The outdrive is then connected to the
neighbor’s grounded underwater metals
(even one several boats away), which may
be stainless steel or bronze. Because the
two different metals are in the water and
connected through the ground wire, it’s the
same as if they were in direct contact. If a
boat’s underwater metal isn’t protected by
an anode, it will use a neighboring boat’s
anode for protection. When that’s gone, it
will use other less-noble metals, likely an
outdrive, to protect itself. The solution is to
install a galvanic isolator, which will prevent
low-level DC current from flowing through
the green wire — effectively breaking the
connection to neighboring boats — but
still allowing shore power current to flow to
ground in an emergency.
SOME INFORMAL RESEARCH spearheaded by our BoatU.S. Technical Services depart- ment showns that AC current may cause corrosion, at least in aluminum, though nor- mally at slower rates than DC current. AC leakages need to be significantly higher than
DC to cause corrosion, because AC switches from positive to negative 60 times per second.
On the positive cycle, corrosion occurs, but on the negative cycle, some re-plating occurs
so corrosion can take significantly longer. However, it’s believed that in swiftly moving water,
as in a marina with current, the re-plating may not always occur, and AC corrosion will be
Studies have shown that in most metals, such as iron and copper, AC current has about
one percent of the effect of DC current as it relates to corrosion, but AC current has about
40 percent of the effect that DC has on aluminum. While this may be important in determining how a corrosion incident occurred, it pales in comparison to what leaking AC current can
do to swimmers in the water. Even very small amounts of AC current can disable a swimmer’s
muscles and cause drowning. New boats are equipped with a device called an equipment-leakage circuit interrupter (ELCI) that prevents shocks to swimmers in the water in the event
of a fault, and also offers some protection against AC current corrosion.
STRAY CURRENT. Another way an outdrive can be destroyed by corrosion is through
a voltage leak in a DC system. This problem is
less common and typically caused by a fault
in a boat’s 12-volt DC system. For example, if
a bilge pump sitting in saltwater were to have
a break in the DC wire’s insulation, current
could destroy metal fittings in the bilge.
Avoiding stray current corrosion means
making sure that there are no wires in the
bilge (all wires should be supported above
the level of expected bilge water), except the
wire to the bilge pump. The latter should be
inspected to make sure it’s in good shape.
Aside from keeping wires out of the bilge,
the most effective way to prevent stray-cur-
rent corrosion is to have all of your under-
water metal bonded to the boat’s ground.
That way, any leaks will be brought back to
your battery before they have a chance to
destroy fittings. Most new boats are bonded
from the factory, but older boats can be ret-
rofitted. It should be noted that a galvanic
isolator won’t normally prevent stray current
corrosion damage because the damaging
voltage exceeds what the isolator can stop.
Charles Fort writes for Seaworthy, the
BoatU.S. Marine Insurance publication dedicated to helping members to avoid injury and
boat damage due to accidents and storms. For
an insurance quote, please call 800-283-2883
or go to www.BoatUS.com/Insurance
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