Hi Michael

Nice to hear from a Mango owner; there don't seem to be too many of
us participating in the forum.

I wouldn't describe myself as an expert, but am happy to help you
with some suggestions. The zincs rapid disappearance definitely
indicates something is wrong. After 18 months my rudder anodes were
slightly used, and of course I changed them anyway. I was puzzled
about why the only anodes are those on the rudder and was worried how
I could place a suitable anode to protect the propshaft and engine.
Investigation revealed that the rudder anodes are connected to the
grounding strap which can be seen emerging by the rudder post (under
the aft bunk), passing along the locker under the aft bunk, then
appearing again in the engine room close to the prop shaft, tacked
along the bilges under the engine, across to the engine earthing
point, and so on through the boat. Thus no other zincs should be
required. I've tested this with a multimeter and can report there was
no resistance between the rudder anodes and the grounding strap
sampled at several places through the boat.

Your situation indicates that there is regular electrical activity
through your anodes into the sea water; perhaps also your anti-
fouling is copper-based (as is common), which might be producing the
paint blistering you describe. It's important to recall that the DC
grounding system to the anodes is intended to connect your anodes
together for protection from galvanic corrosion, i.e. it is not
necessary or desirable in the context of your DC battery circuits,
particularly the services side. With regard to the services side,
unlike a car, where a single wire leads to, say, a lamp and the
return wire is grounded to the car body, on a boat all DC circuits
should return via a negative cable to the battery (bearing in mind
that many services cables lead back to the common earthing point in
the electrical cupboard above the galley, and thence by a heavy-duty
cable to the service batteries). The engine, being an automotive
component, does have a number of electrical contacts, mainly sensors,
which run to earth via the engine casting; however these circuits are
completely separate from the services-side circuits and, when the
engine batteries are isolated, are inert. In operation there should
be no reason for earth leakage unless there is a resistance or break
in the route back to earth, so perhaps you should check your main
fuse (mounted on a tray at the aft end of the engine) and all
connections on the route from there back via the starter-motor to the
batteries (on my boat it's to a services battery negative post and
thus via heavy-duty cable to the engine batteries); check all the
connections around the starter-motor especially as inability of the
commutator cranking current to return to battery negative would
result, presumably, in a heavy discharge via the grounding strap into
the sea (consistent with your description); similarly if the services-
side to engine battery negative connection is not good. (Did you do
your protection check also while the engine was running?) If in
doubt, clean or replace all these cables until your problem disappears.

The services and engine battery banks should have completely separate
electrical wiring systems except for the one heavy-duty cable direct
between the battery bank negatives. There should be one position only
at which the DC system interconnects with the grounding strap, which
is at the engine earthing point; on my Perkins 4.236 this is forward
on the port side of the engine. Thus, when the engine batteries have
been isolated, which is presumably most of the time, there is no
cause for electricity to be circulating via the engine into the
grounding strap and so on to the rudder anodes.

You may want to check your wiring carefully to see if any other DC/
grounding interconnects have been made; check that each grounding
system (DC, AC, HF, lightning) is separate from the others, subject
to sensible interconnects, e.g. if the HF counterpoise is grounded to
the engine or the grounding system, there needs to be a gap of one
tenth of an inch (by fastening to a piece of phrenolic or electrical
circuit board), bridged by suitable capacitors (which are transparent
to HF but block DC); consider cutting the grounding strap from the
aft head intake and from any other "passive" thru-hulls below the
water line (I'm afraid I'm not familiar with a MaxProp). I addressed
this sort of thing in my previous posting.

They're great boats but in all these years many pairs of hands have
been doing stuff. I'm amazed at what I find even professionals have
done wrong. For instance, only recently in hunting down the cause of
excess gassing in the service batteries, I found the regulator on
some new solar panels I had installed last year is undersized; and
just last week I found through detailed enquiry on the manufacturer's
website that the regulator on my wind generator is under-sized too. I
do everything myself now!

Have fun and don't forget to report back if you have a success.

Regards, Roger, Zorba, Mango 28