585 prop noise



I have a Bertram 50 sport fish with a 585 sounder and an Airmar b260 transducer with faring
The transducer is mounted aprox even with the transmission on the starboard side close to the centerline.
If I remove the stbd engine from gear the sounder system works perfectly. In gear screen turns totally red with noise above 15 knots no sign of the bottom, at trolling speed I can manually adjust the unit so I can see a trace of the bottom, a red line on a yellow screen. The depth readout does not recognize the bottom with the starboard in gear above 100 feet. This problem is primarly in the 50hz frequency much less in 200hz
Couple of questions for you.
Do you have pictures of how close the ducer is to the keel?
How does the 585 perform when using only the port engine above 15 knots?
Do you see gaps in the bottom on 50Khz.
Do you have an alternator on the starboard engine?

sorry it took so long to get back.
Transducer is aprox 30"-35" off of centerline , 6" inside of prop shaft (36-41" from centerline.
I cant go 15K on 1 engine, at max speed the sounder would be clear though.
Both engines have alternators, reving the starboard engine in neutral does not affect the sounder.
At speed there are no gaps in the red screen at 50khz. there are screen gaps due to waves in shallow water.
Transducer has the airmar highspeed faring
These types of noise issues can somaetimes be a bear to track down. Do you have a grounding brush on the shaft going to the prop? Can you switch the sounder to another power source? Can you totally disable the alternator on the starboard engine? Any dings on the starboard prop?

The alternator has been disabled, no difference, props are clean and no dings, there is no grounding brush . Power comes from the same panel as mfd12 and navnet1 which work fine
I disconnected the generator alternators and replaced the 12v converter ,no help. The problem isnt affected by engine rpm it is prop rpm only. If port engine only is in gear I can rev the stb engine with no affect on the display. The engines are totally mechanical diesels.
I am running out of ideas to try. I am curious if adding shaft grounding brush would help or not. If possible, it would be nice to temporality try another transducer.