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Steel & Aluminum Hulls
Where wood and FRP vessels require a common bonding
system to connect all metal hardware to the anodes, metal
hulls themselves form the bonding. On steel hulls, it's possi-
ble to weld lugs, projecting from some anodes, directly to
the hull. This, however, makes replacing
anodes a difficult business. A superior
approach is to weld bolts to the hull to
mate with the anode's fastener holes.
Replacing anodes is then as simple as
turning a wrench.
On aluminum craft, the best practice is
to weld either a backing or doubler
plate to the exterior of the hull, or to
weld a thick insert plate into the hull at
the anode location. (The doubler or in-
sert plate should be of the same alumi-
num alloy as the surrounding plate.)
Doubler thickness should be at least 1.5
times the fastening bolt diameter; the
insert plate at least 2.2 times bolt di-
ameter. Both should match the footprint of the anode, or be
slightly larger.) Drill and tap the doubler for the anode
mounting bolts (stainless threaded inserts are even better)
and use standard 316-stainless hex-head machine bolts to
fasten the anode. Be certain to install lock washers under
each nut. Without these, engine vibration and corrosion will
loosen the nuts. (Note: High-speed aluminum hulls and sail-
boats that need to reduce drag can use recessed anodes to
decrease drag, as shown in the illustration, next page.)
To ensure good ele
cal contact and a fir
mounting base, pro
anodes—for steel ve
sels as well as woo
and FRP craft—are
around a steel-bar
core. Fastening bolt
penetrate and land
this core. For alumi-
num hulls, the best
practice is to use a
odes with aluminu
Amount of Zinc for
Metal Hulls
A good estimate of
anode required can
made from the follo
ing formulas:
For steel hulls:
For one year protec
of a hull with an av
age (less than perfect) paint job:
Steel-hull required anode weight (lbs.) = Hull Wetted Surf
For two years of protection use twice as much anode. For
one-year protection of bare metal, u
2.5 times as much.
For aluminum hulls, anodes are bett
selected by their surface area than b
their weight:
For aluminum hulls:
For one year protection:
Aluminum-hull required anode surfa
area (sq.ft.) = Hull Wetted Surface
Remember these formulas give esti-
mates. Boats that operate in warm o
highly polluted water may need mor
protection. Vessels that operate in c
clean waters can often get by with less. The best practice
to test polarization, as described earlier.
Metal Hulls Must be Isolated
Keep in mind that metals more than 200 mV (millivolts o
0.2 volts) apart should not be in contact in salt water. Thi
means that all yellow-metal and stainless fittings must b
isolated from the metal hull using non-conducting insulat
and bedding compound. On smaller vessels, such fittings
can simply be left totally isolated. They will be floating at
Continued next p
Anode Weight Formulas
If you prefer use the formulas for an-
ode weight in a spreadsheet, they are:
Case I:
Anode wgt., lb. =
(0.055 x Dia., in.)
Case 2:
Anode wgt., lb. = 0.96 x
(0.066 x Dia., in.)
Case 3:
Anode wgt., lb. = 0.94 x
(0.066 x Dia., in.)
Case 4:
Anode wgt., lb. = 1.4 x
(0.069 x Dia., in.)