It seems to me that in order to establish a true speed the boat needs to be sailed in opposite directions, to adjust for current. I just bought a Garmin GPS76MAP (in the mail). I suspect that on my home waters, the Hudson River, I can get very fast speeds (over ground), because current is always an issue on this river. The indiginous people called it the river that runs in both directions. Now that I think of it, sailing in opposite directions would be a good way to measure current speed. I'm not an expert, hell I'm not even a novice yet, but I guess the current speed would be 1/2 of the difference between the boat's speed (per GPS) in each direction. Rick Langer M15 #337