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!