Galvanic Corrosion is mysterious to most people. In its details, it can be complex, but the concept is simple, and can be summed up by the following:

This process is invisible, but not mysterious. The prop, (or preferably, the zinc) does not actually disappear, but is being deposited on the shaft and other nearby metal, protecting it. The "less noble" (anodic) metal will disappear from the anode and be deposited on the "more noble" (cathodic) metal. Without zincs, the bronze in your prop. will vanish, coating and protecting the stainless steel shaft. This is not what we had in mind.

Here are some excerpts from the "Galvanic Series" (AYBC Std. E-2):

Metal Corrosion Potential (ref: Silver-silver chloride)

This has many implications. The bottom line: metals closer to each other on the chart are more galvanically compatible, and an assembly made from them will corrode more slowly than if the metals are far apart on the chart. If we must use metals far apart on the chart, galvanic protection, such as zincs, may be in order.

If two metals are placed nearby in seawater, a voltage equal to the difference between their two numbers from the chart above will appear. If the two metals are electrically connected, a current will flow, and galvanic corrosion will occur. The metal nearer the top of the chart (anode) will disappear. How fast this will happen is complex in detail, depending also on the resistance, but it will be proportional to the difference voltage.

If a third metal is connected to the other two, the most anodic (top of the chart) will disappear, depositing onto the others according to the voltage differences. To oversimplify: If one of the metals is zinc, then all metals below zinc on the chart (i.e. all metals of interest) connected to it will be protected. To be effective in practice, the zinc(s) must be close enough, and have enough chemically active area exposed to the seawater, because the path through the water has resistance.

So far we have considered two or three metals alone in the ocean. Once you put your boat in the middle of a bunch of others, and (especially) once you all plug into shore power, you are all part of a giant battery together! You are open to stray currents from other boats, and leakage from the marina's electrical system. Now things get more complicated: your own or someone else's leaky appliance or bad wiring can eat your zincs in short order. An automotive-grade battery charger on a nearby boat is one common cause of problems.

If you seem to be losing zincs too quickly, there are several potential solutions:

1) Check your shore-power battery charger for electrical leakage, and check all wiring in the bilge. These are common problem areas.

2) Consider unplugging the boat from shore power when you are not aboard. A properly-sized solar panel can keep the batteries topped up, as long as nothing is left on. If you want to leave the fridge going, or can't wait around after sailing for the batteries to charge, this won't work for you.

3) Move. Another marina, another dock at your marina, or even another slip on your current dock may be significantly different. The proposed location can be tested in advance, although circumstances do change (i.e. a leaky appliance in a nearby boat may only be used at certain times). If the situation is this bad, keep reading…

4) Install a Galvanic Isolator (GI). The Galvanic Isolator allows the green ("ground") wire from your boat to float a little (1.2 Volts, typically) on either side of the green wire coming from the dock. If the potential on the dock's green wire is different from the water your boat is sitting in, rapid corrosion can result. Isolation transformers (see below) and GIs both protect against this, but to different degrees and in different ways. The GI is a basically simple and cheap device made expensive by the need for multiple regulatory approvals and compatibility with electronics, G.F.I. outlets, etc, etc. The GI will solve the problem, IF it is related to shore power ground, and IF the boat's AC wiring is done correctly, without a connection between the ground and neutral (green and white) wires. This connection is often present in older boats originally wired for 2-prong AC outlets. The basic $99 (defender) GI is worthwhile, but before spending $300 or more for a fully-featured GI, all but the highly weight-conscious should consider an isolation transformer instead.

5) Install an Isolation Transformer. The most complete protection against galvanic issues external to the boat is an isolation transformer. A properly-wired isolation transformer effectively disconnects the boat from the shore, but the fridge, etc, still work. The only connection to shore power is via the magnetic field in the transformer core, but you'll never know the transformer is there. Isolation transformers do the job well, but they cost $500 or so, and are about the size and weight of another battery. To be safe and effective, they must be installed carefully, both electrically and mechanically. There are a couple of good places on the I-36.

6) Hang a "guppy" zinc from you boat when at the dock. This can actually look like a fish, or just be any large bit of zinc with an insulated wire attached. This is hung from the boat, and clipped onto the negative battery terminal when at the dock. It protects nearby metals to some degree, and is cheaper (per pound) than shaft zincs. It is especially effective when used with a Galvanic Isolator or Isolation Transformer.

So galvanic corrosion, while invisible, is not mysterious -- it can be measured, predicted and controlled, if not prevented. Proper boat wiring, a high-quality marine-grade battery charger, and regular zinc replacement will usually be sufficient. To protect your boat from other boats or bad marina wiring, consider a Galvanic Isolator or Isolation Transformer, along with a guppy zinc. If you have questions, or wish to discuss your specific situation, feel free to contact me at the number above.