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Electrolysis and Your Props

© Capt. Matt

Whenever different metals are placed in a conductive liquid, such as salt water, you create a battery. If you connect these pieces of metal together, current will flow. This current, trying to equalize the conductivity of the metals, will be removing metal from one of the metal pieces. This removal is called "electrolysis". If the piece being removed is the zinc in your flashlight, battery is good. But if one of the pieces is your propeller it is bad.

When you pull your boat to do the bottom you may wonder what those pitted, ashen-white pieces of metal are on your shaft, rudder or possibly on the transom. These are called zincs and, as luck would have it, are made of zinc. The zincs you use on a boat are called "Sacrificial Anodes". Zinc is used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller.

To complete the electrical circuit, the zincs must be connected to the items they are intended to protect. Usually this is no problem because the zinc is bolted right to the shaft or underwater housing. Non-metal boats will usually have a copper bonding wire inside that connects all the underwater metal items together so they all share the protection from zinc anodes.

Since engines use the metal frame as the negative battery connection and the engine is connected to the prop shaft, the engine and the negative side of your 12 volt system are also part of this bonding connection. This bonding wire may also be connected somewhere to the rigging. This is not for electrolysis protection but for some protection from lightning strikes to conduct it into the water through the items connected together.

If other currents are allowed to get into this bonding circuit they can easily overpower the small voltage available from your zincs and defeat the protection you need. This is usually the most destructive form of electrolysis and you notice it because your zincs get eaten up very quickly trying to keep up. Zincs should last at least a year if they are working normally, and much longer if you don't have any problems. If they are being "sacrificed" in a shorter period you need to find where the external current is getting in.

The most common source of this external current is the shore power connection, sometimes referred to as stray current. Docks are notorious for bad wiring. Often the ground lead is not connected to ground, but is connected to the neutral and is being used for carrying current to a poorly wired boat.

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