Powering with cranking or trolling battery.....
Once the new is taken off a cranking battery, whenever you crank, the voltage drops until restored by the outboard alternator. When it drops below the minimum required 520C voltage, the head unit restarts, interrupting trail drawing and of course wiping out the previous view. As outboard motors age, the plug wires go after plugs burn, so keep those up to snuff. Sometimes no matter how well they are maintained they will inject electrical noise into a sonar display.
If you use a trolling battery you can get noise from a trolling motor using DC brushes in the motor. They wear down and spark more as they age.
I put an extra battery in the boat to run the electronics. It serves as a backup cranking/trolling battery. You could run them off one garden tractor battery all day, or two days, something easy to find a space for somewhere in the boat. I was fortunate to be able to squeeze in another full sized trolling battery in the battery compartment. If you have a 24 or 36 volt trolling motor then you already have an extra battery that could be used as a dedicated power source if you don't need the higher trolling power. I'd try just tapping one of them anyway to see whether the motor interferes.
You can get more interference from one 520 to the other than those other sources. In that case I set the console unit to 100% ping rate, and the bow unit to less than 50%. If necessary I tilt the transom transducer slightly to the rear away from the bow sound cone. Since I use a skimmer transducer on the bow I can tilt it forward a degree, not enough to affect depth accuracy.
Jim