My two cents on sonar interference:
What to look for when experiencing sonar interference issues:
•Use good quality resistor type spark plugs with shielded spark plug wires
•Air under transducer (bubbles, prop wash, turbulence, etc.) – avoid strake and chine areas
•Transducers mounted too close together
•Transducer cables bundled too close to nearby power cables (coupling and crosstalk)
•Poor grounding of electronics (this is a biggie!)
•Clean battery terminals and contacts, recommend dielectric grease to ensure good contact and prevent oxidation
•Good idea to run dedicated power to all your individual electronics, and not share power off the same branch circuit
•Always best to power an electric trolling motor using its own dedicated battery/batteries
•Excess transducer cabling (keep away from alternators and other sources of EMI)
•Bundle excess cables in a “Figure 8” instead of creating big loops of cable
•Low battery/alternator voltage will cause LOTS of problems!
•Interference from multiple transducers using the same frequency
•Inspect sonar transducers and cables for damage (e.g., debris, cracking, zip ties, etc.)
•Underwater LED lighting can sometimes interfere with sonar
•Sonar settings:
Traveling at high speeds through the water compresses sonar information. Recommend increasing the scroll speed on your display to stretch out the sonar returns for a better picture. Just be sure to revert your settings when you slow down for trolling as too high of a ping rate will blur out returns.
Increase gain, but not too much that you start amplifying clutter
Increase “Ping” rate – this effectively increases beam coverage in the water
Adjust sensitivity every day to optimize the water conditions for the day
All that said, I would avoid using a ferrite core on the transducer cable, as this would likely filter out some of the high-speed sonar data streaming from the transducer to the display unit, resulting in an additional problem. Rather, you'd be much better off placing that ferrite core on the incoming power cable to the sonar display unit.
Hope this helps.