Someone may have a solution to this. I'm in the process of isolating my engine so electrically it is above earth. I've had the alternator modified so it is isolated, replaced single
wire senders/switches with two wire ones, installed a solenoid for the starter, and ran all earth wires what were using the engine for ground directly to the battery. So except momentarily when the engine is starting there is no electrolysis
(steel boat). I used Seabis to find several additional engine-related sources of electrolysis
which I can fix. Except one.
I have an external regulator
(Balmar ARS-5). Rather than run the ignition wire to the alternator directly through the ignition switch (rightly or wrongly, I was concerned about the amount of current), I used a little auto-type relay. Strangely, the ground for the coil circuit in the relay is going to metal. It is showing electrical
continuity with the hull as tested with my multimeter, as well as indicating a critical fault on the Seabis.
The relay itself is air mounted with a plastic case. The only thing I can imagine is that it is coming from the positive side of the coil circuit (some kind of reverse current
flow) that can be fixed with a diode? Or remove the relay altogether or replace it with some king of isolated relay?