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post #19 of Old 10-02-2012
GeorgeB
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Re: Any Ideas

Easy, when you say that the GPS displayed the 10.1 kts max speed was held for twenty minutes –does that mean your boat was traveling at a constant 10.1 kts for that period of time? Or, was that the max recorded speed? If the latter, what was the new max speed after the twenty minutes? I know that on my GPS (both Garmin and Raymarine), I can adjust the sample rate which the unit uses to do the distance and rate calculations. And I know that the relative slow sailing speeds, it is easy to introduce an error with a fast sampling rate. Have you had the opportunity to download the stored memory into your computer for analysis? It is pretty illuminating. The algorithm is using the distance between two points of only a few feet to calculate velocity. It is amazing how much this data fluctuates and it is only after averaging over longer periods of time does the unit achieve it’s high degree of accuracy. On faster moving vehicles like planes and autos, the relative motion type errors become small and inconsequential. On my lowly Catalina, I regularly exceed 10 kts and I occasionally get up to 15. In fact, my max walking pace (as recorded by my Garmin Map 12) can get up to 12 MPH (but I must confess, to do it, I swing my arms while holding the GPS). I still say that your average speed over the four hour trip was admirable and your boat is the opposite of being a slug. But if you are sure you are getting those kinds of speeds over time, you ought to be racing because with your PHRF rating you would pretty much win every race you would enter.
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