Short answer: Zincs wearing out fast usually means one of two things: a large galvanic load, often from boats sharing the marina's shore-power ground, or stray DC current leaking from a wiring fault on your boat or a neighbor's. A reference-electrode test with shore power on and off tells them apart, and the fixes differ: a galvanic isolator for the first, finding the leak for the second.
Long Beach Marine Electric hears about zincs wearing out fast most often from boats in busy marinas, and from boats that recently moved slips. Anodes are designed to corrode so your shaft, prop, rudder and outdrive don’t. When they disappear in weeks instead of months, something is driving them much harder than normal.
Two different problems
Galvanic corrosion happens whenever different metals are connected and immersed in the same seawater. The less noble metal corrodes, and anodes are meant to be that metal. On shore power, your boat’s underwater metal is also connected to every other boat’s through the marina’s green safety ground. The whole dock becomes one galvanic circuit, and boats with weaker protection drain anodes on boats with stronger protection.
Stray current corrosion is DC current leaking from a wiring fault, such as a chafed wire touching a bonded fitting or a bilge-pump connection sitting in water, and leaving the boat through underwater metal. It can strip anodes, props and outdrive legs in weeks. The fault can be on your boat or on a neighbor’s.
From the dock, both look the same: missing zincs. Measurement is the only way to tell.
The reference-electrode test
A silver/silver chloride reference electrode, lowered into the water next to the hull, measures the boat’s potential in millivolts. Cathodic-protection practice holds that adequate protection shifts a metal’s potential at least 200 mV more negative than its natural resting value. Aluminum, for example, rests around -760 mV in seawater and is protected at about -900 to -950 mV, but shouldn’t be driven past about -1,100 mV.
Readings are taken:
- With shore power connected and disconnected. If protection changes a lot, the shore-power ground is involved: a galvanic problem.
- With on-board breakers switched one at a time. A reading that jumps when a circuit is switched points to stray current from that circuit.
The fixes
For galvanic loading through shore power: a galvanic isolator in the shore-power ground blocks the low-voltage galvanic current between boats while still passing fault current safely. A fail-safe, ABYC A-28-compliant unit such as the ProMariner ProSafeFS lists at about $605. Larger yachts often use an isolation transformer. See galvanic isolator vs isolation transformer.
For stray current on your boat: find and fix the leak, usually a wiring fault in the bilge or engine compartment. A DC leakage test with a clamp meter narrows it down.
For stray current from a neighbor: document the readings and tell the marina. An isolator helps with some of it, but the source has to be fixed.
For bonding problems: fittings that have lost their connection to the bonding system aren’t protected at all. A continuity check from each underwater fitting to the anodes finds them.
What not to do
Don’t just add more anodes. More anode surface can overprotect, especially aluminum outdrives, which blister paint and damage the metal. And don’t ignore it. Props, shafts and outdrive legs cost far more than a test.
Bottom line
Fast-wearing zincs mean either a galvanic load through shore power or stray current. A one- to two-hour test at the slip tells you which. See galvanic corrosion testing or call (562) 374-7233.
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