Knot meter calibration

[Mike]

Anyone know the trick to calibrating a knotmeter? I know where to adjust it, but I want it to be accurate. How is it done? Thanks for your help!

 

[Terry Arnold]

calibrating knot meter

With gps so accurate these days, calibration can be done quite easily by simply picking a place and day with little or no current or sea, set the trip on the gps to zero, place the knot meter in calibration mode, motor a distance, (several miles) on the gps, turn around and motor back on your track to starting point, and then adjust the knotmeter measured distance to agree with the gps trip on the gps. By motoring back on your track, compensation for current and different sea conditions is automatically provided.

 

[Bernie LaScala]

Calibrating your knotmeter

Last Saturday, I compared my ST-30 depth/knotmeter reading to my GPS and noticed that it was reading low by about 1 knot. I then eliminated the discrepancy by changing the knotmeter's calibration factor number. This made the knotmeter and GPS agree exactly. In the afternoon, since there was no wind, I became ambitious, donned a mask and fins, and cleaned my entire hull, including transducers and the knotmeter's paddle wheel with a soft scrub brush. Motoring back, I noted that my knotmeter was now reading about 1 knot too high. Moral of the story - clean your paddlewheel before calibrating.

 

[Ron Doescher]

Knotmeter calibration

The other responses were good advice. Just ensure that your GPS is set to display its readings in Nautical (nautical miles; knots), not Statute (statute miles; mph).

 

[Rich Stidger]

Use a Chip Log

I know this is 'low-tech', but I use a chip log. It is a small piece of wood about 1x2x8 inches that is attached to a 1/8" nylon line. The line is about 62' long (1/100 nm). A couple of more feet can be added to allow for the freeboard distance.Tie the line onto your stern or a side stantion (very important).Be sure the line is free to pay out without snags.While moving at a constant speed and course, drop the wood block into the water.Start timing with a watch (stopwatch function is great) upon the 'splash'.Stop timing when the line pulls tight (ie, 1/100 nm has been traveled)Divide 36 by the time recorded to obtain your speed in knots. eg. if the time was 5.5 seconds, 36/5.5 = 6.54 knotsPerform this measurement 3-5 times in succession and average the results to get an accurate speed.Adjust your knotmeter to that same value.Advantages:Low cost, 1% accurate, independent of current, quick check, no GPS neededBe sure your paddlewheel is clean before adjusting the knotmeter per a previous post.