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.