Re: Experience with Garmin GMI 10 and related NMEA 2000 sensors?
GPS (Ground Speed) is MUCH more useful. It is your ACTUAL speed. The water speed is how fast the water is under your hull. If you are at anchor and there is a current, you will display a water speed but not a ground speed. If you sail WITH the current, water speed will be less (possibly even negative) than your ground speed. If you sail against the current, you can show a much greater 'speed' than what you are actually going compared to ground.
I have a Garmin 421s Chart-reader and the GWS-10. I love it. I get (ground) speed, direction, air speed, wind direction, depth, AIS info, everything I need. I'm thinking of adding a GMI-10 to off-load some of the display duties so I can see the chart again....