I am installing a Garmin 3010C chart plotter on a new boat. It uses a
Garmin GSD20 black box to show depth & fish. The GSD20's transducer
has a 50 kHz & 200 kHz signal.

I want to have a separate depth sounder that is independent of the
Garmin. I have been told that two transducers on the same frequency
will interfere with each other. I have searched in vain for a
depthsounder that uses other than 50 or 200 kHz. I did find an Airmar
Smart Sensor that uses 170 kHz or 235 kHz, but it sends its signal to a
plotter or radar screen. I want a simple gauge on the dash as a
backup.

1.Does anyone know of a depthsounder that uses other than 50 or 200
kHz?

2. Has anyone run two transducers on the same frequency with success?
Mine will be about 12 feet apart on the transoms of an aluminum
catamaran.