Determining which sacrificial anode will fully and safely protect a boat
depends on a number of factors. As previously discussed, it is important
to understand the physical properties of each anode material. In addition,
boaters should also understand how each anode works with different hull
compositions, sterndrives and types of water.
- The best way to determine whether a boat is fully protected is by measuring
its cathodic protection voltage or hull potential using a voltmeter. But,
if one isn't available, here are some simple guidelines for selecting the
- The hull material of a boat determines, in part, which anode material
to use. A fiberglass boat having an inboard engine with bronze and stainless
metal parts needs less protection than an aluminum hull or a boat with
an aluminum sterndrive. Zinc or aluminum alloy anodes will work well for
these types of boats. The voltage generated by these anodes cannot overprotect,
i.e., they cannot cause any damage no matter how much anode material is
added. The maximum voltage generated is the voltage of the anode itself.
Magnesium would also work with a fiberglass boat but only in freshwater.
However, aluminum or wooden hulled boats can be overprotected by very active
magnesium. Steel hulls can also be overprotected and the excessive protection
voltage will rapidly lift the paint off the hull.
- Sterndrives and outboard motors, because of their very active aluminum
assembly, are hard to protect. Initially, the anodes for these units were
made of zinc. But corrosion problems in the early 1990s sparked the major
engine manufacturers to start selling aluminum alternatives. The increase
in protective voltage ensures that the sterndrive is protected. Today,
in some cases, using zinc may invalidate an engine's warranty. Again, caution
is needed when using magnesium anodes as they can overprotect.
- Water type is the final influential factor when choosing an anode.
When used in freshwater, a zinc anode forms a coating of zinc hydroxide
that insulates it and stops it from working. Magnesium used in saltwater
can disappear very quickly, and if it is used on an aluminum sterndrive
or outboard motor, it can be very dangerous. If piloting a boat with an
aluminum hull or sterndrive down river and into the ocean, the overprotection
of the magnesium anodes would cause hydrogen bubbles to form under the
paint on the hull or drive resulting in it being literally blown off. Acid
rain and pollution can also increase the conductivity of freshwater to
the point where the same thing will happen.
- Aluminum alloy is the only anode material that is safe for use in all
types of water and accepted by the major sterndrive manufacturers as the
best material to use. It is lighter and protects better than zinc and is
not so active that it becomes dangerous like magnesium. Aluminum alloy
is also environmentally friendly, unlike zinc, which is considered a pollutant.
- Editor's note: Martin Wigg is the president and founder of Performance
Metals. He holds an MBA and a Bachelor of Science degree in mechanical
engineering. Paul Fleury is certified by the American Boat and Yacht Council
as an Electrical Technician and the National Association of Corrosion Engineers
as a Corrosion Technologist.
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