On Aug 20, 10:50*pm, RichH wrote:
Opppps, Brain Fart !!!

Zinc (-1.0v) less reactive than Mg (-1.6v).

Still the rest applies: galvanic isolator, common bonding through the
AC ground, reference cell, etc., etc.

, while you are plugged in to shore you are only using a very small
amount of current; just say, a few watts an ISOLATING transformer
might be the answer?
The transformer might not handle anything really heavy but a small
trickle charger to keep up the battery only takes a few a watts on the
AC side. For example 12 volts at say 3 amps = 36 watts. That's no
more than some laptops, or a model train set!

However to run a fridge all the time might require 360 watts. That's
still not that impossible except that the transformer must be bigger/
heavier. And it must be able to handle the starting loads of a fridge
compressor.

An Isolation transformer has completely separate input and output
windings. So that it prevents the flow of electricity from even the
'ground' system on shore, which is also most likely, connected to
other boats etc.; through the metal parts of your boat into the
water. Even so called freshwater is slightly conductive.

Also if you have two dissimilar metals on your boat which are
connected together electrically both being in the water you have an
electrolytic cell. See note. Current will flow, even so slightly and
the less noble metal will slowly corrode.

Some early batteries, now called 'Alkaline cells' were invented by
LeClanche. They consisted of a glass jar with a weak alkaline solution
of water and sal ammoniac. The positive electrode was usually a carbon
rod which did not suffer any erosion. The negative (and less noble)
electrode was zinc and if was consumed and eaten away. Back then
people could go to a hardware store and buy new zinc (which was
sometimes used for roof flashings anyway) and the stuff to mix with
water to make the alkaline electrolyte. A similarity to two metals on
a boat.

There are two likely conditions.
1) Leakage through boat into the water and/or other boats with their
leakage into the water going to ground through YOUR boat.
2) Electrolytic corrosion on YOUR boat regardless of whether it is l
plugged in to shore. So for example a zinc (or magnesium) clamped and
in metallic contact to say an outboard is designed to be eaten away,
thus protecting the metal outboard motor from being eroded.