How to measure VMG for your own cat

Hi,

This year is the second year I will be sailing my Blade. Last year I had to get used to the boat (coming from a Nacra 5.0), adjusting mastrotation, downhaul, getting to know the feeling with daggerboards, etc.

After this year I kind of know what is a very bad setting and whats not, concerning rotation, daggerboards, mainsheet tension, .... but now I want to start fine-tuning that. Next to that I also want to start working on tilting the mast and working with prebend.

All of these settings should affect VMG of the boat. (for example measuring what the change in upwind angle is while tilting the mast and what the change in speed is).

My main question is now, how to measure it (I don't want this thread to be about the settings themselves).
I have a Garmin GPSIII which is old but can record where and how fast we're going and I downloaded GPSAR.
Is anyone familiar with that or does anyone measure this in a different way?

Regards,
Gill

not sure what GPSAR is... but I have wondered how to do what youa re talking about also. I think any GPS will show you VMG if you pick a point and "go to" it. Even the one I had in 1996 did this.
My only idea is either to sail to weather and pick a point which can be seen or identified as the point when you are downwind. Mark that on your gps. go ddw and mark a spot there. Then you can have your GPS show your speed and VMG to either of those points.
I have seen some GPS that will actually pick a point in a direction 2 miles away or something if you give it a bearing from where you are now.
I am, in the end, too lazy to do this.

Enclosed you can find an example of what this free program can do: http://www.gruesink.demon.nl/appletGPSAR/gpsar.html http://www.gruesink.demon.nl/appletGPSAR/gpsar.html

When you select play it plays, when you select 'grafieken' and the VMG you can see a result.

The track is from the Zandvoort race btw, but not all of the gps recorded the entire race.

Even if I can get my data in this program I would know how to interpret or compare different results...

Regards,
Gill

Sounds more like you are out to find a polar diagram for the Blade F16? A polar diagram would give you the target speed/angle for any given course. You can generate a polar diagram in gpsar I think, but how do you know you have the boat at its optimum VMG <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />
There was some talk about getting GPS units on the top performers at the GC at Zandvoort, but I guess it never happened? Getting quality data like this, and making it available for members as a membership advantage would be something new and very valuable I think! It removes the uncertainty of going to an event and finding that you are totally off pace. It also makes it easier to experiment like you want to do.

It is a little tricky to measure VMG even with a GPS and GPS Action Replay, without some nice tools that record and correlate wind speed and angle, GPS, and boat speed data. Leeway angle, current/tide and wind shifts will make GPS data a bit inaccurate.

However if you are aware of its limitation, you can still get useful information out of it.

A very basic polar plot (atleast with the upwind and downwind VMG data) can be generated with GPSAR, but you need to clean the data (track) up a lot first. Take out all the tacks and gybes, the pre-start, and any other stuff ups. Then you need to pick a "median" wind direction that "balances" both sides of the produced polar plot.

The problem you then have is correlating the data between races, and even during a race, because we all know that the wind speed changes during a race, hence the boat speed will change and make the polar plot even less useful.

With all this in mind, take the GPSAR polar plot with a pinch of salt.

An alternative use of a GPS track is to put it into Google Earth and see how your track changes at various locations around the course. For example, seeing how land masses affect your track (variations can be caused by either wind and/or current and/or wave affects).

The "VMG" features on some non-sailing GPS units are of little use to sailing. They only give you "VMG" to a point (the one that is marked), which will change between tacks/gybes if you are far from an imaginary line that is drawn through the mark and is inline with the wind. Boat (real) VMG is measured against an imaginary line that is only drawn inline with the wind, so it doesn't matter where on the course you are, it will be the same before and after you tack/gybe (if your keep the same angle to the wind and same boat speed before and after).

Imagine you are approaching the layline with the "VMG to a point" selected on your unit and the point is the top mark. The closer you get to the layline, the slower your VMG to that point will get (the mark is getting closer to abeam (90 degrees) of your direction of travel). The instant you tack onto the layline, your VMG will equal your boatspeed exactly, which is of little use to us.

Agree with Nick. Makes you wonder how reliable polar diagrams the "big boaters" are touting really are <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I really think we should do an effort to get an GPS on board the top boats at the GC. Would be great in so many ways for the class.

I used to sail with GPS all the time and read the plots but stopped doing it because the information was quite meaningless.
The only interesting information was the speed of my tacks and gybes (fullspeed to fullspeed time), and the tacking angles during various conditions.
IMHO a GPS is also too slow to use it as a realtime speedometer. Best thing to improve speed is with two boat testing.

Agree that a GPS is not perfect, but it will help you with the "whoops, we are totally off the pace" syndrome when you go to an event. Not everybody can go two boat testing, so they need something else to help them measure their efforts. I know, becouse we went the solo route on the Tornado. Very slow progress and always uncertain wether we were competitive or not when we got to an event. Having something to measure with would at least have given us an idea. We did compare our plots with Mike Dobbs on the Tornado, and it was very helpful for our own boatspeed/VMG and for strategic analysis. Since the F16 class is spread around the globe, many sailing alone, gpsar and GPS would in my opinion be helpful.

I did have a good look at Gills tracking at Zandvoort and was suprised how often Matts course changed whilst running back up the course towards Zandvoort compared to Gills. Then I remembered Gill was flying a kite and I beleive Matt had a problem and thus sailed the long run under main and jib alone.
I would also agree that GPS isn't that useful a tool when it comes down to a training aid...meaning it's best used for navigational purposes but the fact that it can re-call certain data as described above can help understand where gains and losses have been made but only by comparison.

vmg as displayed on the velocitek gps unit is very usufull and as a training aid when sailing alone essential as theres no other way to physically assess your performance on the water , it also clearly tells you what angles pay downwind and wether its worth trapping when solo or sailing deeper , simple speed function is more usefull than you may think too as you can come off the pace without noticing , say a 10% diff when beating at 10 knotts is hard to tell by human analogue , new velocitek gps have twin displays and can also show compass headings too , and at the end of the day its a gadget !!!!

Could be better with integral mp3 player and fek orf speakers <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I have to agree with using the Velocitek units. They've been great through the years. See their ad on Sailing Anarchy. Ed

okay, so where (and how) do you mount your Velocitek (pics please) so that you can see it on both tacks without manipulating the unit?

I have the older (S-10) unit that mounts on the boom with bar clamps. I don't think that unit is available anymore. The best way to mount the new unit would be the same as mounting a Tac Tic, on the spin pole about a foot or so in front of the mast. I don't know if they have the mount or if you would find that elsewhere. I would look into it more but if I go to their web site one more time I know I'll break down and buy the new model. Ed

Rolf,

I agree we should get some GPS available on the next GC.
Does anyone subscribed at the moment has a GPS that can be used for tracking?

1 Mark Pressdee
2 Hans Klok
3 Simon Longstaff
4 Gill De Bruyne & Kathleen Vandenbulcke -> YES
5 John & Rob Alani
6 Paul Warren & Ann Powter
7 Geert Ruesink & Joanna Lienti -> YES
8 Nick Moore
9 Gary Maskiell
10 John Terry

I guess if you make a polar diagram everytime you sail, you will get a chart that is more or less reliable after some time?
And using that chart I guess you can calculate the best angle to sail upwind or downwind?

But the most interesting about a GPS is comparing the same race.

Regards,
Gill

If you measure every time you sail, you will get a better polar diagram for your skill, not neccessarily for how fast you could go <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />


If you lack GPS units at the GC08, I can lend out my old Garmin eTrex. As long as I get it back when the event is over, no worries.

I also have a spare GPS data logger that I'd be happy to lend out.

Paul

My S10 velcros to a bracket mounted to the underside of the boom. It pivots from side to side, and I just pull gently on it as I cross the tramp. Works well.

Dave

I used the Velocitek device for training in Singapore before the A-Cat worlds. Since I was always out by myself needed to be able to guage what my speeds were and what were appropriate targets. Built myself a fair picture of what to expect.

For example, when I really should be on the wire and when not. But also when to go wild and when to stay flat.

Used the unit at the worlds and based on my training and utilisation of the my own developed "polars", was on the pace.

Thoroughly recommend the Velocitek. You will find it very useful for downwind with the kite. Eg, when to have the crew on the lowside, on the highside, and when on the wire. In my experience I see a lot of boats try and heat it up with the crew weight not in the right place. Eg having crew on the low side and skipper sitting in middle of tramp will get the hull out of the air early and you will be able to go downwind at the same speed as the other boats but at a deeper angle, thus achieving better VMG in lighter marginal conditions. Did this many times in Singapore and would smoke everyone.

I personally believe that that little jave applet we saw earlier is the main attraction. It is sort of a poor mans virtual spectator. Afterall halve the benefit of a global Challenge is the promo value that comes with it. Being able to follow the races second by second using GPS tracks is a huge boost to promo.

I can lend mij Garmin Geko GPS logger to Hans Klok if he doesn't have one.

Also you guys much remember to bring plenty of battery chargers and a splitter so you can devide the power output of a single walloutlet into bank of battery chargers !

Wouter

those with velociteks certainly have a good grasp of what the velocitek vmg function can do for you above the other gps varients ,they give you a realtime blow by blow account of what is and what isn't good relative to the race course and conditions on the day.

Polars whilst giving target speeds and angles cant take into account differing conditions ie wave patterns , tide , wind , shear and any other variable , polars are at best average performance targets based on historical data . Velocitek VMG is realtime and constantly updated and is the only thing you need to look at and whats more ITS DEAD SIMPLE .

There are ways of setting up vmg on other gps devices , but for a whole host of reasons this info can be totally irrelevant and the setting up impractical on the water .

What you guys need to consider is wether you allow VMG calculating GPS as Velocitek to be used on the race course or do you want to use data logging for later playback.

I have used many GPS units (garmin foretrex 201 - relegated to my bike, Garmin various handhelds and raytheon plotter on the cruiser ) and have found the Velocitek with its VMG function to be the most performance advantageous of the lot , infact I now have the S1 which has dual displays which you can set as you like , compass (in tiktak format or true ) , VMG , true speed .

speed alone is better than nothing as it help keep you on the pace

I think you will all agree that Rohan Veal has made good use of his sailing with Velocitek.

As for mounting , I've made a simple rotating bracket mounted on my spi pole with lines taken to my rotation spanner so it self tacks , prototype worked a treat from first day .

I definately think we should not use GPS units while racing. Just for logging and later analyzis. The game is racing, and a compass is all we should use, the rest is skill and experience. Having VMG, even with its limitations, available removes part of what I think makes the game interesting.

We had the A Cat north american championships at our club last year and one of the guys who is a sales rep for Velocitek (anyone can be) had 6 boats run the units during racing. At the bar later he was able to run all six tracks at the same time showing the race as it eveolved. Very cool. For the GC08 you might want to approach Velocitek and see what you can work out. Maybe all boats carry some Velocitek advertising in trade for them supplying a certain number of units. Possibly work out a reduced price for regatta participants to purchase units. The possibilities are endless, or they might just say no. Of course when we did it the units were half of what they cost now. Oh well, worth a shot? Ed

I think you can do that with any GPS, right, using GPSAR?

Indeed,

Every GPS able of tracking can be used in GPSAR and can do playback.

Is there any rule on GPS/VMG units during racing in any racing rules? (F16, Tornado, F18, ...).

Gill

I'm might be wrong but in the UK GPS and electronic compasses are a no-no for the A Classes but digital watches are OK. They are anti-gadgets because they'd like to believe in pure sailing.

Tornado class also limits you to an electronic compass + emergency communication (not to be used except in emergency or before/after racing).

Velocitek devices are OK to use in the A-Class and other classes unless the class specifies differently.

The A-Class Association clarified this last year and published on their website. The text is:

"Dear Piet,
As far as I am aware there is no general Racing Rules of Sailing prohibition of electronic equipment including GPS.
If a class association wishes to control the permitted or prohibited equipment then these should be specified in the class rules.
Regards
Simon Forbes"

The linky is:

http://www.a-cat.org/id113.htm

I think these devices are great and shouldn't be limited.

I like these words written by William Arthur Ward:
THE PESSIMIST COMPLAINS ABOUT THE WIND;
THE OPTIMIST EXPECTS THE WIND TO CHANGE;
THE REALIST ADJUSTS THE SAIL

Mark,

any chance of getting GPS units onboard to capture data on the top four or five boats every day at the GC-08? It is an extra effort, but it would be very good to have those logs next winter <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

If it would be seen as an unfair advantage to have these units on some boats but not others the display can be taped off. As long as the unit is on the boat and turned on it is archiving the data for display later. Ther is some other outfit that will put their "bug" on each boat and collect all of the data from a regatta. I think this was done for the Key West Race Week. Sound a bit more involved but may be worth checking into. I think they give you a finished package that can be sent around ekectronically.

One thing about the Velocitek verses a regular GPS is that I think the refresh rate is faster and as someone said just the speedometer alone was worth it, it is for me.

Ed

hi

GPS unit now are very cheap, so why band them, if they help you go faster. Most f16 sailors are unable to get two boat training and these tools make up for this.

I do not worry about if the next boat has a gps.
I worry that the helm&crew are lighter then me and the sails are newer then mine, and the boats in better condition then mine.

John,

I dont think anybody suggested banning the use of GPS units for training or similar. But at a largish regatta it might be sensible to have a rule on GPS units and similar to make the competition be about sailing and skill. The tape idea came up becouse a couple of us would dearly like to see some GPS plots from the top boats at Mumbles GC'08. If we agree to race without GPS units, taping the display is just a way to not give anybody an advantage (which they dont need anyway, since they are in the top <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" /> ).

Rules are rules, if there would be a ballot on this issue I would never vote for it.

Basically I believe that banning GPS/log devices would hurt inexperienced sailor most because there is one less tool to be used to observe how your trim changes affect your speed when driving straight line. More experienced sailors don't need these as much because they know in advance how trim should be changed, also more experienced sailors tend to have better feel on their boat and speed. Bottom line is that knowing your speed is most important thing when developing your sailing skills and these devices cost lot less than a good coach, rib and a video camera <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />.

In a race situation a beginner would have a hard time using the GPS in a meaningful way in my opinion. When surrounded by other boats they are the measurement on how your speed is. Having a GPS then is not really useful for a beginner in my opinion as he has more than enough to do with keeping track on what is happening around him and trying to get his boat going. A really experienced sailor would as you say not need the GPS to keep track of his speed, but for determining VMG and deciding on shifts, favoured sides etc. it could be useful. A beginner would not be in a position to really take advantage of this as he simply dont have the time to concentrate on this.

We have trained rather a lot with GPS units to try and find best trim. We have also raced with it in distance races but then mainly to find the fixed marks. Workes incredibly well in that configuration.

My personal opinion is that racing is a game and a large part of that game is determining shifts, favoured side, best trim, best VMG etc. etc. Sailing with a GPS removes some parts of the game which I wold rather keep. For training, I find a GPS useful, but a coach is infinitely better!

Speed is only one tool for following how you do, good part with it is that it's still there when others have pulled away and you are sailing behind. It doesn't take lot of time to look at it sometimes and combine this knowledge with how others are doing around you then it starts to be more useful. This data can be used for finding better trim between races which I see quite important for inexperienced sailors like me.

Anyway I understand your points on VMG and shifts but I still don't think that they are that important for good sailors mostly because sifts can be detected with land marks or compass and the VMG is only needed for sailors that don't know how their boat should feel.

The Velocitek may be able to help you pick shifts, but it won't take the shift into account when calculating the VMG. You have to do that manually by pressing buttons.

So with a shifting breeze, which is almost always the case, even if the wind strength stays steady and your boat speed stays the same, the indicated VMG will change, while the actual VMG stays constant.

What do you guys think about this thing?
I dont think that any device on the market has more features.
At €550 its not exactly cheap, but on the other hand a separate GPS and Tacktick adds up to the same amount.

http://www.nauteek.com/EN/index.php?page=3&prod=2

Hi guys, another cat n00b comes looking for knowledge...and hopefully another hull sometime soon.

After reading the discussion of measuring VMG vs. plotting polars, one would think those to be apples and oranges: VMG is measured with respect to a fixed point such as a mark, while in a polar we need to know course vs. that fickle wind direction. To plot decent polars, one should measure the direction and speed of the wind coming at the vessel. Wireless wind buoy? Seems insane, but the components would cost less than that Nauteek thing. Dealing with the difference in wind speed and direction between a buoy and a boat would be a problem though.

A question is, what would be more valuable in improving one's technique: Ordinary GPS showing VMG to a given waypoint, or a comparison of one's boatspeed to a polar?

VMG much better. Polars are better for yachts, and due to the masses of lead they drag around can be sailed close to their max and it is harder to drop away from targets.

Cats on the other hand can accelerate/decelerate due to a multitude of factors. Training with the Velocitek it is normal to see a +/-1.5 knot difference in boat speed. Yacht would be +/-0.2 knot from polars.

This is prob not technical enough for many of those on this forum, but just my opinion from sailing various types of boats and using various types of devices.

VMG in terms of sailing isn't measured to a single mark, that is just how some non-sailing GPS devices define it.

Even the very first popular sailing GPS, the Velocitek, required two points to be marked that defined a wind angle. One point directly to windward of the other. It didn't matter where these points were marked, as long as they reasonably defined the wind direction. If the wind direction changed, you'd have to mark two more points that defined the new wind direction. The old devices required you to sail to two points and press a button to mark it, but now the latest device only requires you to dial a wind direction into it, no point marking required.

VMG is extremely closely tied to polar plots and wind direction.

This is the best diagrammatic definition of VMG I can find at the moment...
Velocitek VMG

You can easily calculate VMG from a polar plot, just draw a horizontal line from the very top peak and bottom peak of the curve across to the vertical axis. Where they cross are the upwind and downwind VMG targets. Go back to the very top and bottom peaks and read off the actual boat speed to find what most ppl call the target speed, the actual speed you try to sail at to maintain optimum VMG.

Hmm, thanks ncik, that's a good diagram and explanation, and with a windsurfer in it. Does this mean sailors rely on windsurfers for technical instruction and advances, as we always suspected? (Note: JUST KIDDING!)

The Velocitek diagram assumes that the windward and leeward marks are lined up with wind direction, thus ignoring the shifts. I still see a difference between two things:

1. VMG as measured with a GPS only, with respect to fixed point(s). Here, how you play the shifts is critically important, often more so than boatspeed.

2. Velocity with respect to actual, changing wind direction. This would take out the effect of wind shifts.

I suspect that training for #1 would make one a better tactician, and #2 a better trimmer. I further suspect that most experienced sailors know quite well the significance of both these measures but don't really care for describing them mathematically. However, if one is interested in perhaps designing a system to improve training, relating the experience to some math is necessary. Your help in linking the math[s] (my thing) to what matters on the water (just learning) is appreciated.

Mathematically, VMG is the vector component of your boat speed that is directly into the wind.

Velocity Made Good = Boat Speed * cos(True Wind Angle relative to your boats direction of travel)

VMG should not be mixed up with shifts and other stuff, it is just a measure of how fast you are sailing upwind or downwind. The words "upwind" and "downwind" are critical to understand, marks of the course are not important to VMG calculation, shifts are not important to VMG calculations.

1. VMG shouldn't be related to fixed points, it isn't necessary and only confuses the issue. Boatspeed and playing the shifts are both important. 99% of the time you should be sailing as fast as possible, ie. max VMG.

2. That is what VMG is useful for, it ignores windshifts.

It is interesting that generally with cats that have tacking angles higher than 90 degrees, you have to sail away from the mark you are trying to make when getting close to the layline.

Try drawing a diagram, it makes understanding VMG much easier.

OK, got it. It was really just a matter of definition.

Now back to Gilo's original question: How does one best measure VMG? Unless the wind is unusually steady in both direction and speed and there's little current, GPS based measurements might not be very good for trimming. They'll mostly tell you how well you're playing the shifts -- valuable information in its own right, but not the poster's original intent.

This kind of leaves those who have to train with a lone boat in a difficult spot. To know VMG accurately, you have to know both vessel velocity and wind velocity with respect to the Earth...at the position of the vessel. Hmmm...

GPS based VMG can still be "useful" but you need to know its limitations and pitfalls.

It becomes more useful the longer you use it. Keeping a sailing diary with all the usual things and adding GPS tracks will help with any training.

There's more than just polars that can be obtained from GPS data.

It gets even harder when you get to measuring instantaneous wind angle because of wind shear, mast rotation (if you have a mast mounted wind sensor), pitching, etc...

A previous thread

...and another...

Hey, those other threads are interesting. I'll enclose an old GPS plot just for fun -- I did it on a windsurfer back in 2000, the plots with a Matlab script, overlaid on aerial photos. Hopefully someone will find it amusing. Orange is about 25 kts, BTW. The associated polar plot is scattered all over the place, of course, because the wind direction changed so much from one end of the trip to the other.

Anyway, all this discussion has gotten some ideas going; I'll try to do some research and experiments with wind calibration this (Northern) Summer. Thanks for the links and tips.

ncik,

In the second thread you refer to you have a polar plot from gpsar. Is that plot formed with data from multiple days or just from one track? (I don't know if multiple tracks are possible).

And if I would go out sailing 6 hours with a gps, would a plot indicate more or less the correct VMG? (if the main wind direction doesn't shift).
Would it be better to sail a normal racecourse for 6 hours or would it be better to just sail around all kinds of directions to measure VMG?

Regards,
Gill

As a tool to evaluate new equipment or try new rig settings, VMG can be very usefull. The Velocitek units are great because they have a simple VMG readout. It is not exact as the wind often is variable, but it provides instant readings as opposed to looking at plots later on.

As a race tool I find the use of a GPS a big detriment. It is just 1 more thing keeping my head from being in the race and stuck in the boat. VMG does not tell you the favored side of the course, if the wind is variable, the readings are misleading, and it definitely does not help you be tactical with respect to your competition.

A lot of classes have restrictions in their rules about extra electronic or measuring gear. Some only allow the use of a simple compass, while others are even more restrictive and only a countdown watch is allowed. I am not necesarily in favor of banning anything as I feel they are not a help anyway, but it is something to consider in the championship rules.

Matt

That polar was generated from GPS data collected in one afternoon of racing, but can't remember if it was one or two races. It took a while to get something that looked "clean"; tacks and gybes were removed, pre-start and post-finish was removed, messy bits were removed. Then I adjusted the wind angle to "balance" both sides of the plot.

I've thought about how to generate a cleaner polar plot from GPS data and came up with the following method.

- Pick a time close to high or low tide.
- Head out to a big open area with a fairly steady breeze and conditions.
- Go head to wind and measure the wind speed.
- Start recording with the GPS (or use this point as the beginning of you polar generation "run")
- Bear away and get up to speed on a close hauled pinching course.
- Bear away again, reasonably steadily, and work hard to obtain your maximum speed as you're bearing away.
- Keep bearing away until you gybe, then round back up again to close hauled pinching.
- Go head to wind and measure the wind speed again. (don't worry about direction too much, just get it from the plot when you balance it)
- This should take about 2-5 mins depending on conditions.
- Use only this loop to generate the polar.
- Take more time when coming up from a reach to close hauled because you will bring "latent" apparent wind speed with you.
- Setting the kite will change this procedure a bit.
- Do it again, maybe in the other direction so it is more of an experimental process and is repeatable.
- Change some settings and repeat, including measuring the wind speed before and after the loops.

When it comes time to process the data on the computer you will easily see the times you measured the wind speed, set the kite, etc.

That is probably a better methodology than just collecting heaps of GPS race data.

Yes, the Nauteek SC200 looks a lot better than the Velocitek S1. It's more money...but seems to have better on the water functions and display modes, better waterproofness (not in a separate box) and easier to use buttons with menus. It determines true upwind direction (TUD) by dividing your tacking angles or you can enter a compass heading.

There's a new firmware out that allows waypoints & routes...so it now becomes useful for distance racing. Also has a man overboard mode to help you get back to the same place you dropped your crew :-)

Backlighting is a nice feature as well.

Seems to be under $600 in the US from a couple of distributors.

Gill,

Checking VMG is probably not really interesting, during a regatta, like Matt said already. Of course the GPS doesn't know the changes in wind direction.
You would need a wind indicator on you boat and feed that information to the GPS. That could make it accurate and is what they do on the "big" boats. Still waiting for someone to do this for a beach cat, and affordable <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Ncik’s way of getting VMG is quite good, I think, but will take some time.
An other thing we can do is get a track, pick a part with just one tack and 2 legs , with as little changes in direction as possible. GpsAR should calculate the right VMG. Then take the average VMG of both tacks and you should be very close.(if there is no current)

GpsAR takes an average wind direction for the complete track, so VMG can be a fair bit off when the wind shifts during a regatta. So better not take the average of a complete track.

When someone is interested, I’ll take an extra GPS with me to Mumbles.

And while we’re at it, I wouldn’t be too happy if a GPS would be banned. In my case (Garmin foretrex 101) it’s also my starting watch and that works pretty well. And the GPS won’t help you winning a regatta anyway. Just nice for analyzing what happened, after the race and for training purposes.

Geert
Blade 9

...and seeing how fast you can go flat out!