CROWLEY’S TIP #11: When your vessel is connected to shore power, it can cause corrosion to your vessel and others.

Continuing our series on corrosion, we’ll talk about the risks when connected to shore power. Then we’ll explain how to test installed corrosion systems to ensure they are providing the best protection.

Whenever a boat is connected to shore power, the hull and drive system is connected to the shore grounding system and to other adjacent vessels (also connected to shore power) via the grounding conductor in the shore power cable. This connection, while required for safety, creates a galvanic corrosion cell involving the dissimilar metals between boats and between a boat and the shore grounding system.

Shore power corrosion protection
When a boat is connected to AC shore power, the grounding lead provides protection from electrical shock, but it also connects underwater metal parts from your boat to metal parts on neighboring boats that are also using shore power. As a result, destructive low voltage transient galvanic currents can flow between them. These currents can cause severe corrosion damage just in a matter of hours. The increased corrosion potential is more than sacrificial anodes and even a Mercathode can handle. The best remedy is a galvanic isolator. This is a solid state device that is series-connected into the boats green safety grounding circuit inside the boat. The isolator filters out destructive low-voltage transient galvanic currents while maintaining the integrity of the boat’s safety grounding circuit.

Corrosion protection testing
To make sure that the drive unit is getting the best possible corrosion protection, it’s very important to have the system tested on a regular basis. The first test measures hull potential. This requires a Mercathode reference electrode tester and digital multi-meter. If the unit is equipped with a Mercathode system, make sure the battery has a full charge. This test should be performed after the boat has been in service for one or two weeks because new boats may give inaccurate readings. Don’t rock the boat as you board as this may alter the readings. Boats should be moored for at least 8 hours prior to conducting the test. This allows the sacrificial anode or Mercathode system to polarize the surrounding water. When you’re ready to begin, plug the negative meter lead into the negative receptacle on the meter. Then, attach the other end of the lead to the engine ground. Plug the end of the reference electrode tester into the positive receptacle of the meter. Set the meter to read between 0 and 2,000 millivolts. Lower the reference electrode tester into the water within six inches behind the propeller. Then, check the reading.

In fresh water, the reading should be between 750 and 1,050 millivolts. In salt, polluted or mineral-laden waters, the reading should be between 850-1,100mv. If readings aren’t within normal limits, additional troubleshooting should be done. Make sure the test is conducted again if corrective measures are taken.

Mercathode System Testing
If your boat is equipped with a Mercathode system, there is a simple way to test if the system is operating properly. The goal is to check the output from the Mercathode controller. When the boat is in the water, disconnect the orange wire that comes from the anode at the mercathode controller. Set the digital reader to show milliamps. Connect the black meter lead to the terminal on the controller. Connect the red meter lead to the end of orange wire. If using the blue or black Mercathode controller, the reading should be 25 or less milliamps, to as much as 200 milliamps in saltwater areas. If using the red Mercathode controller, the readings should be less than 25 milliamps in fresh water areas to as high as 400 milliamps in saltwater areas.

Stay tuned for our next installment when we talk about continuity testing and stainless steel corrosion.