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Possible to go downwind faster than the wind?

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Re: Possible to go downwind faster than the wind?

Postby spork » Sat Oct 23, 2021 8:25 am

Regis-de-giens wrote:
Thu Oct 21, 2021 3:23 pm
This mechanical link with soil, combined with the calculated blade angle, offers a surface that goes a bit rearward vs the soil...
It's true that the propeller produces a sort of virtual surface that can be thought of as moving backward relative to the vehicle, but not relative to the ground. It's still advancing relative to the ground, but slower than the true wind.

But that's not really the best way to visualize this in my opinion. That's what helps some folks get past the idea that the wind is "pushing" something that's moving faster than the wind itself, but it's not quite how it works. If we analyze the flow through and around the propeller disk, it's identical to that of a Cessna in straight and level flight. The necessary advantage comes from the fact that the propeller is trying to advance through the air more slowly than the wheels are rolling over the ground - so we can trade a smaller force over a larger distance for the larger force (propeller thrust) over a smaller distance.
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Re: Possible to go downwind faster than the wind?

Postby spork » Sat Oct 23, 2021 8:27 am

Regis-de-giens wrote:
Thu Oct 21, 2021 7:43 pm
This video is really excellent !!! thank you ; very clear and finally presents the good physics and good counter arguments of non - believers ; i strongly encourage you to listen to it.

thanks again rnelias

It is an excellent video, and of course it has a LOT more exposure than mine. I'm really glad Derek decided to take this on. But for folks that are still struggling to get their head around it, I recommend this video where I explain the mechanism to a bunch of sailors at the St. Francis Yacht Club...


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Re: Possible to go downwind faster than the wind?

Postby Regis-de-giens » Sat Oct 23, 2021 9:44 am

spork wrote:
Sat Oct 23, 2021 8:25 am
Regis-de-giens wrote:
Thu Oct 21, 2021 3:23 pm
This mechanical link with soil, combined with the calculated blade angle, offers a surface that goes a bit rearward vs the soil...
It's true that the propeller produces a sort of virtual surface that can be thought of as moving backward relative to the vehicle, but not relative to the ground. It's still advancing relative to the ground, but slower than the true wind.
I agree with you, backward relative to wind is true and sufficient to understand and make it work. Relative to soil is just "easier" to catch the principle IMO because it "works" in whatever cart speed; i will try to detail my reasoning.

I also agree on your presentation of distance vs force that is different on wheel and on the propeller ; this provides the "energy conservation" explanation and it works as well ....

... but ... :wink:
I still believe (95% sured, personal statement, open to your arguments ), that the propeller surface HAS TO be driven backward faster that the wind. Actually the parameter of the ratio between the cart speed and the propeller "surface" (*) sets the ratio speed vs wind that the cart will ever reach (ideal , in case of no drags nor mechanical friction of the cart. Do you agree ?

for the next part of my message: lets call above case " ratio 1:1": meaning the propeller surface goes backward exactly as fast as the speed and remain at zero speed vs the ground. this is a rigid mechanical link with soil, so it remains true whatever the cart speed.

... so, ratio 1:1 (so backward speed is equal to the ground) is a good particular case where the speed could be infinite if no friction , hence there is no theoretical speed threshold for the cart ... the propeller will offer a surface facing the wind whatever the cart speed and whatever the wind positive value

now imagine that this ratio is lower, say 1:2 , meaning the propeller goes only half way back vs the soil. when cart will reach twice the wind, then the propeller will reach wind speed exactly, and turn "with no thrust in the air", like a Cesna propeller that would have no angle of attack ... cart cannot run faster, ideal threshold is reached, end of the story ...

so to me, having "just" the explanation that the propeller turns in a "lower" wind vs the cart is not complete... it only works until this ratio (speed vs wind) is not reached; and this ratio (*) is set by design but can be changed .

(*) parameter fixed by design, combination of :
- propeller blade angle
- propeller diameter
- ratio of transmission wheel // propeller
- wheel diameter

interesting to know your thoughts about it

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Re: Possible to go downwind faster than the wind?

Postby tegirinenashi » Sat Oct 23, 2021 3:18 pm

Slightly off topic, but more practical question would be how to sail on a broad reach with VMG faster than wind speed. It looks like the racers can approach absolute speed 3X of wind speed, which amounts to somewhere around at least 2X VMG. They seems to have line tension and hence the stance almost as if they were sailing close hauled. Their kite seems to be flying in almost the same position, as opposed to simply drifting in the front. Here is my visual understanding how it works:
vmg1.png
where the "15" is the wind vector, the "30" is the board speed vector, and the green vector is the apparent wind. So, a sailor on broad reach would have the kite almost at the other broad reach...

To summarize: can I sail faster than wind?

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Re: Possible to go downwind faster than the wind?

Postby PullStrings » Sat Oct 23, 2021 3:54 pm

1234567Simon wrote:
Thu Oct 21, 2021 11:12 am
Hey!
If I start looping, I am getting an extra pull and Start going faster, BUT: is the windspeed the limit to go full downwind?
Hey!
I think if you are stuck in a death loop in 30 mph of wind you will dragged straight downwind at also 30 mph
Just sayin
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Re: Possible to go downwind faster than the wind?

Postby Regis-de-giens » Sat Oct 23, 2021 9:39 pm

tegirinenashi wrote:
Sat Oct 23, 2021 3:18 pm
Slightly off topic, but more practical question would be how to sail on a broad reach with VMG faster than wind speed. It looks like the racers can approach absolute speed 3X of wind speed, which amounts to somewhere around at least 2X VMG. They seems to have line tension and hence the stance almost as if they were sailing close hauled. Their kite seems to be flying in almost the same position, as opposed to simply drifting in the front. Here is my visual understanding how it works:

vmg1.png

where the "15" is the wind vector, the "30" is the board speed vector, and the green vector is the apparent wind. So, a sailor on broad reach would have the kite almost at the other broad reach...

To summarize: can I sail faster than wind?
your sketch is correct ; you just need to "draw" the l/D ratio of the kite which changes the angle a little lower than 90 degree toward the green arrow. But still you can have this red force that has still a downward component, therefore the vmg drawn on your diagram is higher than wind, while red arrow still pull downward ...
so it works ! with a certain angle to the wind and like the example you drawn with the angle you choose. not in a direct downwind , never with a kite as I said earlier
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Re: Possible to go downwind faster than the wind?

Postby nothing2seehere » Mon Oct 25, 2021 12:34 pm

Just to check then, are we happy that the AC75 boats that ran in the Prada cup where the wind speed was typically 10-18 knots (lower limit 6.5 knots) couldn't go downwind any faster than the windspeed? Only from the footage it looked like the boats were doing close to 40 knots even on the downwind legs. Or is the difference VMG vs direct downwind?
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Re: Possible to go downwind faster than the wind?

Postby Peter_Frank » Mon Oct 25, 2021 2:22 pm

nothing2seehere wrote:
Mon Oct 25, 2021 12:34 pm
Just to check then, are we happy that the AC75 boats that ran in the Prada cup where the wind speed was typically 10-18 knots (lower limit 6.5 knots) couldn't go downwind any faster than the windspeed? Only from the footage it looked like the boats were doing close to 40 knots even on the downwind legs. Or is the difference VMG vs direct downwind?

You can go way faster than the wind in downwind VMG yes, also on surf gear and not only AC75s.

In this thread we are talking about a vessel going dead downwind, using a kite - is it possible to go faster than the wind now?

8) Peter

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Re: Possible to go downwind faster than the wind?

Postby Regis-de-giens » Mon Oct 25, 2021 9:54 pm

Non mon chéri ... :heartface:

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Re: Possible to go downwind faster than the wind?

Postby UKSurf » Wed Oct 27, 2021 11:54 am

Peter_Frank wrote:
Mon Oct 25, 2021 2:22 pm
You can go way faster than the wind in downwind VMG yes, also on surf gear and not only AC75s.

In this thread we are talking about a vessel going dead downwind, using a kite - is it possible to go faster than the wind now?

8) Peter
For light winds, say 10 knots, what would you say the best angle off the wind is to maximise the Downwind VMG on a foil - I appreciate it depends on equipment and waves etc just looking for a rough idea.

For those reading this thread VMG stands for velocity made good or the component of your velocity in the direction of the true wind (downwind or upwind) - I had to look it up.


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