Saturday, November18, 2017 L-36.com

Knot Meter Calibration



Introduction

In many area calibrating a Knot Meter is a simple matter of setting it to read what the GPS reads. But if your boat is in areas where there are tides, local current can make this method inaccurate. The following method can be used to calibrate a knot meter even if there are very strong currents.

Calibration Procedure

The steps to calibrate your knot meter were given to my by my friend John Hughes. These steps assume you have a motor and a GPS. Here they are:
  1. Pick a nice calm day, no wind. The wind will interfere with your measurements because it will push your boat.
  2. Travel into the current (or against it). The faster the better. Let your boat get up to speed. Mine takes 1 minute.
  3. Note the GPS reading and set the knot meter to equal the GPS reading.
  4. Reverse course which is best done by starting your turn in one direction and going hard over the other way when you are half the distance it take you to pivot. When you are done, you should be traveling down your bubbles.
  5. Note the GPS reading and the knot meter reading. If you have current, they will be different.
  6. Set the Knot Meter to half the difference between the GPS reading and the Knot Meter reading. In other words, if the two readings are off by 2.6 knots, change the knot meter reading by 1.3 knots toward the GPS reading.
  7. Make note of the remaining difference which in the above example should be 1.3 knots.
  8. Reverse course again.
  9. Note the GPS reading and the knot meter reading.
  10. The difference should be the same but of opposite sign as it was in step 7
  11. You are now calibrated.

Example

  1. You are going 6 knots by the GPS and the Knot Meter says 5 knots. You think there is a 1 knot current.
  2. Set the Knot Meter to 6 knots.
  3. Reverse Course
  4. Your knot meter still says 6 knots because your engine speed is the same and there is no wind.
  5. Your GPS, however, says 4 knots.
  6. This is a difference of 2 knots. Half of that is 1 knot. Your actual speed is 5 knots.
  7. Your knot meter is now high so decrease it by 1 knot to 5 knots.
  8. Well, it was correct to begin with but it is still correct and now you know it is.

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Disclaimer:
The information on this web site has not been checked for accuracy. It is for entertainment purposes only and should be independently verified before using for any other reason. There are five sources. 1) Documents and manuals from a variety of sources. These have not been checked for accuracy and in many cases have not even been read by anyone associated with L-36.com. I have no idea of they are useful or accurate, I leave that to the reader. 2) Articles others have written and submitted. If you have questions on these, please contact the author. 3) Articles that represent my personal opinions. These are intended to promote thought and for entertainment. These are not intended to be fact, they are my opinions. 4) Small programs that generate result presented on a web page. Like any computer program, these may and in some cases do have errors. Almost all of these also make simplifying assumptions so they are not totally accurate even if there are no errors. Please verify all results. 5) Weather information is from numerious of sources and is presented automatically. It is not checked for accuracy either by anyone at L-36.com or by the source which is typically the US Government. See the NOAA web site for their disclaimer. Finally, tide and current data on this site is from 2007 and 2008 data bases, which may contain even older data. Changes in harbors due to building or dredging change tides and currents and for that reason many of the locations presented are no longer supported by newer data bases. For example, there is very little tidal current data in newer data bases so current data is likely wrong to some extent. This data is NOT FOR NAVIGATION. See the XTide disclaimer for details. In addition, tide and current are influenced by storms, river flow, and other factors beyond the ability of any predictive program.