Bil wrote:


If the water is truly sweet (ie no salt), zinc anodes are not v
effective.

Magnesium is better than zinc as a sacrificial anode in sweet water.
Your prop manufacturer will be able to advise further.


It is a matter of degree, of course.
Just about any dissolved minerals will
make "freshwater" conductive and a zinc
anode effective. The more pure the
water, the less the opportunity for
corrosion involving the prop/shaft
galvanic couple. The magnesium will go
quite quickly if the boat ventures into
seawater.

If the zinc is (has recently been)
active, it will have a shiny appearance.
If it has been inactive, it will be
dull, off-white (covered with zinc
oxide), and possibly fouled. If
inactive, it must be scoured to remove
the oxide. It will not clean itself and
will be ineffective until it is cleaned.

If the manufacturer believes the
symptoms point to insufficient anodic
protection (which seems unlikely to me),
some simple tests can be conducted to
test that theory.

Chuck

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