Professor of physics bets 10k he's right

[TexasLawyer2000]

Originally Posted by IDoPhysicsPhD

Actually you're the one that is very wrong... in multiple ways. But the most important part is that the wind on the car is only coming in one direction (parallel to car and not at an angle). Your triangle example has the boat going in another direction than downwind.

Precisely.

Originally Posted by turf3

Yes, because that's the direction that boats sail fastest. They don't sail fastest downwind. If the boat is headed directly downwind it's impossible for it to go faster than the wind velocity. In fact, directly downwind, it's impossible for it to equal wind velocity, because sails aren't 100% efficient and because there's drag on the hull against the water.

If the boat is sailing with the wind on its quarter, its velocity in the wind direction is still less than the wind velocity, but it also has a component perpendicular to the wind velocity, and its total velocity is the sum of those two vectors. Being a vector sum, it's the root sum of squares. If the sail plan is efficient (so its parallel velocity is a large fraction of the wind velocity), and if the keel is effective at minimizing sideslip (so the angle between the direction of the boat and the angle of the wind is "large", then the total velocity of the boat will exceed the wind velocity. Just draw it out and you'll see.

Ice boats can exceed the speed of water boats for a given wind speed, as their runner blades are extremely efficient keels, and the drag of their runners on ice is very low.

I should point out that if you want to cavil at "vehicle" being used for a boat, there are vehicles that are built along the plan of the iceboat, but use wheels and run on pavement. They have many of the same characteristics of iceboats - I expect they don't reach the same speeds due to the increased rolling resistance of tires and wheels compared to runners on ice, but I'm sure they too can exceed wind velocity in their groundspeed, depending on how the wind's quartering to their direction.

This is a good explanation of why your previous posts were wrong. Thanks for the clarification.

[turf3]

"Can a vehicle powered only by the wind go faster th[a]n the wind?"

I've just explained why and how the answer is "yes".

If you want to expand the question to "is it possible for a vehicle powered only by wind to go faster than the wind, in the downwind direction? - well, that's a different question.

[Dane_in_LA]

Guys, you're in agreement. Stop bickering and go sailing.