Knot Meter Calibration

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Postby Guest » Sat Aug 24, 2002 9:52 am

I know relative speed changes are most important, but I'm interested in trying to calibrate my knot meter. I do have a hand-held GPS and have used it as a reference. Readings just feel too fast. Any advise?

Postby Guest » Sat Aug 24, 2002 9:57 am

The user's manual for your knotmeter should detail calibration procedures. This usually involves motoring over an accurately measured distance (often a "measured mile") in an area with no current and then making the necessary adjustments.

Postby Guest » Sat Aug 24, 2002 6:13 pm

Measured mile is the hard way when you hav a handheld. But if you have current (tides) you will need to do runs in several directions.

I have raytheon stuff. It hasd a mode called calibration factor. its just a fractional multiplier of the indicated speed.

I go into calibration mode and raise or lower the CF factor until displayed speed matches (approximately) the GPS. I'll do it in a couple of directions to get a feel of the accuracy.

With mine, switching from fresh to salt water affects it. Other times it just seems to drift. I try to check it everytime I trailer the boat.

J80 290
Ghost Dancing

(BTW, I had a guy in a slip next to me tell me he could consistently pull 8 - 9 knots upwind in his O'day 272. If you want to impress your friends . . .

Postby Guest » Sat Aug 24, 2002 8:05 pm

Good Lord,

Were playing around with these J/80's when an O'day 272 can PLANE UPWIND........

Must have been the skipper of the boat that made that statement cause only someone who lives in fantasyland would say that. My current boat (J/35) can't even do that, boy do I feel stupid.

RS (Hull #544 soon)

Sounds like he should be reading this thread on calibration......

Postby Guest » Sat Aug 24, 2002 8:37 pm

I use the measured mile method with a handheld GPS. While mototing up the lake I set a waypoint, reset my knotlog and motor at a fixed heading for a mile on the knotlog. At that point I read the distance to waypoint indicated on my GPS. If the GPS says 1.16 mi then the correction factor for the log is 1.16.

The last time I did this I also noted that the bearing to waypoint did not match the reciprocal of the course that I had steered, indicating that my fluxgate compass sender was not quite aligned to centerline of the boat. I corrected this electronically (sailcomp).

This was done on a lake so current was not a factor. Probably have to do a reverse run in current to get the real picture.

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