PhilGoetz comments on The Correct Contrarian Cluster - Less Wrong

38 Post author: Eliezer_Yudkowsky 21 December 2009 10:01PM

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Comment author: wedrifid 23 December 2009 06:28:08AM *  2 points [-]

Contrary to what the article says, sailboats can't travel downwind faster than the wind (except briefly, when the wind changes).

The article explicitly refers to 'tacking sailboats', which can in fact travel faster than the wind in the downwind direction.

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.

The energy comes from harnessing the difference the difference between the velocity of the wind relative to the velocity of the ground. It may be helpful to refer to the 'propeller' as the 'propellee'. It is there to make sure the wind always has something to push on that is at roughly the same speed as the ground and only uses energy based on losses to drag and friction.

Comment author: PhilGoetz 23 December 2009 06:51:13AM 1 point [-]

The article says: "It should be obvious that there's some way to go downwind faster than the wind, because as so many people pointed out, sailboats do it." Sailboats do not go downwind faster than the wind. I have gone downwind hundreds or thousands of times on many different types of sailboats, and I have never seen the wind indicators streaming behind me as I did so.

Tacking sailboats are going upwind, not downwind.

The energy comes from harnessing the difference the difference between the velocity of the wind relative to the velocity of the ground.

Well, that's obvious. By definition of "wind power".

It may be helpful to refer to the 'propeller' as the 'propellee'. It is there to make sure the wind always has something to push on that is at roughly the same speed as the ground

The propeller is not at the same speed as the ground.

and only uses energy based on losses to drag and friction.

?

Comment author: JGWeissman 23 December 2009 07:31:54AM 3 points [-]

I have gone downwind hundreds or thousands of times on many different types of sailboats, and I have never seen the wind indicators streaming behind me as I did so.

Sailing downwind faster than the wind looks and feels like sailing upwind. How often have you, when the tell tales are streaming aft, checked to see if a stationary flag was blowing in the opposite direction?

You could also have sailed on many kinds of boats whose hulls experience too much water resistance before achieving the speed of the wind to accelerate further with the power provided by that wind.

Comment author: wedrifid 23 December 2009 07:08:57AM *  3 points [-]

The article says: "It should be obvious that there's some way to go downwind faster than the wind, because as so many people pointed out, sailboats do it." Sailboats do not go downwind faster than the wind. I have gone downwind hundreds or thousands of times on many different types of sailboats, and I have never seen the wind indicators streaming behind me as I did so.

Assume a uniform wind of 20 km/h flowing in the direction from A to B and that B is 100 km from A. Fred is an expert sailor and has a top line sailboat. While Fred is stationary at A, he notices Joe floating past him in a hot air balloon going at the 20 km/h wind speed. Assuming no changes to the wind is it possible for Fred to catch up to Joe using only sailboat before Joe reaches B five hours later?

  • If you answer 'no' then you are incorrect.
  • If you answer 'yes', then understand that this is what people mean when they say it is possible to go downwind faster than the wind.
Comment author: PhilGoetz 23 December 2009 03:03:48PM *  0 points [-]

Has this actually happened? Does this prove anything if it did, given that winds at altitude and ground level are vastly different?

It is impossible for any existing sailboat to have a downwind component that is faster than the wind. If it were possible, you could sail the boat with no wind at all. (This argument does not apply to the ground vehicle under discussion.)

Will you at least agree that it is impossible to sail with the boat pointed directly downwind faster than the wind in a conventional sailboat (including racing sailboats)?

A sailboat can reach faster than the wind because the mass of the wind is greater than the mass of the sailboat, and the energy in the wind is transferred to the boat. Moving downwind faster than the wind is very different, and requires a different mechanism. And you cannot use apparent wind to explain moving downwind faster than the wind, because the apparent wind would be in the opposite direction.

Comment author: JGWeissman 23 December 2009 11:25:18PM 1 point [-]

Does this prove anything if it did, given that winds at altitude and ground level are vastly different?

The greater wind at higher elevation would only be an advantage to the hot air balloon.

It is impossible for any existing sailboat to have a downwind component that is faster than the wind. If it were possible, you could sail the boat with no wind at all.

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.

Will you at least agree that it is impossible to sail with the boat pointed directly downwind faster than the wind in a conventional sailboat (including racing sailboats)?

Yes.

A sailboat can reach faster than the wind because the mass of the wind is greater than the mass of the sailboat

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.

And you cannot use apparent wind to explain moving downwind faster than the wind, because the apparent wind would be in the opposite direction.

It is not exactly the opposite direction, and even a small deviation can be significant.

Comment author: wedrifid 23 December 2009 04:26:01PM 0 points [-]

Has this actually happened?

There are potential ambiguities in the language used. Considering a specific example like this allows us to establish whether we are disagreeing about the physics itself or just using different words. I get the impression that we are disagreeing on the nature of physics itself. Fred can win.

Does this prove anything if it did, given that winds at altitude and ground level are vastly different?

I didn't want to dwell on technicalities and hoped 'uniform' was sufficient to convey my intended meaning.

Will you at least agree that it is impossible to sail with the boat pointed directly downwind faster than the wind in a conventional sailboat (including racing sailboats)?

Yes.

A sailboat can reach faster than the wind because the mass of the wind is greater than the mass of the sailboat, and the energy in the wind is transferred to the boat.

Hence the applicability of the 'sailboat' analogy to the vehicle in question.

Comment author: Vladimir_Nesov 23 December 2009 03:20:39PM *  0 points [-]

Does this prove anything if it did, given that winds at altitude and ground level are vastly different?

Possible world too convenient!

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