water, the best prevention is dry storage.
But for those whose boats stay in the
water, there is only one thing that can
be done: You must actively or passively
interfere with your floating battery’s use
of expensive unintended anodes. The
most common way of doing this (
passively) is to introduce a new metal into
the “cell,” one that is an even better anode
Anode care: Make sure
they’ll make the sacrifice
Be happy to see your anodes disin-
tegrate, rather than any other metals
on your boat. If they’re in great shape
(and attached properly) after a year, it’s
likely that the anode contains impuri-
ties or has a bad connection and is not
working. Replace it, and make sure it’s
Any anode should be replaced when it’s
about half gone, which is usually every
year. Most inboard/outboard and outboard
manufacturers recommend that anodes
be replaced every six months if your boat
is used in saltwater.
An anode has to have a good electrical contact to the surface to which it’s
attached. Be sure the surface is clean and
unpainted. The anode fasteners must be
tight to make an effective connection.
Never paint the anode itself. This effectively “turns it off” and “turns on” your
less-noble but much more expensive
Your anodes can warn you of a potential
stray-current corrosion problem in your
boat or marina. If they suddenly start to
corrode much faster than usual, have the
problem checked by a specialist.
If your boat is bonded, be extra vigilant
about the anodes. A bonded boat has all
of its thru-hull fittings electrically connected to the underwater machinery, and
anode failure can cause galvanic corrosion to your boat’s thru-hulls.
Don’t use antifouling paint that contains
copper on an aluminum outboard or
outdrive. Instead, use a paint specifically
formulated for outdrives.
(less “noble,” i.e., more likely to be damaged) than, say, a bronze propeller.
Zinc, aluminum, and magnesium
are metals that are willing to sacrifice themselves for a more noble cause.
When these metals – usually zinc – are
electrically attached to the piece to be
protected, they become the anode and
stop the expensive underwater hardware
– now effectively the cathode – from
disintegrating. In effect, they become the
negative part of the battery cell.
This can also be actively accomplished
by “forcing” a current into the galvanic
cell. The more-noble metal can then
relax and stop trying to destroy the less-noble fitting. This is called an impressed-current system and is used by Volvo and
Mercury in their twin counter-rotating
prop outdrives. The large amount of
stainless steel in the two props is far more
noble than the aluminum outdrives, and
a sufficient passive zinc anode system
would be difficult to fit. One potential
problem, though, is that the boat’s battery must be fully charged for the system
Protecting your boat
Most boats have sacrificial anodes
installed from the factory. Inboard/outboard and outboard-powered boats usually have several installed at different
areas of the lower unit. (Make sure
you know where they all are.) Inboard-powered boats will have them on prop
shafts, rudders, and trim tabs, and
these may have anodes on the transom.
Impressed-current systems use permanent anodes, but sacrificial anodes may
be fitted to the boat as well. A fiberglass
sailboat will typically have a collar anode
on its prop shaft and possibly one for its
rudder shaft. Depending on which metals are installed underwater, there could
be one for the keel as well. Remember,
if your boat-sized battery cell uses up its
anodes, the least-noble metal is next in
line, and chances are that it’s a lot more
expensive than an anode.
Which type of anode is best?
As mentioned above, anodes can be
made of zinc, aluminum, or magnesium, and each type has different uses.
The majority of anodes are the familiar
“zincs.” While zinc anodes have worked
well for years, aluminum anodes are even
more effective because they can create a
higher voltage (driving force) and have a
much higher capacity (useful life) for the
same weight. Aluminum anodes also do a
better job of protecting in both freshwater and saltwater. The majority of boats
should be fitted with aluminum anodes.
Magnesium anodes are even more
powerful than aluminum (and more
expensive) and are best used only with
the advice of a specialist because in some
situations, they can overprotect and cause
paint blistering and other problems.
Top: Collar anodes are used to protect prop
shafts. Above: Anodes should be replaced
before they’re about half used up. The anode
at top is half used; the one at bottom is new.
ALUMINUM ANODES DO A
BETTER JOB OF PROTECT-
ING IN BOTH FRESHWATER