The following instructions are intended to systematically eliminate each potential source of stray current that may be effecting your vessel. The instructions indicate readings for seawater. If you wish to conduct the stray current testing in freshwater, refer to the protected range in the Freshwater Assessment Table.

For swing moored vessels or vessels moored in a marina follow these steps:

1. Identify stray current.

  •  Shore/marina mains power must be connected and switched on
  • Onboard power must be switched on

Follow the instructions for taking a set of readings around your vessel using Rust Seeker™. If you are confident that your anodes are correctly designed and installed, and your coating is in adequate condition, but your readings are still more positive than the protected range of -800mV to -1100mV (in sea water), this may indicate that stray current is effecting your vessel.

This initial set of readings will act as the baseline for further testing, so record your results from around your vessel. Note the location and or individual components effected. For example, the port side propeller shaft. This may help to later identify that the stray current source is from a vessel moored to your port side or that the stray current is from an earth leakage through the port side engine or gearbox discharging through the port side propeller shaft.

2. Is shore/marina mains power the source of the stray current?

  • Disconnect shore/marina mains power
  • Onboard power must be switched on

Retest your vessel and/or components. If the stray current source was from shore/marina mains power you should see your vessel’s potential slowly shifting more negative to the protected range of -800mV to -1100mV. If there is no change in potential readings, shore/marina mains power is unlikely to be the cause.

IMPORTANT NOTE: If your vessel or its components have been subjected to stray current, it may take some time for the excess stray current to dissipate from your vessel after isolating its source. Do not isolate each potential source of stray current and imediately take your readings, allow some time for the excess stray current to dissipate before taking your readings.

3. Is the marina cathodic protection system, another vessel or submarine power cables the source of the stray current?

  • Motor or sail your vessel into calm water away from any external source of stray current
  • Onboard power must be switched on

Retest your vessel. If your readings shift more negative towards the protected range of -800mV to -1100mV, then there is a strong chance that you have identified that the stray current was coming from an external source from within the marina/mooring environment.

It would be advised to immediately consult with all other vessel owners in your local vicinity and also with the management of the marina making them aware of the potentially harmful stray current situation.

4. Is onboard power the source of the stray current?

  • Vessel must be away from any external sources of stray current
  • Onboard power must be switched off

Retest your vessel and/or its components. If the readings shift more negative to the protected range of -800mV to -1100mV the stray current source is likely to be from your vessels' onboard power.

Return your vessel immediately to the marina and consult with a marine electrician to help identify the source of the onboard stray current. Ensure that all electrical systems on your vessel have been installed correctly with the wiring system installed as “insulated return” (two wires).

If you have carried out the above tests, but still cannot determine the source of the stray current, seek the advice of a qualified marine electrician or a cathodic protection expert.

“As with Cathodic protection, although the principles of electrolytic corrosion are simple, affecting a cure is often difficult and it is wise to call in a specialist”
Metal Corrosion In Boats. Second Edition 1998,

Chapter 8 – Electrolytic, Selective and Stress Corrosion and Corrosion Fatigue.
Author: Nigel Warren

Earlier it was noted that a positive shift in your vessel's potential could be the result of loss of or spent galvanic anodes possibly combined with a poor coating system. You may need to dive overboard to visually check that your anodes are still in place and if they are, check that they are not spent to a point that they are not offering enough protective current to your vessel and need replacing.

IMPORTANT NOTE: Stray currents can cause serious damage to your vessel’s hull, underwater components, coating systems and anodes. If when conducting an underwater inspection of your vessel you ever discover rapid depletion of anode material or serious localised damage to the coating system this may be a direct indication that your vessel has been or is currently being subjected to stray current.

If your vessel has been subjected to Electrolytic corrosion through stray current, you may need to have your vessel inspected more closely for serious damage to underwater metallic components and fittings. You may also need to replace your anodes and repair any damage to coating systems. This may involve having to remove your vessel from the water for inspection, maintenance and repair work.

Establish the life span of your anodes and coating

Use the Rust Seeker™ booklet to record your vessel’s readings every 2-3 months. As your anodes corrode and your coating deteriorates, the readings will shift towards the unprotected range, indicating when it is time to replace your anodes or repair your coating.  This will allow you to accurately predict future anode and coating replacement cycles.

Also, by recording your readings every 2-3 months, you will notice any out-of-the-ordinary shift in readings which may indicate the presence of stray current.