Garmin GMI 20

[Alphadug]

Hi All, this may be an old topic. Does anyone know how to calibrate the water speed on a Garmin GMI 20 unit? Mine is networked to the GPSMAP820 and all the other instruments. Can't find any info on the Garmin site or in the meagre user manual. At the moment it reads about 2 knots high. Looks nice but is obviously wrong as I'm sure my Vision 42 isn't/can't do 9.5 at 2500 RPM!
Thanks, Doug

[tiger79]

Download the manual for the GPS820, and look for the section on calibrating the water speed.

[Alphadug]

Thanks, last place I would have looked. It's buried at the bottom of a page. Hopefully it works. I will try it on our next outing and let you know.

Thanks again,
Doug

[Alphadug]

The GPS unit doesn't have a speed calibration choice. I can calibrate the water temp though! Anyone have any other ideas? My speedo reads 2 knots high, boat goes really fast now:)

Thanks,
Doug

[Symphony]

Don't think you can calibrate GPS speed. It is derived from the time the boat takes to move from one GPS derived position on the ground to another. It gives you speed over the ground. The log speed is derived from the movement of the electromechanical impeller through the water. Calibration of this is adjusting the way the software reads the signals from the impeller.  The two "speeds" are only the same if the water is static in relation to the ground, that is there is no current moving the water in relation to the ground.

Some argue that speed through the water is of limited use for cruising, but (if it is accurate) it allows you to get a good idea of the strength of the current (if any) that is affecting your boat when compared with the speed over ground. It also helps you judge the impact of sail trimming for example on the performance of the boat.

I largely ignore log speed now on my Garmin, partly because I also have difficulty in calibrating it and partly because the impeller gets easily fouled. My sailing is largely along the coast where tides can be significant, but are very predictable so passage planning concentrates on going with the tide. Speed over the ground (GPS) gives a good measure of progress and I can judge how well the boat is sailing (or motoring) without the log speed through the water.