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may be over protected, others under protected,
still others may not be protected at all.
An even greater advantage is that a zinc control-
ler/monitor can be installed in the bonding sys-
tem between the zinc (or zincs) and the protected
fittings. (The bonding wire is run from the zinc to
the controller/monitor and then to the main
bonding conductor.) Controller/monitors allow
you to adjust the resistance in the bonding sys-
tem, so you can speed up or slow down how fast
Dry Roller
's zincs corrode, and how much they
polarize the other fittings. Keeping tabs on
's bonding system voltage, enables you to
be sure that its zincs are working properly; to
know when they need replacement; to use the
controller to adjust for the zinc's decreased effec-
tiveness with age; to eliminate overprotection;
and to spot stray-current corrosion, should it
start. This is a lot of pluses, especially on large
craft, where corrosion damage can quickly add
up to expensive headaches.
One controller/monitor is the Siemens CM2 Cor-
rosion Monitor,
. An-
other source is Caltheco
Sizing up the Zinc — Wood and FRP Vessels
For wood and FRP hulls, there're four standard
variations of shaft assemblies.
Case I:
Centerline propeller on a single-screw vessel, no
exposed shaft and an all-wood or all-fiberglass
rudder assembly.
Case 2:
Same as Case 1, but with a bronze or stainless rudder, rud-
der post, and fittings.
Case 3:
Long exposed shaft and strut assembly, and a wood or fiber-
glass rudder assembly. (Double the amount for twin screw.)
Case 4:
Long exposed shaft and strut assembly, with bronze or
stainless rudder, rudder post, and fittings. (Double the
amount for twin screw.)
Wood and FRP vessels are occasionally fitted with mild-steel
rudders and fittings. These should
be attached to the
bonding system, but kept isolated, and protected with sepa-
rate anodes of their own. A better solution would be to dis-
card the mild steel gear and replace it with stainless or
By referring to the charts, you can select the correct weight
of anode for your boat. Say your
Daffy Dancer
was a 60-f
fiberglass cruiser.
is driven by a single 40-inch pr
projecting out of the deadwood (no exposed shaft). Its ru
der is foam-core fiberglass with a bronze rudder post and
fittings. These characteristics make
Daffy Dancer
a Case-
vessel. Simply enter
’s prop diameter on the horiz
tal axis of the "Anode Weight vs Propeller Diameter" char
and run up to intersect the Case-2 line. Run across and r
off the weight of zinc anode required—about 13.4 pounds
This isn't an exact science only an estimate, so you'd refe
the manufacturer's catalog and order the nearest size up
from the size indicated. (In large vessels you may have to
use multiple anodes.) If
Daffy Dancer
were a wood craft;
remember that overprotection can damage a wood hull e
pecially. You should check the voltages generated by the
zincs as soon as you get the chance. (If you have a contro
monitor system installed, you have a built in continuous
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