We have been collecting GPS data for the last couple of races and displaying it overlayed on google earth. Very informative. You can see the influence of nearby land, how slow you are out of tacks/mark roundings, general tendencies in wind shifts(not entirely accurate).
If you have a GPS onboard, there are some pretty cheap ones available now with count-down timers etc, you can upload the raw data to
GPS Visualizer and it will process the data and give you a download for a google earth file. You can colour code it to speed for a better representation. Only problem with it I have found is that if there is useless data in your track file, it is difficult to remove in google earth, best to use your GPS software to cull bad data first.
Don't fall into the trap of using your GPS to keep track of your progress during the race, keep your eyes out of the boat and on the wind and water, it is faster. Also, my understanding is that having a GPS onboard is not allowed under the current dinghy racing rules so only use it if you are prepared to be protested and disqualified.
I've added an example screenshot from Google Earth. Magenta is fast, blue is medium, green/yellow is slow. With a few assumptions, you can bisect the tacking/gybing angles to get wind direction and heading angles and being general with the wind speed can give you VMG. Analyse enough of these with different setups and relative headings and you can develop a basic polar plot.
