Garmin 240 puts snow on the humminbird screen. Turn 240 off Humminbird clears up. Why don't the Humminbird interefere with the Garminc 240?

With the Garmin 240 off, crank the big engine up, the snow appears.

All electronics are on an electronics only battery.

I think there is noise coupled in the wiring from the batteries/transducer/etc. to the cockpit because of wires being parallel in the cable channel.

Impossible to separate wires via the cable route.

Rule of thumb. Signal wires should not run more than 12 inches in a parallel path to power wires and should be separated by at least 6 inches.

Any suggestions?

By the way folks, in electronics there are all sorts of thumb rules.