Hi Dennis

Yes, I've spent quite a bit of time looking at the grounding systems
and there a few good articles in magazines and on the internet, but
some how they never quite get down to specifics of every situation.
There are four grounds which need to be kept separate, subject to
certain approved interactions: AC, DC, HF, and lightning. Regarding
the DC ground, which is the subject of your enquiry, the logic not
many years ago was to connect together every metal thru-hull, which
Amel have done elegantly with the grounding bus you describe, which
connects to the zinc anodes mounted either side of the rudder (at
least it's so on my Mango); this system still holds good if you spend
your whole time in remote anchorages, but entering a modern marina
exposes your boat to stray electrical currents from and between other
boats (DC) and pontoon services (AC). This can set up a galvanic
corrosion reaction, in which the current enters by one thru-hull and
exits by another, taking with it some metal. This is also a problem
where two or more metals are in contact in an electrolyte, e.g. sea
water.

Therefore the common wisdom has altered since your boat was built to
say that thru-hulls should not be linked together. Bronze thru-hulls,
such as for toilet water intakes below the water line, are considered
stable and may happily be left alone, i.e. not connected via a strap
to any other thru-hull which would expose it to the corrosion effects
described above.

There are some situations where connection is unavoidable, such as
stainless steel (itself an alloy) prop shaft mounted in bronze tube
and with copper propellor. It's essential to ensure the grounding bus
is connected to both the prop shaft and its mounting tube which, as I
recall, is visible inside the engine room, between the engine and
stern gland. If you look further you will find that the engine water
intake cock is connected by water to the raw water manifold and to
the engine itself and, if I observe it correctly, to the diesel
supply system via that fluid and to the fresh water heater. Some of
the bus gets pretty inaccessible in these places. I'm not 100% sure
of the electrical connectivity via fluids of some of these items and
would be glad of contributions by others. It seems to me that whether
the engine etc should be connected to the grounding strap, and thus
the zinc anodes, is a question if there is more than one connection
to the sea water, i.e. via the raw water intake and what else? I
believe the prop shaft is isolated from the engine by a flexible
coupling. The exhaust water exits above the water line (as do all
basins and toilet water exit points), so does not count. Thus if the
only connection point between the engine and sea water is the raw
water inlet, it may be correct that the ground strap was severed.

If your model of Maramu has a ground strap connection to a keel bolt,
unlike my Mango where the rudder anodes are the single point of
protection, then you may need to restore the connection and make sure
the keel bolt is protected by an anode, but it may be your analysis
of this is not correct.

Regards, Roger, Zorba, Mango 28