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post #1 of 4 Old 08-24-2009 Thread Starter
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Knot/log calibration

I replaced my Signet knot/log meter and have little luck in calibrating it. I live on the mouth of Lake St. Claire and there is always a strong current entering the Detroit River. Anyone know of an asy way to calibrate it. I have a handheld GPS with a speed in m.p.h. function to compare the knot speed to.
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post #2 of 4 Old 08-25-2009
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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.
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post #3 of 4 Old 08-25-2009
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Is the Signet user manual silent on this topic? Ramarine has a semi-automated method of calibrating the transducer to bias current. I will attach a URL to the .pdf. The relevant section is on page 31, "Speed Calibration".

The process could be simulated with your GPS. Power into the current at a set RPM and note the SOG: turn and return on the same leg and note SOG. Calibrated speed through the water = the resulting average of the two SOG readings and 1/2 the difference is the current offset.

Is the boat Pequod?


Last edited by wwilson; 08-25-2009 at 10:35 AM.
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post #4 of 4 Old 08-29-2009 Thread Starter
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No, It's not Pequod. They have a formula on the paper sheet manual that came with it. Ihe difference in speed of travelling in one direction and then back the other but it calls for little or no current. I might try staying abreast of a fellow sailor and set it up to match the speed on his knotmeter. Thanks for the above mentioned formula, also.

Last edited by ahab211; 08-29-2009 at 11:52 PM.
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