Trolling motor manufacturers are now able to produce a more efficient trolling motor relative to power management. In doing so, the motors sometimes produce electrical interference for the depth sounders. If your trolling motor is causing interference with your Humminbird unit, you may need to add the US2 Ferrite Ring. If your trolling motor is a PowerDrive V2 or Terrova model, you can also add the Ferrite Bead to the positive and negative motor output leads on the coil cord closest to the control board. All 2013 Terrova and PowerDrive V2 models will include a factory installed Ferrite Bead.