Depth Finder Delima

[bwkre 11]

So this is what I have. 14' deep wide aluminum boat with an Evinrude 9.9 hp. I have an Eagle Cuda 300 with a Skimmer transducer. The transducer is transom mounted on the port side. I has worked great for the last two seasons and still works great with the gas motor. Enter the Shakespeare Glider 55 Trolling Motor. I fabricated an aluminum mount to bring the transom level on the starboard side and mounted the trolling motor on it. When I use the trolling motor the depth finder goes crazy. Erattic depths and fish locations. At first I thought it was EMI and / or RFI. But after adding several ferrtie torid coils to the motor and depth finder and using an isolated battery I find it hard to believe it is electronic noise. Monitoring the AM radio band showed very little RFI and a scope on the power wires showed next to nothing for EMI. That to me confirms it is not interference from the electronic pwm circuit in the trolling motor.

One thing I did notice was that the interference disappeared when the trolling motor was out of the water and turned on. It got me to thinking about turbulence in the water. I found that the Evinrude prop turns clockwise and the trolling motor turns counter clockwise. It appears that the trolling motor might just be creating greater turbulence on the port side of the prop toward the transducer. I did notice changes in depth finder operation when the trolling motor was turned to correct drifting, etc. I just never dawned on me until a couple days ago.

My next experiment is to relocate the transducer to the bow ( temporary ) to see if the interference disappears.

Has anyone ever heard or experienced the effect of prop rotation on a transducer?

Or does anyone have any suggestions.

I'm in the process of fabricating an aluminum mount to try moving the trolling motor to the front just off centre of the bow. I'm just not sure if I want to leave it there permanently.

Any help would be appreciated.