Hi Mary, I haven't logged on this week until today. This has turned into quite a thread!
My idea, being the lazy type, is to measure the distance between 2 existing marks, and then have a stopwatch (or two) on the boat. Using the gps to measure the distance between the marks would seem to be the most accurate method, and then once the co-ordinates are posted here we can have 20 pages of arguments about the calculations
I refuse to believe that I can't average more than 20mph over a mile or so on a reach, but the data won't lie I guess. Maybe it just "feels" like 30 mph?
Al