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.
