Hull Potential Survey: Checking Corrosion Protection with a Reference Electrode

D'Antonio's method: boat afloat in calm water, shore power unplugged (not just switched off), and no onboard equipment running. He explains why unplugging matters: the cord's green ground wire stays connected to the dock even with the breaker off, and that connection is what you'll test separately in Step 9. Fresh anodes need a little time in the water to settle, so don't survey the day of launch.

On a bonded boat, all underwater metals are wired together and to the anodes. Find a clean point on that circuit inside the boat: a bonding bus, a terminal or a bolt on the bonding strap. D'Antonio suggests cleaning it with a Scotch-Brite pad if it's dirty. He also notes the engine's negative terminal or bus is generally accepted as the reference point for the grounding and bonding system. If your boat isn't bonded, you'll measure each metal separately (Step 7).

Set the meter to DC volts (millivolts if it has that range). D'Antonio's hookup:

• Reference electrode lead to the meter's negative (COM) socket.
• Positive lead to the clean bonding point (or the metal you're testing).

With that hookup, a protected metal reads as a negative number. Some articles connect the leads the other way round (Power & Motoryacht's does), and then the same readings show as positive. It's the size of the number that matters; just use the same hookup every time and write it down.

Lower the electrode over the side about two feet below the surface, near the middle of the boat, without letting it touch the hull, the anodes or the dock (D'Antonio's guidance). You may need a long lead; he suggests at least 20 feet of marine-grade wire. Give the reading a few seconds to steady.

Write down the number. Here are the seawater ranges D'Antonio gives (Ag/AgCl electrode, electrode on the meter's negative), with what other published sources say alongside:

For reference, D'Antonio gives these approximate unprotected (resting) voltages: silicon bronze about -260 to -290 mV, aluminum alloys about -760 to -1,000 mV, and 316 stainless in still water about -430 to -550 mV. He explains that to be protected, a metal has to be driven at least 200 mV more negative than its resting voltage. BoatTEST's summary of ABYC standard E-2 states the same two anchors: a minimum negative shift of 200 mV from the least noble protected metal, and a reading beyond -1,100 mV as a sign of excessive protection. It also notes that zinc itself sits at roughly -1,050 mV, and that pushing hull potential that far reduces how well antifouling paint works.

The sources agree on the shape (at least 200 mV of shift, nothing beyond about -1,100 mV, a lower band for wood) but not on the exact lower limit for fiberglass boats, so treat the numbers as guides, not pass/fail lines. Your engine or drive maker, or the full ABYC E-2 standard, may state slightly different numbers, and the maker's figures win for their equipment (for example, an outdrive with an active protection system). A reading that is borderline, or that changes a lot between surveys, is worth a professional's look.

In plain terms:

• Under-protected (number too small, closer to zero): the anodes aren't doing enough. Usual causes are worn-out anodes, too few or too small anodes, the wrong anode metal for the water (zinc works poorly in fresh or brackish water, D'Antonio notes), poor contact between anode and metal, or a broken bonding wire. Expect pitting, dezincified bronze and eaten aluminum over time.
• Protected (inside the range): the system is working. Record the number as your baseline.
• Over-protected (number too large, beyond the range): too much anode or a misadjusted active system. On fiberglass boats, Practical Sailor and D'Antonio say this mostly damages paint around the fittings (bottom paint lifting or a growth-covered "halo" around bronze fittings). It's a real problem for aluminum, which is attacked by the alkaline conditions overprotection creates, and for wooden hulls, where the alkali softens the wood around bonded fittings. Practical Sailor notes that on wooden boats the fix is usually to limit bonding and anodes to the shaft and propeller area.

D'Antonio recommends testing every underwater metal separately even on a bonded boat, because any big difference points to a bonding fault. Move the positive lead (with an extension) to each seacock, strainer, shaft, rudder stock and strut you can reach from inside, keeping the electrode in the same place:

• Bonded fittings should read very close to your Step 5 number. Power & Motoryacht's writer got readings close to his base number at each fitting, and says the exact voltage matters less than spotting an obvious odd one out.
• A fitting that reads far less negative isn't connected to the bonding system, or has a bad connection. On an unbonded boat, a metal with no anode reads its resting voltage.
• Propeller and shaft: D'Antonio notes the shaft connects to the bonding system only through the transmission's oil, a poor conductor, and shaft brushes rarely make a good enough connection, so the prop and shaft often need their own shaft anode and their own reading.

With the electrode still in place and the positive lead on the bonding point, switch the boat's DC circuits on one at a time (bilge pumps, electronics, lights) and watch the meter. A reading that jumps noticeably when one circuit comes on can mean that circuit is leaking current into the bonding system or the water. Write down which circuit and the change, switch it off, and have a marine electrician trace it. Don't open wiring to chase it yourself.

Now the shore power check, from D'Antonio's procedure: with all readings taken, plug the shore cord in at the pedestal and the boat but leave the power off, then read again at the same bonding point. If the reading changes by more than a few millivolts, the boat is not isolated from the dock's ground, which means it needs a galvanic isolator, or the one it has isn't working or is installed wrong. See the galvanic isolator test. An isolation transformer is the other option for some boats (isolation transformer basics).

Also watch for signs of trouble from the marina or neighboring boats: readings that drift or swing over a few minutes without you changing anything, or anodes that suddenly wear much faster after a new neighbor arrives. Power & Motoryacht warns that stray current from your boat, another boat or the marina's power system can destroy anodes and underwater metals in weeks. That's a job for a professional with the right gear; tell the marina too.

• Under-protected: replace worn anodes, add or relocate anodes as the boat or engine maker specifies, clean and tighten anode mounting points, and repair bonding faults (see sacrificial anodes and bonding system). Use one anode metal suited to your water throughout; BoatUS warns that mixing anode types means one protects the other instead of your fittings.
• Over-protected: don't add more anodes. Remove extras the maker doesn't require, or have an active system checked. On aluminum or wood, get professional advice before changing anything.
• Shore power change in Step 9: have the galvanic isolator tested or installed.
• A fitting that isn't bonded: have it bonded properly, or ask whether it should be isolated instead (some non-metallic or isolated fittings are designed that way).

After any change, give the anodes a few days and survey again.

Survey at launch (after the anodes settle), mid-season and before haul-out, and any time you add underwater metal, change anodes or move slips. Note each reading, the date, where the electrode was, whether shore power was connected, and water type (salt, brackish, fresh) in BoatMend. Trends tell you more than any single number.