Julius,
On which transducer does this happen: trolling motor or transom?
Does this happen only when the boat is moving, main motor running, over a certain speed or when the trolling motor is running?
If it is the transom mounted transducer doing this, can you post a pictures of how it is mounted? We would need to see an overall picture of your boat’s transom, at least one from the left and one from the right side of the transom (looking across to the opposite side) and all at about the same height as the transducer.
Can you post some Screen Snapshots of what this looks like on the Di sonar and also one of the Si sonar with the Si Range set to 20 feet?