Forums > Windsurfing   Gps and Speed talk

What is an "Alpha"?

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Created by TheSailingMoose > 9 months ago, 27 Dec 2014
TheSailingMoose
VIC, 142 posts
27 Dec 2014 7:50AM
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Looking at the speed categories on the GPSTC website...peak speeds, nautical miles and one hour averages are all pretty self explanatory but what is "Alpha Racing 500m"?

bLeSsEd
7 posts
27 Dec 2014 6:17AM
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Its basically 250m sailing, gype, 250m back, must be within 50m of starting point.........

Stuthepirate
WA, 3130 posts
27 Dec 2014 12:07PM
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bLeSsEd said..
Its basically 250m sailing, gype, 250m back, must be within 50m of starting point.........







sboardcrazy
NSW, 5988 posts
27 Dec 2014 4:21PM
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Please everyone its' gybe not gype ( noticed a few people spelling it gype..)

easty
TAS, 2213 posts
27 Dec 2014 7:18PM
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sboardcrazy said..
Please everyone its' gybe not gype ( noticed a few people spelling it gype..)


...it's gybe....not its' gybe.
Grammar nazis!

sboardcrazy
NSW, 5988 posts
27 Dec 2014 7:58PM
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easty said..

sboardcrazy said..
Please everyone its' gybe not gype ( noticed a few people spelling it gype..)



...it's gybe....not its' gybe.
Grammar nazis!


decrepit
WA, 7695 posts
27 Dec 2014 6:59PM
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bLeSsEd said..
Its basically 250m sailing, gype, 250m back, must be within 50m of starting point.........



Basically yes, but it's less than 250m legs, as total distance mustn't be over 500m, and you need some room to gybe in.

sailquik
VIC, 3539 posts
16 Jun 2015 10:41PM
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Yes the speed is the average speed over the points measured. It can be any distance less than 500m.

But note that because the Gybe is the slowest past of the track, the more straight line reaching you do at higher speed, the faster the average will be. So in practice, the fastest Alpha 500's will always be as close to 500m as possible.

If you calculate the Alpha 1000m and Alpha 250m from the same track segment, (assuming that you continued long enough to go 1000m at approximately the same speed), you will almost invariably find the Alpha 1000 faster and the Alpha 250 slower.

So I can say truthfully that I have done a 30 knot Alpha! But it was an Alpha 1000!

For those who don't know, the speed of the Alpha is measured with the Doppler data, but the proximity circle is measured using the Locational data (trackpoint).

This is one of the reasons why Alpha speeds are not considered accurate enough for official records with the current technology. Better locational (and Doppler speed) accuracy from future GPS devices with higher hz data may change this.

Dylan72
QLD, 543 posts
21 Jun 2015 10:57PM
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The 500 is a maximum.

On ka72, until recently, we had a minimum distance based on 2x the radius, so your Alpha was the best gybe based on travelling between minimum of 100m and maximum of 500m.

Some "modern" GPS devices cause kind of a speed distortion when gybing blowing out the distance calculation (mainly affects devices that output FIT files) so I've decided recently to increase the minimum distance travelled, because some awfully short gybes were being detected as valid alphas. I think the new ka72 standard is 2.2x radius for the minimum distance travelled.

The Alpha is the most painful division to get right. Your inbound and exit tracks could pass within 50m of each other, but you might not get an alpha because there are no actual trackpoints that are within 50m of each other. Sometimes you get lucky, sometimes you don't. I have a feeling that some software works with this, but I could never be bothered. Alphas are slow enough to work out already.

(In the WindWanderers online speed comp, there is a 5m Alpha challenge division. You have to start and finish 5m apart. Otherwise the distance is still 500m maximum. That is more of a classic "Alpha" shape.)

Windsurfunstu
NSW, 176 posts
22 Jun 2015 1:12PM
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sailquik said..
Yes the speed is the average speed over the points measured. It can be any distance less than 500m.

But note that because the Gybe is the slowest past of the track, the more straight line reaching you do at higher speed, the faster the average will be. So in practice, the fastest Alpha 500's will always be as close to 500m as possible.

If you calculate the Alpha 1000m and Alpha 250m from the same track segment, (assuming that you continued long enough to go 1000m at approximately the same speed), you will almost invariably find the Alpha 1000 faster and the Alpha 250 slower.

So I can say truthfully that I have done a 30 knot Alpha! But it was an Alpha 1000!

For those who don't know, the speed of the Alpha is measured with the Doppler data, but the proximity circle is measured using the Locational data (trackpoint).

This is one of the reasons why Alpha speeds are not considered accurate enough for official records with the current technology. Better locational (and Doppler speed) accuracy from future GPS devices with higher hz data may change this.


So assuming your inward and outward tracks are roughly parallel and say you do travel 1000m, what determines the start and end points? Is your alpha calculated on the minimum or maximum distance?

I guess parallel tracks are not that likely over a long distance so another, and perhaps better question would be, is the start and end point determined when your outward track becomes further than 50m apart from your inward track, provided its within the min/max distance? I think I may have answered my own question (scratching sore head)

decrepit
WA, 7695 posts
22 Jun 2015 8:04PM
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most software picks the fastest part of the tracks that fall within the criteria. so if one leg is faster than the other, and the legs are under 50m apart it will use more of the fast track up to the 50m apart and under 500m total.

Windsurfunstu
NSW, 176 posts
23 Jun 2015 9:45AM
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Thanks Decrepit, makes sense. All I need to do now is sail faster



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"What is an "Alpha"?" started by TheSailingMoose