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Post your polar and VMG

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Frank Rosin
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Re: Post your polar and VMG

Postby Frank Rosin » Wed Apr 12, 2017 10:58 am

I got no VMG records, but by feel and just normal gps recording I would assume that the wind speed / VMG ratio is getting more extreme the lighter the wind.
As foiling speed does not change as much.
Makes sense?

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Re: Post your polar and VMG

Postby evan » Wed Apr 12, 2017 11:52 am

That makes sense indeed, with a 17m it is possible to even have an upwind VMG higher than the actual windspeed and a downwind VMG of more than twice the windspeed!

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Re: Post your polar and VMG

Postby Lokihel » Wed Apr 12, 2017 12:02 pm

I'd like to see someone having a VMG twice the true wind speed when it is blowing 30 to 40 kts.


Obviously in light winds it is easier to have a high VMG / true wind speed ratio.

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davesails7
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Re: Post your polar and VMG

Postby davesails7 » Wed Apr 12, 2017 12:33 pm

Frank Rosin wrote:
Wed Apr 12, 2017 10:58 am
I got no VMG records, but by feel and just normal gps recording I would assume that the wind speed / VMG ratio is getting more extreme the lighter the wind.
As foiling speed does not change as much.
Makes sense?
Me too. In anything over 10 kts of wind, my speeds are about the same. My downwind angle does get better as wind speed increases though.

I'll try to get some polar from my GPS tracks this weekend. Interested to see how they turn out.

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Re: Post your polar and VMG

Postby cosmo » Wed Apr 12, 2017 2:01 pm

You will get the twice the wind speed in the downwind vmg if you keep your 17m kite in 30 knots!
Seriously. We use some "wind to kite size" algoritm ;-) so is logical that the speed was always the same. If we use always the same kite in every speed of the wind (imposible) our speed will go some kind of proporcinal increase.

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Re: Post your polar and VMG

Postby plummet » Wed Apr 12, 2017 2:47 pm

So the high aspect foil/kite combo gets 30 deg from direct upwind?

Low aspect hf high aspecr for 40 deg.

Anyone of medium aspect foil with lei got stats?

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Re: Post your polar and VMG

Postby evan » Wed Apr 12, 2017 4:53 pm

There is a better angle on the race-gear indeed, but do not read the polars as absolute values as the wind shifts at least 10-20deg thus making the best up- and downwind angles on the polar way better than if you measure the angle between tacks unless you are on a spot where the wind is absolutely free of windshifts. Best I got between tacks is around 40deg upwind and 150deg downwind.

To get an idea, this is a polar of last tuesday on a very gusty inland lake on the 14m aero:
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alexrider
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Re: Post your polar and VMG

Postby alexrider » Wed Apr 12, 2017 7:10 pm

Pertinent observation explaining the optimistic polar. :idea:
To avoid this, a quick fix would be to split the sessions into shorter ones during which the wind may be assumed to be free of windshifts.
If there is a way to figure out the actual wind direction in real time during (from a nearby weather station for example) then it would be easy for the developer to integrate the values in the program, instead of using a fixed, averaged out one. But even this would have a severe limitation since the wind direction at the land station is unlikely to be the same as where the rider is.
The only way out of the conundrum would be to carry an anemometer measuring the wind speed as well as the apparent direction (as in race yachts), from which together with the GPS readings, the true wind speed and direction can be calculated. But what a bore to have to carry yet one more gimmick. :-?
evan wrote:
Wed Apr 12, 2017 4:53 pm
There is a better angle on the race-gear indeed, but do not read the polars as absolute values as the wind shifts at least 10-20deg thus making the best up- and downwind angles on the polar way better than if you measure the angle between tacks unless you are on a spot where the wind is absolutely free of windshifts....

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Re: Post your polar and VMG

Postby evan » Wed Apr 12, 2017 7:19 pm

I am talking about shifts inside gusts that last only seconds, a minute at most, that do not show on a landbased station wind direction that averages over multiple minutes unless it is showing the wind direction-range like the gusts vs average wind. But readings like that are sadly very rare.

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alexrider
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Re: Post your polar and VMG

Postby alexrider » Wed Apr 12, 2017 7:38 pm

Thank for the clarification. The sort of gusts that the best sailors take fully advantage of to win races.
evan wrote:
Wed Apr 12, 2017 7:19 pm
I am talking about shifts inside gusts that last only seconds, a minute at most, that do not show on a landbased station wind direction that averages over multiple minutes unless it is showing the wind direction-range like the gusts vs average wind. But readings like that are sadly very rare.


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