My stray current, prop pitting experience.

When I first launched the Navigator back when I bought it, it was a brandy
new boat with new, shiney 30x30 props.
My first slip in Scituate was located right beside the marina main shore
power cables. After the first season we hauled the boat for hull
maintenance before taking it on the trip to Florida.

When it was hauled, we noticed that the edge of one of the props had some
very small, sharp edges pitted into the bronze. Somebody mentioned that it
looked like electrolysis and I should keep an eye on it. Although the boat
is equipped with an automatic electronic system that is supposed to
neutralize stray currents, along with the standard zincs, we suspected that
the close proximitry of the props to the submerged marina power cable was
likely the cause, particularly because only one prop (the one closest to the
cable) had any evidence of pitting.

When we got to Florida, I arranged for a monthly, in-water hull cleaning
service (required in the warm, Florida waters). I happened to be on the
boat one day when the diver was working on my boat and he surfaced to tell
me that, BTW, he was feeling a very mild "tingle" whenever he touched my
props or shafts. Not good. I investigated everything I could think of,
including disconnecting shore power and having the diver touch the prop.
Under this condition he couldn't detect any tingle.

I ended up calling Navigator, who in turn got me in touch with Volvo, the
engine manufacturer and installer. The engineer at Volvo told me that by
design, Volvo isolated the engines, shafts and props from the boat's bonding
system to prevent stray currents. H told me to look for any accessories
added to the boat that bonded the engine ...either through the battery or
directly.

It turns out that when the bow and stern thrusters were installed, their
dedicated batteries were connected to the main engine batteries for
recharging. The ground and negative leads of the thrusters were
electrically connected and the ground was tied to the boat bond, thereby
bonding the engines. The problem was resolved by adding a second, dual
battery charger dedicated to the thruster batteries.

One of those boat mysteries resolved and recorded in my head.

Eisboch