I am surprised you have not received any replies yet - this place is full of radio
/ electronic Wallas.
Anyway I will start the ball rolling even through I am not familar with the actual unit you are having the problem with:
Try to ascertain where the RFI is being generated - motor
, electronic controls, etc. Then try to find out what is radiating the RFI - the dc power leads, the actual wiring
from the motor
, something in control board (microprocessor etc).
of thumb - TRY to surpress the RFI at its source rather than try to stop it getting into the receivers (or whatever).
of thumb - try the easy / cheap
I am not sure of your technical level of experience but maybe you can try some of the following random ideas and just see happens:
Connect the unit to a COMPLETELY fully charged seperate battery
with very short leads - note what happens.
Fit a 3 wire RFI filter into the power lead (in series with the unit).
Temporary enclose the whole unit with plenty of Alfoil and ground that out with a short lead to the boats main earth system.
Probe around the power and any other accessibile wiring
with a digitial AC volt meter selected to the millivolts and you MIGHT get some clues.
If I thought the motor was the source, I would be pulling it apart and fitting some caps (with very short legs) across the brushes
If I thought the electronic controls were the problem, maybe try fitting ferrite beads on all the wiring in and out of the circuit board or enclosing the board in some metal container (again grounded out).
Something to start with