Hmmm... Thought I wrote this reply already but must not have saved it.

I have been using the HF rig on the boat successfully for Sailmail/Winlink for years, but it is incredibly tolerant of bad signals. Now I'm getting on the morning Northwest Boater's Net at 3.865 mhz. That works fine on the hook with most systems off. Now that I'm using that band more, I have the incentive to decrease the noise.

Keeping RF and DC grounds separate on the Icom M-802 seems about impossible. I do have a line isolator on the coax going to the antenna tuner. But Icom insists that foil needs to go to the ground on both tuner and transceiver. The ground on the transceiver is case ground. It in turn is in common with the negative power lead and a bunch of other stuff. That makes some ground loops.

I'll try some cheap shielding in the area of the autopilot and use my handheld transceiver as a noise sniffer. Should provide hours of entertainment {grin}.

Some day, I might even try shielding the linear drive leads, attaching the shield at only one end as suggested.

The other day I was reminded how far a little noise goes in the 75/80 meter band. I have a newly installed Carolina Windom dipole 70' above ground in the trees at the house. I have had great noise-free results. The other morning noise was suddenly gobbling what had been perfectly clear signals the previous day. I went around the house shutting things off and found that dimmable fluorescent lights in the kitchen were the culprit. Once these were off, the noise was gone. Gives some perspective to signal-to-noise ratio. It doesn't take much RF noise in the vicinity to wreak havoc on a boat. The noise source is just so close.