong for you either to pull the sail in or to steer windward, so unless you jibe, you're stuck going the direction you're going until the wind dies down).
I always trim the sails for the apparent wind. If the apparent wind is backwinding the sails, I will trim them in, so that they work properly and provide forward thrust. As the boat accelerates on a straight line course, the apparent wind will shift forward and I will trim in the sails.
In the situation you described, the apparent wind is travelling from the tail of the sail to the front, in the opposite direction you would need for it to provide thrust. There is an apparent wind, and you do trim the sail in response to it, but it doesn't provide thrust when you're going downwind.
No, in the situation I described, the apparent wind flows from the luff (leading edge) to the leach (trailing edge) of the sail. I have actually done this. I will see if I can produce a diagram later tonight.
Edit: Here is the diagram:
You're right.