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You cannot use a GPS to calibrate a knotmeter, unless there is no current. You might use a Dutch log.
The Dutch Log can be used to calculate boat speed through the water. Position one crewmember on the bow with some floating, organic material; position another crewmember in the cockpit with a stopwatch. The crewmember on the bow tosses the material forward and shouts “mark” when the material passes the bow. The crewmember in the cockpit times the passage from bow to stern. Apply the following formula:
Speed (knots) = 0.6 X boat length (feet) / time (seconds)
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Last edited by jackdale; 08-25-2009 at 12:53 AM.