Hi speed freaks. I have a GT-31 that I can wear in its aquapac pouch for official data recording, but a wrist watch would be much more convenient for reading "on the run". I previously posted a question about the Garmin F45. I now see that no Garmin IQ windsurf apps will work on the F45, therefore I need to get something more capable. I understand a top-end watch would do what I want, but I can't justify $1000 (or even $500). Please recommend a suitable watch (doesn't have to be Garmin) for monitoring my windsurf speed. Thanks in advance for any input.

Take a look at the gpstc Garmin datafield thread (search Garmin on this forum) and you'll find a list Andrew put together, some lower priced ones there doesn't need to be a Fenix. The sunnto watch also has some apps available for windsurfing speeds, you can customise the screens too.

The Garmin Vivioactive 3 is currently on special for $248. It's on the list of Garmin devices that work with the GPSTC Data Fields developed by Andrew Haigh (tbwonder) mentioned by Dezza above. The Garmin and the data fields provide good on the water feedback across all GPSTC disciplines.

I recently heard of people having problems with the vivoactive 3 and my datafields, so I would not recommend buying that model.
I don't see many bargains available at the moment. I see the Fenix 5X is going for around $530, which is good for a Garmin with full maps.

Since I want maps, I got a Fenix 5X (renewed) for about USD$275. Not a bad price. Since it has datafield memory of 128k, it can accept all the water-sport datafield downloads out there.

But, oh my goodness, what a complex beast it is. That's the price I pay for having maps. The user manual is 46 pages, for crying out loud.

If I want just plain ole GPS that records speed, distance, track, max speed, not for gpstc purposes certainly, but for my own archiving purposes, and for a good on-water display of speed, distance, and time of day, I use a VERY SIMPLE Timex Ironman GPS, which runs about USD$75 on amazon. I see the track on a map offline with Timex Connect and Strava, as well as other numbers such as max speed, distance, elapsed time, etc.

I have a VA3 that I use for windsurfing. I do not have the datafield mentioned setup. I just have an activity type setup and just use standard datafields (which give you distance, max speed etc. but not windsurfing specific ones like 5X10, Alpha etc.) I mostly am interested in distance and max speed.

I will say that with a VA3 (or any type of garmin touch screen display probably), you need to set everything up before you hit the water. You don't want to be making adjustments with the screen when it and your fingers are wet. It is almost impossible and normally involves me going in and drying stuff. I normally set it to start recording before I get wet, and lock the screen. Unfortunately the lock screen can put a padlock symbol in the centre of the screen if the screen detect something (like water on it). Then I just stop it when I come in. It works fine using it like that but not ideal. A couple of times I forgot to start it and had to come in.

I would recommend a watch with buttons rather than touch screen for windsurfing. Probably a Forerunner would be cheapest with buttons.

Got a Garmin Forerunner 245 Music when it was on special late last year for under $300. It currently retails for $590 or so. Took me a few days to get it properly sorted after downloading the GPS data fields from Garmin and courtesy of the generous TBWonder. Use it for on water feedback of all the GPS fields and use my old GT31 for recording to KA72 and the GPSTC site. Don't think it would be worth buying for the full retail price, but it was too good to resist the 50% off price. Keep you eyes peeled for anymore bargains like this. It will do the job you want.

Big sale on Garmin watchs at www.bikebug.com/

Forerunner 45 for <$200. 245 for cheap also.

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WaynoB said..
Got a Garmin Forerunner 245 Music when it was on special late last year for under $300. It currently retails for $590 or so. Took me a few days to get it properly sorted after downloading the GPS data fields from Garmin and courtesy of the generous TBWonder. Use it for on water feedback of all the GPS fields and use my old GT31 for recording to KA72 and the GPSTC site. Don't think it would be worth buying for the full retail price, but it was too good to resist the 50% off price. Keep you eyes peeled for anymore bargains like this. It will do the job you want.

I like many others took advantage of the special on the Forerunner 245 Music. The on water feed back with tbwonder's data fields is great. What's been interesting is the comparison between the Garmin watch results and the results from my Mini Motion. The data field results vary by up to 0.5kts both up and down from the Mini Motion results across all disciplines. Is this a watch accuracy issue or a data field calculation issue?

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John340 said..

WaynoB said..
Got a Garmin Forerunner 245 Music when it was on special late last year for under $300. It currently retails for $590 or so. Took me a few days to get it properly sorted after downloading the GPS data fields from Garmin and courtesy of the generous TBWonder. Use it for on water feedback of all the GPS fields and use my old GT31 for recording to KA72 and the GPSTC site. Don't think it would be worth buying for the full retail price, but it was too good to resist the 50% off price. Keep you eyes peeled for anymore bargains like this. It will do the job you want.

I like many others took advantage of the special on the Forerunner 245 Music. The on water feed back with tbwonder's data fields is great. What's been interesting is the comparison between the Garmin watch results and the results from my Mini Motion. The data field results vary by up to 0.5kts both up and down from the Mini Motion results across all disciplines. Is this a watch accuracy issue or a data field calculation issue?

You could put two of the same type of device on your body in different spots and get 0.5knots difference. And how do you know which is right?

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John340 said..

Is this a watch accuracy issue or a data field calculation issue?

There is no easy answer. It doesn't matter how many GPS's you have they will all read differently. I have seen files from 2 mini motions worn on the same arm, with 2 sec peak results 1 kt different, even though the reported "accuracy" was only around 0.2 kts. The Garmin accuracy is not recorded, so it is impossible to be absolutely certain what the true speeds are.
Looking in detail at each category.
For the two second peaks there isn't really much datafield calculation to go wrong, all GPS's have spikes, I would generally go with what the Motion records as that has 20 data points to work with.
The 5*10 average, should be very close for the Garmin and Motion, again not to much datafield calculation going on.
Again Nautical mile and distance should be very similar, generally I have found this to be true.
After several weeks at LG observing alphas recorded on the Datafields and those subsequently reported by analyzing Motion data, I am generally seeing the Datafield software reporting alphas between 0.5 and 1kt higher. I am starting to wonder if there is a bug in my code or perhaps a different way of calculating the alpha, I plan to investigate this shortly.
I am also seeing the "hour" category report higher results, perhaps by 0.2 to 0.4 kts, this could be related to the alpha problem. As if each alpha is reporting high, by the time the hour is up perhaps 30 alphas have been completed.

Yes Bikebug has 245 Musics for around $350, that looks like a good deal at the moment.

there is another potential issue: Garmin Connect is not compatible with some phones, so it is worth trying to install the App on your phone before buying the watch

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tbwonder said..
I have seen files from 2 mini motions worn on the same arm, with 2 sec peak results 1 kt different, even though the reported "accuracy" was only around 0.2 kts.

Please send me these logs: julien@motion-gps.com thank you. I take pride in the devices giving the same results so it's a shocker and I hope it's a bug somewhere.

Possibly the way they are worn, two loggers (not mini motions but similar), on my head give results within 0.05kts

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vosadrian said..


You could put two of the same type of device on your body in different spots and get 0.5knots difference. And how do you know which is right?

Based on what evidence?

I have often worn multiple doppler devices (eg GT31, GW52, GW60, Motion). The difference in 2sec results is always within 0.15kts and usually less than 0.1kt. They are worn on either my upper forearm or near my shoulder, so they are unaffected by under or over hand grip positions.

The difference in the 2sec, 5x10, alpha, hour & nm for the Garmin Forerunner 245 has been up to 0.6kts. Hence my question, is this a watch error or data field calculation error.

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John340 said..

The difference in the 2sec, 5x10, alpha, hour & nm for the Garmin Forerunner 245 has been up to 0.6kts. Hence my question, is this a watch error or data field calculation error.

It is most likely a watch error. As I said above I plan to check certain aspects of my alpha code. The Motion has a much larger antenna , 10 hz logging and Doppler, so is far more likely to be correct. julien, I will send logs when I get home in 2 days.

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John340 said..

vosadrian said..


You could put two of the same type of device on your body in different spots and get 0.5knots difference. And how do you know which is right?

Based on what evidence?

I have often worn multiple doppler devices (eg GT31, GW52, GW60, Motion). The difference in 2sec results is always within 0.15kts and usually less than 0.1kt. They are worn on either my upper forearm or near my shoulder, so they are unaffected by under or over hand grip positions.

The difference in the 2sec, 5x10, alpha, hour & nm for the Garmin Forerunner 245 has been up to 0.6kts. Hence my question, is this a watch error or data field calculation error.

My point is simply that two identical device will only read exactly the same very rarely. There is always error. There is more chance that two different devices will read differently as calculation algorithms will be done differently even if they get identical signal. The truth is that there is no way to 100% know which is correct. If you use many devices and one is outlying the others, then it is most likely incorrect... but still not 100%.

Am constantly amazed by the accuracy of these little GPS watches and loggers. By my calculations, the '.05kt max. difference' on his 2sec that Decrepit has experienced between two loggers on his head equates to... wait for it... 51mm, app 5cm, or 2 inches, or app 0.2%.

Am constantly amazed by the accuracy of these little GPS watches and loggers. By my calculations, the '.05kt max. difference' on his 2sec that Decrepit has experienced between two loggers on his head equates to... wait for it... 51mm, app 5cm, or 2 inches.

here's a comparison I did a while ago between the big motion and my home made 10hz logger.



So well under 0.05, the worst is 0.009 And the plus/minus are also similar. I'm sure these numbers, can't be knots if I was sailing, could be a car test I did, in that case they are next to each other on the roof rack. or if sailing, on my head in km/hr????. But I suspect this was an early car test.

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vosadrian said..

John340 said..

vosadrian said..


You could put two of the same type of device on your body in different spots and get 0.5knots difference. And how do you know which is right?

Based on what evidence?

I have often worn multiple doppler devices (eg GT31, GW52, GW60, Motion). The difference in 2sec results is always within 0.15kts and usually less than 0.1kt. They are worn on either my upper forearm or near my shoulder, so they are unaffected by under or over hand grip positions.

The difference in the 2sec, 5x10, alpha, hour & nm for the Garmin Forerunner 245 has been up to 0.6kts. Hence my question, is this a watch error or data field calculation error.

My point is simply that two identical device will only read exactly the same very rarely. There is always error. There is more chance that two different devices will read differently as calculation algorithms will be done differently even if they get identical signal. The truth is that there is no way to 100% know which is correct. If you use many devices and one is outlying the others, then it is most likely incorrect... but still not 100%.

But doppler devices are more accurate than track point devices and doppler devices provide a range of accuracy. Track point devices do not, hence why track point devices are not approved by GPSTC.

Here is an example of differences recorded from 2 mini Motions mounted in the same "aquapac" and worn on the upper arm.
These are not my devices or my logs.
The first pic shows a typical comparison of the two logs. It is hard to see much difference between the two red and blue lines. I have done a 10 point moving average of the data to smooth the graphs. The green line is the difference *10 in knots between the two logs, so typically 0.1 or 0.2 kts. This is what we have come to expect from Motion devices.


The second pic from the same session (and also the fastest 2 sec peak for the session) is below:



This time the red and blue lines are clearly different to each other. The green difference line is at times greater than 1kt. One of the Motions is clearly not measuring the correct speed.
The Blue line device had a lock on 17 satellites throughout the 2 sec peak of 41.276 and reported an error of +- 0.163
The Red line device had a lock on 15 satellites throughout the 2 sec peak of 41.728 and reported an error of +- 0.199

Files sent to Julien

Could the user please redo a comparison without the Aquapac?

Both devices have disappointing accuracy compared to the Australian logs I have on hand. The noise in satellites used is also surprising: it's usually a straight line around 18 if you exclude falls in the water.

One device speed accuracy spikes on this run but also every time he turns.

They are waterproof, they are solid, and if something happened I'd replace it. Don't place anything around it. Worse, don't use anything that could retain water.

I'll gladly investigate further without the Aquapac.

There's a surprising number of you using two. o_O

And Julian, this is why those with your logger are so happy with them and so happy to promote their use... because of your your support for your product and your product's users!
Keep up the good work; we really do appreciate you and your product.

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tbwonder said..

The Blue line device had a lock on 17 satellites throughout the 2 sec peak of 41.276 and reported an error of +- 0.163
The Red line device had a lock on 15 satellites throughout the 2 sec peak of 41.728 and reported an error of +- 0.199

So the sum of the errors is less than the difference in the 2s peak.... should not be possible if the errors were accurate. This is the same magical error parameter that GPSTC devices must have right??

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vosadrian said..So the sum of the errors is less than the difference in the 2s peak.... should not be possible if the errors were accurate. This is the same magical error parameter that GPSTC devices must have right??

User error!

In any case where the the difference is greater than the error, the results should be discarded.

In the scores of side by side Motion (and other model) data I have (every single session I sail) I have never seen such differences. My differences between devices ranges from 0.000 to 0.05. i have a mountain of data on this.

incorrect use can stuff anything up, especially your favourite mickey mouse watch.

Wear it as intended by the designer, as has been clearly explained a number of times in this forum.

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sailquik said..

User error!

I know that the recommended way to wear the device is on the upper arm using the strap provided. However often people are wearing it in other ways. Eg like a watch or in an aquapac or similar, or even stuffing it inside their wetsuit.

My understanding was that if the device was not getting a good view of the satellites, for what ever reason, then this would be reflected in the error data or in the number of satellites being tracked.

Julien you state that you are disappointed with the accuracy and there is noise in the satellites and spikes in the data, however most users do not have the skills to analyze this, they tend to look at the reported error figures which in these two cases are both below 0.2 kts.

If I was using a single device and the reported error was better then +_ 0.2 kts I would be happy to post it.

How can a user identify easily that there are other issues with the file?

The error data comes from the ublox chip, there's nothing Julien can do about that.
I too have noticed differences greater than the combined error data, in my comparisons, which is a bit disappointing. I've also noticed it in locosys SDoP data. So I think the error numbers are a calculation that don't take every single factor into account, but do give a good indication most of the time.

I'm afraid if we want the very best accuracy we have to wear at least two devices.


and in the case above maybe average them?

I recently did a couple sessions with a new GW60 one my wrist and Mini Motion on my bicep if that's a help. The top speeds were extremely close.

Of course, wearing two devices would be the gold standard for verification, and that is what is required for GPSSRC Records. but for the average windsurfer, that is just too hard.

Personally, it has always been my practice to wear at least 2 GNSS devices at all times. I certainly would not want to be in a position of having one fail and not having a record of my session - and sadly, I have seen quite a few cases of just that. It also give me that peace of mind re verification, so I KNOW that my results are real. That is just my personal choice. I know it is not for everyone.

I also wear my best two devices in my Helmet and have proven over hundreds of session that this is the ideal positioning. That is not for everyone either, but I have also proven to myself that two devices worn in a good, stable, waterproof upper arm bag, positioned properly is 99% as good. 99% of sailors can do this quite easily and happily.

I still maintain, that if you have results outside the error margin from two devices worn at the same time, it is because they are not positioned side by side in the same plane and one or the other is compromised.

A lot of research and engineering has gone into optimising the performance of the approved devices, and a lot of testing and comparison has been done to verify their accuracy so we are playing with the best possible starting point. After that, humans inevitable find an infinite number of ways to compromise the data. All we can do is try to educate people to wear and position the devices in the best way to get the designed accuracy.

Wearing under any material that can absorb water, wearing so that the antenna is not pointing directly towards the sky, wearing in any bag that can slip around under your arm or partially fill with water are all things that should be avoided with any GPS device and things that can severely compromise accuracy and reliability.

Every other GNSS device is subject to the same variables created by less than ideal wearing/positioning/signal blocking. The GW-60 9and every other wrist worn device) is a case in point with the under over grip issue. There are some factors we can control and some we can't. If there is a compromise, that is where they are.

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remery said..
I recently did a couple sessions with a new GW60 one my wrist and Mini Motion on my bicep if that's a help. The top speeds were extremely close.

I would like to see the data files to add to my collection of research if you are happy to send them to me.