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29.892 knots
30.786
33.93
37.649
41.808
43.615
41.003
37.348
28.712
22.631
..... Anyone care to comment.

Now we can conclude beyond reasonable doubt "10 seconds" is an inferior measurement for speed claims.

The unlearned speed peasant me dares to mention 500 meters is the perfect speed filter!

Ah....what? Oh you mean 20 seconds!

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icesurf said...

Now we can conclude beyond reasonable doubt "10 seconds" is an inferior measurement for speed claims.

The unlearned speed peasant me dares to mention 500 meters is the perfect speed filter!

Michael,
I know you already know this, but the trouble is the GPS cannot record an exact 500metres as it is time based. Unless you want to revert to an archaic system of timing gates then this is completely unrealistic for what we GPS speedsters love to do. We shouldn't be too rash just because of one debatable track. Remember this has been recorded with older technology too (GT-11). I reckon the 5 x10 sec average is the best possible system for accurately measuring one's ability to go fast (consistantly) and having two GT-31 for a 1x10sec record seems fair from this snail's perspective.

PS - getting cold in NZ yet?

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sausage said...

MarkvanOsch said...

decrepit said...

so is that trackpoint or doppler???

Just checked for myself, here's the doppler table.
Still a big jump going from 27 to 38
So the biggest acceleration is 10kts in one second, the biggest decceleration is 5kts in 1 sec

27.14
27.33
38.80
42.20
42.94
44.77
39.60
36.45
31.82
30.63
29.24
28.07
27.78
27.27

Hi Decrepit,
how did you get this list of doppler speeds? In your list the decceleration is less than in other lists mentioned. It is different from the screenshot by Sailquick, also doppler speeds list on the right in realspeed: /

Thanks,
Mark

Mark,
I just figured out Decrepit has accidently listed the doppler figures in reverse.

Sausage, well spotted!!!! It's a strange thing with gpsarpro, to find the spot in the table I had to click at the top of the doppler speed column to get the table in speed order. That gave me the run number, then to go back to time sequence I hit the run number. for some reason that puts the whole table back to front!!!!!
This is the correct table, sorry for any confusion.

28.07
29.24.
30.63
31.82
36.45
39.60
44.77
42.94
42.20
38.80
27.33
27.14

Andrew. you're way ahead of me, I've no idea how to view acceleration in any software.

PS. I notice Nebs has given the official OK on the GTC site.

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decrepit said...

PS. I notice Nebs has given the official OK on the GTC site.

There's more to the story than meets the eye:
We couldn't prove that the track was false (although it did look suspicious). This means that it's up to the poster's discretion. Frank decided to keep his numbers up, as he felt that he did in fact do that speed. We support him in his decision, and we would have supported him if he decided to use the next highest speed instead.

One of the rationales for the existence of the KA72 website is that it allows you guys to do exactly this kind of analysis. No automatic system is capable of replicating the kind of indepth analysis you've done on a track with such a marginal result in it. We try our best, but at the end of the day there is no substitute for eyeballing the data.

I hope it was useful for the exercise.

Reached 160,000 uploaded kilometres today.

Cheers,

Dylan.

The site is fantastic Dylan, but individual tracks are hard to find, there's just so many there now.
A search function of some sort would help

I tell you, the peer review system works well for GPS tracks

Thanks for all the help that was available from the web community analysing that one, it was a doozy!!! Think I might get myself one of those fancy GT31's soon I shudder to think what the community would have thought of a trackpoint-only file

And it wouldn't be possible without Dylan's wonderful KA72 program

Top job mate

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decrepit said...

The site is fantastic Dylan, but individual tracks are hard to find, there's just so many there now.
A search function of some sort would help

I'll roll up my sleeves and get something done.

Watch this space...

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nebbian said...

I tell you, the peer review system works well for GPS tracks

Thanks for all the help that was available from the web community analysing that one, it was a doozy!!! Think I might get myself one of those fancy GT31's soon I shudder to think what the community would have thought of a trackpoint-only file

And it wouldn't be possible without Dylan's wonderful KA72 program

Top job mate

Really shows the worth of wearing 2 GPSs doesn't it. If there'd been 2 separate files to check, that would have eliminated any doubt.

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sailquik said...

Ah....what? Oh you mean 20 seconds!

Unless you doing 50 knots

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sausage said...
PS - getting cold in NZ yet?

Yeah finding really hard to reacclimatise.
5mm wetsuit season soon.

The owner of this track should really know better than to claim the speeds as legit.

1/ The claimed acceleration may be possible?

2/ The claimed deceleration isn’t possible!
From observation, travelling at speeds of 35+ knots down wind in about 20-30 knots of wind you are generating large amounts of apparent wind. As you travelling faster than the wind your momentum & the apparent wind propels you to the next puff/ gust of wind. If there wasn’t another puff/ gust of wind deceleration would be gradual unless you spin out as mentioned. Everyone who has travelled downwind next to a sand bank knows how hard it is to slow down & stop, takes effort to decelerate. At 40 knots of speed on smooth water its even harder to decelerate!

The GPS has filters that "smooth" the data before it is recorded. You can see this when you have a crash- it still takes a couple of seconds for the speed to reduce to zero on the gps log, even though you actually stopped in a splitsecond. When analysing transient GPS data you have to consider that these filters are smoothing the data before its recorded. Sudden changes and spikes are spread out in time.

So, to get that result after the filter, the real accelleration/decelleration and max speed would have to have been higher, but over a shorter period.

The decelleration on the tracklog is similar to braking from 100km/hr to a stop in ~6.2 seconds. If we consider the filter, then the actual accelleration/decelleration would have been even more aggressive than that.

I cant say it didnt happen but given that the decelleration appears similar to a crash that must have been one hell of an awesome recovery. Also to accellerate at that rate for so many seconds is quite extraordinary given that such accelleration is usually afforded by body inertia for a fraction of a second before you are yanked off your feet.

From time to time the sirf-based GPSs give an "out-there" reading. I have seen this plenty of times. Every now and then that "out there" data is going to land somewhere in the grey area. Using two GPSs helps to highlight where one of the GPSs might have lost its brain temporarily, so you can treat that data as suspect (from both units). Not perfect but a lot better than running a single GPS, for confidence in the validity of data.

When using one GPS, we have to trust ourselves to be objective. If it seems highly unlikely it probably is.

After reading all the facts & informative and expert responses (here and elsewhere), I think I'll stop sitting on the fence. Time to call this particular myth track busted I suppose.