Regal 3300 Owner's Manual page 241

Cosmetic Care & Maintenance
Typically electrons flow from the anode (the aluminum drive unit),via
the external conducting path to the cathode (stainless steel propeller).
If there is a very large anode connected to a small cathode, the anode
will corrode very slowly. If a very large cathode is connected to a small
anode, the anode will corrode very quickly. Obviously, if you do not
control galvanic corrosion, over time the aluminum will corrode away.
The first sign of galvanic corrosion is paint blistering (starting on sharp
edges) below the water line- a white powdery substance forms on the
exposed metal areas. As the corrosion advances, the exposed metal will
become deeply pitted as the metal is actually eaten away.
Another condition which will increase galvanic corrosion is the removal
or reduction in surface area of the sacrificial anodes. Never add
aftermarket products that are connected to the engine ground such as
stainless steel steering aids and trim planes.
Zinc connected to aluminum will form a corrosion cell but the
aluminum (drive) becomes the cathode and the zinc (anode) corrodes.
Even though your boat may not have shore power aboard current from
nearby vessels with shore power can produce stray current galvanic
corrosion. Stray current corrosion occurs when metal with an electrical
current flowing into it is immersed in water that is grounded (lake,
ocean, pond). The current can leave the metal and flow through the
water to ground. This will cause rapid corrosion of the metal at the
point where the current leaves.
When a vessel nearby is plugged into shore power, they can potentially
tie your aluminum drive unit to their boat via the green grounding shore
power lead. Your aluminum drive unit could be the receiving end of
a large galvanic cell (a battery) interconnected with nearby vessels or
even through the marina's metal structures via their electrical system.
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