Wind powered cart goes faster than the wind, directly downwind

[RichardS]

The wheels and the gearing force the propeller tips to move at and angle to the wind. Just like the dagger boards on the sailing boats.

Errrr ...... OK.

Richard

 

[johnalison]

Except that it's the wheels that are turning and providing motive power to the fan rather than the other way around.

Richard

I don’t like that. The energy is being extracted from the wind by the fan, or the vehicle would remain stationary. Boats don’t have wheels and can still achieve the same trick.

 

[Mark-1]

I dont really see what there is to explain.

See post 65. But I don't feel strongly about it, just nice to have a third set.

The polars and the footage all agree with each other.

See post 79.

Your own

Not mine, I just linked to one I found on SA. Didn't even get a comment at the time I posted it.

 

[Buck Turgidson]

I don’t like that. The energy is being extracted from the wind by the fan, or the vehicle would remain stationary. Boats don’t have wheels and can still achieve the same trick.

err....
No, the energy is being extracted by the wheels which drive the prop which produces thrust.

 

[RichardS]

I don’t like that. The energy is being extracted from the wind by the fan, or the vehicle would remain stationary. Boats don’t have wheels and can still achieve the same trick.

Boats can't achieve the same trick .... and that explains the difference. Boats cannot sail directly downwind at faster than the wind speed.

You could presumably build a boat with a large propeller in the air which could so the same trick but the prop in the water would be driving the prop in the air.

Richard

 

[flaming]

19:26. Gybe not tack.
Starboard foil goes down so Starboard is the leeward side on the new gybe - it is also the side nearest the leeward mark.

Yes, I can't reconcile that with the Polars you found, but it's not really something that can be mistaken. The weight of the wind is on the port side. Bearing away would take it further downwind, not upwind.

I have no idea what it is you are think you are seeing. That's a perfectly normal gybe for an AC75. During which speed never dropped below 30 knots and the apparent wind stayed forward, with the boat passing through the apparent wind as if it had tacked, not gybed.

 

[flaming]

Boats can't achieve the same trick .... and that explains the difference. Boats cannot sail directly downwind at faster than the wind speed.

You could presumably build a boat with a large propeller in the air which could so the same trick but the prop in the water would be driving the prop in the air.

Richard

The bit you are missing is that it is the same trick because the propellor blades also do not travel directly downwind, but form a helix. If you imagine the cart doing this on a pitch black night, and you watched from directly above, a light fixed to the end of one of the propeller blades would appear to describe a gybing back and forth course very similar to that of the AC75s, just with lots more gybes! The only difference is that the propeller blade is doing it rotationally, rather than just back and forth.

 

[JumbleDuck]

I don’t like that. The energy is being extracted from the wind by the fan, or the vehicle would remain stationary. Boats don’t have wheels and can still achieve the same trick.

See my post a couple of pages up. It's easy to confuse the shaft work required to turn the fan (which comes from the wheels) with the thrust work done by the wind on the fan (which moves the car and turns the wheels).

 

[Mark-1]

I have no idea what it is you are think you are seeing. That's a perfectly normal gybe for an AC75. During which speed never dropped below 30 knots and the apparent wind stayed forward, with the boat passing through the apparent wind as if it had tacked, not gybed.

Yes, it's gybe. But it shouldn't be if there's a 22kt wind component it should be tacking. More obviously its on a port tack but the apparent wind should be coming from the starboard side where the leeward bouy is (the leeward bouy is effectively a windward bouy to the boat). On that course the boat should be on the other tack.

In standard monohull terms the weight of the wind is pushing on the port side where the true wind is coming from. If the boat is sailing with the stated VMG DD it should be taking upwind and the weight of the wind should be on the starboard side.

 

[Mark-1]

See my post a couple of pages up. It's easy to confuse the shaft work required to turn the fan (which comes from the wheels) with the thrust work done by the wind on the fan (which moves the car and turns the wheels).

There's a ratchet. Only one of those can happen, not both.

 

[Mark-1]

The bit you are missing is that it is the same trick because the propellor blades also do not travel directly downwind, but form a helix. If you imagine the cart doing this on a pitch black night, and you watched from directly above, a light fixed to the end of one of the propeller blades would appear to describe a gybing back and forth course very similar to that of the AC75s, just with lots more gybes! The only difference is that the propeller blade is doing it rotationally, rather than just back and forth.

They can't be gybes, if you're sailing upwind you're tacking not gybing. If you're gybing you're, by definition, going DDWSTTW.

 

[JumbleDuck]

There's a ratchet. Only one of those can happen, not both.

As I said, it's easy to get confused. Both happen.

 

[JumbleDuck]

They can't be gybes, if you're sailing upwind you're tacking not gybing. If you're gybing you're, by definition, going DDWSTTW.

Flaming has already explained the terminology.

 

[Mark-1]

As I said, it's easy to get confused. Both happen.

It's not easy to get confused about presence of a ratchet!

 

[Mark-1]

Flaming has already explained the terminology.

If there's alternative agreed terminology maybe you and Flaming can link to your source for it so I can share in the knowledge.

 

[RichardS]

The bit you are missing is that it is the same trick because the propellor blades also do not travel directly downwind, but form a helix.

But that's not the sailing boat, as described by johnalison, doing the trick.

Richard

 

[JumbleDuck]

It's not easy to get confused about presence of a ratchet!

That's irrelevant. You are confusing two completely different types of work!

 

[Mark-1]

That's irrelevant.

Ok, I'll take your word for that.

 

[flaming]

Yes, it's gybe. But it shouldn't be if there's a 22kt wind component it should be tacking. More obviously its on a port tack but the apparent wind should be coming from the starboard side where the leeward bouy is (the leeward bouy is effectively a windward bouy to the boat). On that course the boat should be on the other tack.

This is, I'm afraid, rather comprehensively incorrect.

I'm afraid you have something of a misunderstanding about what is happening. The apparent wind is the vector of the boat's forward motion and the true wind. You are, I'm afraid, rather mistaken about where the apparent wind should be coming from. There is nothing about the motion of the boat that will swap the wind to the other side of the boat compared to a slower boat. All it will do it drag it forwards.

You are also wrong is stating that the leeward mark is in effect a windward mark. That is just demonstrating that you have not understood what is happening. It's just vectors, and no vector with the boat sailing at any angle to the wind can ever result in the wind flipping by 180 degrees.

 

[flaming]

But that's not the sailing boat, as described by johnalison, doing the trick.

Richard

No, because the point is that the cart isn't doing the trick either... Because the sails on the cart, the propellers, are also not travelling dead down wind. Which is how it works...