Wind powered cart goes faster than the wind, directly downwind

[Mark-1]

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.

If you stand on on a hill watching boats sailing in the same direction as the wind the ones with DD VMG lower than the wind speed will appear to be sailing broadly downwind, the ones with DD VMG higher than the wind will be sailing up wind. (Yes, I know.)

That's why 90 degs to the wind is the fastest point of sail. As soon as you bear off from there you lose wind speed. (Assuming flat water.)

Indeed you say: "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."

But I think we've identified a point where we disagree and are never going to agree, so we can stop wasting each others time.

The great news for both of us is we can prove ourselves right easily:
Apparent Wind Calculator

 

[johnalison]

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).

The wheels may act as an intermediary source of energy, but this energy has to come from somewhere in the first place, which is the wind.

 

[johnalison]

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

Richard

If I had any hairs left to split, they would be shredded by now.

 

[JumbleDuck]

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).

The wheels may act as an intermediary source of energy, but this energy has to come from somewhere in the first place, which is the wind.

Yes, I know. That's what I said, right there in the post you were replying to!

 

[RichardS]

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...

I've absolutely no idea what you mean.

The cart is doing the trick. It is travelling directly downwind faster than the speed of the wind. That is the trick!

Johnalison said "Boats don’t have wheels and can still achieve the same trick" but no boat on Earth can achieve this trick.

Surely this is kindergarten stuff?

Richard

 

[flaming]

I've absolutely no idea what you mean.

The cart is doing the trick. It is travelling directly downwind faster than the speed of the wind. That is the trick!

Johnalison said "Boats don’t have wheels and can still achieve the same trick" but no boat on Earth can achieve this trick.

Surely this is kindergarten stuff?

Richard

Ah I see your confusion, you are looking at it from the point of view of the cart pointing DDW, not the ability of a wind powered craft to get to a destination DDW from it's starting point faster than the wind, which is the trick that Johnalison was referring to.

 

[flaming]

If you stand on on a hill watching boats sailing in the same direction as the wind the ones with DD VMG lower than the wind speed will appear to be sailing broadly downwind, the ones with DD VMG higher than the wind will be sailing up wind. (Yes, I know.)

No.... No they won't... This is, I think, your fundamental misunderstanding of what is happening. They are still travelling downwind. Upwind and downwind always have been, and always will be, defined by the true wind.

Their APPARENT wind will be forward of the beam, but the boat is still travelling downwind.

 

[RichardS]

Ah I see your confusion, you are looking at it from the point of view of the cart pointing DDW, not the ability of a wind powered craft to get to a destination DDW from it's starting point faster than the wind, which is the trick that Johnalison was referring to.

In reply to my post Johnalison said "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." so absolutely no reference to to VMG faster than the wind.

Richard

 

[flaming]

In reply to my post Johnalison said "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." so absolutely no reference to to VMG faster than the wind.

Richard

To which you replied

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

So you'll excuse me if I thought you were referring to the trick being the DDW bit...

So if you are now saying the trick is not VMG downwind faster than the wind, what are you saying the trick is? Extracting energy from the wind?

 

[RichardS]

To which you replied

So you'll excuse me if I thought you were referring to the trick being the DDW bit...

So if you are now saying the trick is not VMG downwind faster than the wind, what are you saying the trick is? Extracting energy from the wind?

In the absence of a clear statement to the contrary, I have assumed that the "trick" relates to the title and subject of the thread which is travelling directly downwind faster than the wind.

If the "trick" is simply extracting energy from the wind, then that's hardly a trick as mankind has been successfully doing that for thousands of years.

Richard

 

[flaming]

In the absence of a clear statement to the contrary, I have assumed that the "trick" relates to the title and subject of the thread which is travelling directly downwind faster than the wind.

If the "trick" is simply extracting energy from the wind, then that's hardly a trick as mankind has been successfully doing that for thousands of years.

Richard

Well yes, but I explained way back in 87, and 100, why the cart and the boat are actually doing the same thing from a physics point of view.

Both are getting downwind faster than the wind, the cart does it by revolving its "sails" around an axis, the boat does it by gybing back and forth.... Same effect, and the parts doing the work on the cart are not travelling DDW, but in both cases the average course of the sails is DDW.

But as Johnalsson said.... This is rather splitting hairs.

 

[johnalison]

I think we have got confused by semantics. I thought that by ‘trick’ I meant VMG, but even so, the prop-driven car is still doing the same thing, as someone elegantly pointed out, if you see the propeller blades as acting like sails.

 

[JumbleDuck]

I think we have got confused by semantics. I thought that by ‘trick’ I meant VMG, but even so, the prop-driven car is still doing the same thing, as someone elegantly pointed out, if you see the propeller blades as acting like sails.

I think that is a misconception. I'm pretty sure that, in theory, the cart could do its stuff using a centrifugal fan with horizontal axis and suitable ducting.

 

[RichardS]

I think we have got confused by semantics. I thought that by ‘trick’ I meant VMG, but even so, the prop-driven car is still doing the same thing, as someone elegantly pointed out, if you see the propeller blades as acting like sails.

But that is the misconception which I have already explained. The propeller is not acting like sails because it's the wheels that are driving the propeller.

Richard