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I just uploaded a file about galvanic corrosion. Maybe it is helpful.
Regards Claus, S/V Jonathan, SM #196
--- In firstname.lastname@example.org, amelliahona <no_reply@...> wrote:
Since Bill & Judy Rouse posted related to their bilge
grounding strap failure I have inspected my strap and found it
intact. My boat is of about the same vintage so I am trying to
figure out why their strap failed, then the AC pump impeller
went to pieces and the relation to the general theory of grounding
(bonding) on the Amel boats.
I would welcome input from everyone in trying to understand the
rather seemingly black art of bonding in sea going vessels.
This is my understanding.
1. The iron keel and all other metallic items that are submersed
in salt water are bonded (connected) to all other metallic items in
the boat that are salt water immersed. This includes the various
pumps, valves, through hulls,engines, transmission, Herth drive,
2. These are then bonded to the rudder post to which the
sacrificial zinc anodes are attached.
3. The purpose of bonding is to provide an electrical equipotential
or low resistance path for any stray current to flow to the
sacrificial anodes on the rudder. This stray current may be from
electrical component stray current, marina leakage current or
dissimilar metal induced stray current.
4. a). I believer, but I am not sure, that the bilge bonding
strap is made from solid copper because it connects to such a
large potential current sink (the keel) and lives in a hostile
environment where a cable with a terminal end would not
fair well. b). The engine compartment bonding wires are all tied
together at the bilge copper strap just as a centrally located
5. Like a wet cell battery, with two dissimilar metals submerged
in an electrolyte solution current flows when a complete circuit
exists. Our boat is a giant wet cell battery with dissimilar metals
submerged in an electrolyte solution. Add to this the additional
hazard of stray current leakage from marina wiring or onboard
electrical appliances and it is no wonder our zinc electrodes go
away so fast.
6. So, causes of increased zinc anode consumption might include:
a) More time in marinas with their well know propensity to have
lots of stray current, b) electrical components onboard the boat
that leak electrical current, such as inverters, electrical appliances,
7. Any item that becomes isolated from the bonding grid by
becoming disconnected or having a high resistance attachment
to the grid due to corrosion of the bonding wire to
protected item joint will no longer be protected by the zinc
sacrificial anodes and will proceed to corrode.
That is my rudimentary understanding of the system. Other
thoughts are welcome.
So Bill, I think the keel becoming disconnected from the bonding
system would probably NOT have led to the problem with the AC
pump and/or sea water manifold. More likely there is a high
resistance connection between those items and the bonding
system or you have additional current leaks. I think it should
be possible to measure for a high resistance connection by
using a digital ohm meter and checking the resistance between
the rudder post in the aft cabin and the pump or pipe you're
My time on the boat in marinas is very limited because I
only get on the boat 3 months a year and when I am in the
marina and not on the boat I disconnect from shore power
and use solar panels to keep the batteries up. Six months
of the year the boat is on the hard, My limited exposure
is what I am thinking helped me still have a good bilge
bonding strap. I would have rather had more miles on
the boat by now. I envy you.
Regards, Gary s/v Liahona Antigua