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Post your polar and VMG

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tegirinenashi
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Re: Post your polar and VMG

Postby tegirinenashi » Mon Apr 10, 2017 6:10 pm

I'm still confused. Draw a 15 kn wind vector pointed downward (180 degrees). Draw 30 kn kiteboarder vector pointing 150 degrees. The difference (apparent wind) would be pointing almost backwards to HFKB direction of travel. It is significantly more than 90 degrees.

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Re: Post your polar and VMG

Postby gmb13 » Mon Apr 10, 2017 6:14 pm

Please take into account that the speeds on the dead down wind (180 deg) are during gybes. I don't hold that course for a long time. When racing your spend most of the time on max upwind and downwind VMG courses.

--
Gunnar

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tegirinenashi
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Re: Post your polar and VMG

Postby tegirinenashi » Mon Apr 10, 2017 6:15 pm

Ah, the kite pull is 90 degrees to apparent wind -- what a newbie mistake! (And the problem of apparent wind facing against the direction of travel is the same for going upwind).
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Last edited by tegirinenashi on Mon Apr 10, 2017 6:25 pm, edited 3 times in total.

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Re: Post your polar and VMG

Postby gmb13 » Mon Apr 10, 2017 6:17 pm

alexrider wrote:
Mon Apr 10, 2017 2:03 pm
A bit off topic (but I'm allowed because I'm the OP :P ): what's the advantage of shorter lines wrt performance? Seems that racers all opt for that.
Being able to hold more power for max speed and angle downwind, but still be able to hold the kite on the upwind course.

Bonus is also "easier"/faster tacking, transitioning from reaching to beating and less drag from the lines.

--
Gunnar

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Re: Post your polar and VMG

Postby gmb13 » Mon Apr 10, 2017 6:21 pm

gmb13 wrote:
Mon Apr 10, 2017 6:14 pm
Please take into account that the speeds on the dead down wind (180 deg) are during gybes. I don't hold that course for a long time hence this is not a top speed. When racing your spend most of the time on max upwind and downwind VMG courses.

--
Gunnar

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Re: Post your polar and VMG

Postby evan » Mon Apr 10, 2017 9:23 pm

Wrap your head around this: In winds under 20kn your VMG can be faster than the windspeed so when a gust hits you on an upwind leg you can head downwind after the gust passes and catch that same gust again. Downwind faster than the wind, something most sailors would call witchcraft a decade ago.

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Re: Post your polar and VMG

Postby alexrider » Tue Apr 11, 2017 4:56 am

Good explanation. May I suggest to edit just one word to make it even more understandable? Underlined and coloured. "Upwind" suggests you are going upwind. "Close-hauled" not necessarily if used as "sail trimmed in tightly".
Lokihel wrote:
Mon Apr 10, 2017 5:56 pm
It is all about apparent wind speed.

I am a bit lazy to do the math to show it right now, but basically the faster you go, the more wind you create, so the faster you go.
Essentially, you're still sailing 'close-hauled', although you are actually travelling downwind.

I am not sure what the limit is on a hydrofoil, but on a high performance sailboat you want the apparent wind to remain a 90°.
As you gain speed, the apparent wind will shift 'forward', allowing you to bear a little downwind. You can then very easily be 150° off the true wind.
You have to start by again speed though. If you start by pointing 150° off the true wind you will struggle to build speed.

Because of that magic, performance sailors can have a VMG downwind greater than the true wind speed.
This means they can get downwind faster than the wind that is powering them.

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Re: Post your polar and VMG

Postby alexrider » Tue Apr 11, 2017 5:09 am

There was a long and interesting DDWFTTW (dead downwind faster than the wind) thread in kiteforum. Perhaps time to revive it?
viewtopic.php?f=1&t=2361300&hilit=DDWFTTW
For those who have problems understanding how it's possible to sail dead downwind faster than the wind, there's more in the following articles:
https://www.wired.com/2010/08/ddwfttw/
https://www.lhup.edu/~dsimanek/museum/DDWFTTW.htm
A as matter of fact, it's not conceptually harder than understand how it's possible to go upwind. I suppose it's just a matter of visualising the wind triangle.

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Re: Post your polar and VMG

Postby Lokihel » Tue Apr 11, 2017 5:17 am

alexrider wrote:
Tue Apr 11, 2017 4:56 am
Good explanation. May I suggest to edit just one word to make it even more understandable? Underlined and coloured. "Upwind" suggests you are going upwind. "Close-hauled" not necessarily if used as "sail trimmed in tightly".
Lokihel wrote:
Mon Apr 10, 2017 5:56 pm
It is all about apparent wind speed.

I am a bit lazy to do the math to show it right now, but basically the faster you go, the more wind you create, so the faster you go.
Essentially, you're still sailing 'close-hauled', although you are actually travelling downwind.

I am not sure what the limit is on a hydrofoil, but on a high performance sailboat you want the apparent wind to remain a 90°.
As you gain speed, the apparent wind will shift 'forward', allowing you to bear a little downwind. You can then very easily be 150° off the true wind.
You have to start by again speed though. If you start by pointing 150° off the true wind you will struggle to build speed.

Because of that magic, performance sailors can have a VMG downwind greater than the true wind speed.
This means they can get downwind faster than the wind that is powering them.
:thumb:

Agreed.
Not very good at remembering some technical words, hence the ' ' around some words.

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Re: Post your polar and VMG

Postby bohme » Wed Apr 12, 2017 10:44 am

My polars (maximum)
15kn wind
11m elf joker5 on 20m lines.
Spotz 2
GT-31 gps
I can't jibe yet.

GpsAR can show apparent wind. Best AWA (Apparent Wind Angle is) were around 20° upwind, and 30° downwind.

But the holy grain for me is not going faster than the wind downwind on VMG. It is going faster than the wind, upwind!
Gunner's plot shows a maximum upwind VMG of 18kn in 12-15kn wind! That is impressive...

Jacob
Attachments
upwind.PNG
downwind.PNG
Polar_kiteforum.png


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