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