I posted this on another bulletin where a contributor had bonded all
his underwater metal with 1" copper straps and needed to test. Copied
here for anyone having electrolysis problems.

An ohm meter is a poor way to check bonding connections. I use a 12
volt lamp and make a probe using a screwdriver. Hook a long enough
wire to the positive terminal of the starting battery and then poke
the screwdriver into each of the through hull fittings and make sure
the lamp lights. An ohm meter is only testing the circuit at micro-
amps.

The following applies mainly to boats left in the water. Trailered
boats rarely spend enough time in the water for electrolysis to be a
problem.

ELECTROLYSIS 101. I've worked with electrolysis for 18 years
including live-aboard on a steel yacht for 14. We have sold thousands
of our galvanic isolators and handle numerous emails/phone calls daily
on electrolysis situations.

BONDING STRAPS

Although your copper straps look impressive they are totally
unnecessary. The wire at the most only carries a few milliamps and
there is no concern for voltage drop so the copper only needs to be
heavy enough to resist deterioration from oxidation. A 10 gauge
copper wire is more than adequate. I use non stranded 10 gauge bare
copper wire for my installations, being careful to secure it so it is
not subject to flexing from boat motion. Non stranded wire has much
less surface area and avoids the cavities between the strands where
moisture and corrosion/oxidation can progress.

THEORY

Electrolysis only happens when two dissimilar metals are immersed in
an electrolyte and connected together. The dissimilar metals have
different electrolysis voltages so if you connect them together
current flows through the connecting wire one direction and through
the water the other. As the current leaves one metal to travel to the
other, it causes metal to come off one surface and be deposited on the
other like battery plates so the higher voltage metal suffers
electrolysis. Electrolysis can also occur when an adjacent structure
or boat is injecting DC current into the water and that current goes
in one end of your boat and out the other on its way to the
destination. This can cause electrolysis even though your boat is not
an offender.

PROTECTION

There are basically 2 ways to reduce electrolysis. The PREFERABLE one
is to DISCONNECT the electrical circuit. If this can't be done, the
second method is to provide a sacrificial anode (Zinc) so it
deteriorates rather than your expensive equipment.

a) DISCONNECTING

Electrolysis cannot occur on an isolated piece of metal in salt
water. It is all at the same voltage but if it is isolated no current
can flow so there is no electrolysis. When it is connected to another
piece of metal, ESPECIALLY if the other piece is a different metal,
you just created a shorted battery and electrolysis will start. By
following the wrong advise and bonding everything in the boat you are
creating batteries where it is unnecessary and making electrolysis
problems worse.

ANN-MARIE'S RULE #1. Only bond underwater items that are showing
symptoms of electrolysis. If it ain't broke, DON'T fix it. Once you
bond it unnecessarily you have CREATED the circuit rather than
DISCONNECTING it. You are now stuck with providing Zincs since you
have removed the first line of protection.

Although some through hulls appear to be isolated due to sitting in
fiberglass and using non-metallic tubing, they quite often are not and
WILL need bonding. For example the raw water cooling inlet for an
inboard engine is in fact connected to the engine block by the salt
water in the tubing and may need bonding so the current flows though
copper rather than the water and in/out of the through hull.

b) SACRIFICIAL ZINC

There are situations where different metals cannot be disconnected and
you are stuck with putting zincs on them to provided a target for the
electrolysis. Zincs have a higher electrolytic voltage than marine
metals so it is the first to deteriorate.

Keep in mind that the zinc will CREATE electrolysis (favorable
electrolysis since the zinc is being eaten up). Putting zincs where
they were not needed will still cause the zincs to erode away since
they create the battery situation where none may have existed.

The range of protection a zinc can supply to other metals bonded to it
is limited by the conductivity of the water. In salt water you get
coverage for a radius of about 4 to 6 feet. A zinc on the stern of a
14 ft boat where everything is bonded is only protecting half the
boat. This range gets even less in fresh water and may reduce to only
inches. Often magnesium is substituted for zinc in fresh water to
provide an even higher voltage to push through the fresh water.

USE A CONDOM

Over 90% of our customers electrolysis problems are created by the
shore power connection. You could theoretically disconnect the ground
connection in the shore power and avoid electrolysis (and in some
cases this is a solution) but ABYC regulations require the AC ground
be connected to the DC ground so an electrical fault on the boat won't
electrocute swimmers in the vicinity.

When you connect your underwater metal to the shore power ground you
have "bonded" with every other boat on the docks who have the same
connection. Now electrolysis currents are free to flow anywhere in
the marina and it only takes one boat with a 12 volt DC leakage to eat
up every boat within a wide radius even though the offender has zero
electrolysis evidence.

Your protection is a Galvanic Isolator in the ground connection of the
shore power lead. You don't have to purchase ours (although they are
typically less than 1/2 the price of our competitors). Any Galvanic
Isolator that meets AYBC specifications will do - they are not rocket
science and extremely reliable. We have sold thousands, all on
unconditional warranty and have never had a return due to failure.

Feel free to ask general interest questions here or email me if you
prefer.