Like duckpaint, I could not calibrate my GNX 20 for speed through the
water using the instructions on the device display. I called
Garmin Support and was told that those instructions are incorrect.
Here is the correct Garmin GNX 20 speed calibration procedure: (Do on a calm day with no currents)
First, go to the speed calibration section of the device menu. ( Menu > Setup > Data Sources > Select Data Source > Select
Depth > Select numeral representing Speed
Sensor
Now ignore instructions on device and follow these:
1> Bring the boat to a steady speed between 4-6 knots.
2> Maintain that speed.
3> Note that speed. (Use a
GPS, such as
Navionics or other
GPS app on your
phone.)
4> Enter the noted speed into the GNX 20.
5> Maintain speed until calibration is complete.
That should do it; your GNX 20 should now report an accurate speed through
water. That will, of course, not always be the same as the speed over ground from the GPS; there will be variances due to
wind, tides and other currents, turbulence around the speed sensor and
hull, etc.