for a GPS for this purpose if you have a mile course and the GPS is accurate to say 50ft then you have 1% error possible on distance. But the good news for measuring things like skeg effectiveness is you DON'T really need to know the distance at all. If you have a fixed route (say one pier to another) then all you have to do is measure TIME accurately to determine that say a skeg is 2% faster or slower. So measuring is pretty darn accurate -- the problem is now the paddler which will vary by fatique, motivation, etc. For that problem the best you can do is have many trials alternating between skeg or not. The variations in the results will give a clue to how accurate you can declare your final conclusion to be.
Full Size Sail Rig
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