If your anodes disappear in three months, the reason is not "bad anodes". Anode consumption rate is the most honest indicator of an electrical problem on your boat or on the pontoon you are lying to. This page explains how the measurement is made and what it reveals.
1. Why It Is Measured
Seawater is an electrolyte. When dissimilar metals, a bronze propeller, stainless shaft, aluminium transom, steel hull, are connected through it, the most active one is sacrificed. That is what an anode is for. The problem is protection that is insufficient or excessive; both cause damage.
- Insufficient protection: pink discolouration on propeller and shaft (dezincification), pitting in the hull, losses at rudder and trim tab fittings.
- Excessive protection: paint blistering and alkali damage, particularly on aluminium. "More anodes is better" is wrong.
- Stray current: from shore power or an onboard fault; it can strip anodes in weeks and affects neighbouring boats too.
2. How the Measurement Is Made
A zinc reference electrode is immersed and the other lead of the voltmeter is connected to the vessel's bonding system. The potential read tells you the state of protection:
| Reading | Meaning | Action |
|---|---|---|
| Above the protection threshold (insufficiently negative) | Not protected | Add anodes; check bonding continuity |
| Within the target band | Protection working | Monitor; set an anode replacement interval |
| Excessively negative | Over-protection | Reduce anodes; paint damage risk on aluminium |
| Value shifting rapidly | Stray current suspected | Isolate charger, pumps and shore line in turn |
The measurement is not taken once. It is repeated while systems are isolated one by one: charger off and on, shore lead connected and disconnected, pumps off and on. That is how a leak is found.
3. The Causes We Find Most
- Earthing fault on the marina supply, a neighbour's fault consuming your anodes.
- An ageing charger, or the absence of an isolation transformer.
- Broken bonding, the anode is there but not electrically connected to what it should protect.
- Wrong anode material, zinc for salt water, aluminium for salt and brackish, magnesium for fresh. Zinc barely works in fresh water.
- Propeller and shaft outside the bonding system, common after a coupling has been replaced.
4. Who Needs It Most
- Aluminium hull owners, stray current can become hull damage within months.
- Boats consuming anodes quickly, if you replace anodes twice a season, the cause should be found.
- Newly purchased boats, as part of the pre-purchase survey; previous owners' electrical additions show up here.
- Boats lying long-term in a marina, stray current does its worst work on a boat that sits.
- Marinas and yards, pontoon-level measurement stops one faulty vessel from affecting the whole berth.
Frequently Asked Questions
5. Book an anode and stray current measurement
Systems isolated one by one, the cause is found by record, not by guesswork.
Related Pages
Prepared by SurveyYat, ABYC Professional Member, 149 GT Yacht Captain. Last updated: September 2026.
Scope: The survey is non-destructive and nothing is dismantled; details on the survey types page.
Findings we often see on site
- Leakage current on shore power: A steady current measured on the earth conductor with shore power connected.
- Spent anodes and low protection: Saildrive and shaft anodes largely wasted; protection measured with the reference electrode is inadequate.
Continue with this topic
- Hull potential readings: Reference electrode method and reading the values.