01 May

The Devil Is In The Details

Knowing what kind of corrosion can be present aboard will help save your boat from disaster.

By Capt. Ken Kreisler

Great floods have flown from simple sources.” So said William Shakespeare and is a fitting opening for our conversation on the potential negative effects of corrosion aboard your boat and the possibility of big troubles because of it. How big? Great floods indeed.

Let’s jump right into this by getting a few facts straight. Steering clear of any chemistry

This kind of damage, and much more, can occur when your underwater metal is not protected.

lesson, electrolysis involves only one metal and a major change occurring in an electrolyte with chemical properties that make it capable of conducting an electric current. A good example would be when a lead-acid battery discharges and produces a significant alteration in the concentration of the battery acid.

So when, years ago, one of my dock mates called me on the VHF asking for a tow, he was wrong when he stated, “Electrolysis caused that blade on my prop to weaken and finally break off.” No shipmates, his problem was most likely brought about by galvanic corrosion or, in part and sometimes in collusion with, its equally evil relative, stray current.

With galvanic corrosion the deterioration occurs between the dissimilar metals as they react while immersed in salt water. This is caused by the current—the movement of an electric charge—that flows between the two, each acting as anode or cathode, depending on its place on the galvanic chart of metals in sea water, and as a result of either being physically or electrically connected.

The easy-to-read galvanic chart.

The easy-to-read galvanic chart.

The long and short of it here is not so much with the why of the process but more of preventing the result. For example, my buddy’s bronze propeller blade gave up more of itself, corroded as it were, then let’s say the stainless steel shaft it was attached to because it is a less noble, or more reactive metal than that of the shaft’s material.

While the effects of galvanic corrosion will more than likely occur over a protracted period of time, the serious effects of stray current can be seen in as little as several days. Should you have, for example, some faulty wiring lying in the bilge or a damaged float switch sending current into the water, or the same condition existing on another vessel in your marina, and even with issues from the dockside shore power, regardless if you have a galvanic isolator in use, your boat is in serious danger. A shaft, rudders, props, outdrives, lower units, and bronze through hull fittings are all at risk.

If you even suspect this may be the case, and unless you are an ABYC certified marine electrician, the best advice is to take a big step back and get the experts in post-haste. (

The all too familiar bolt-on zinc anode is necessary for protection against galvanic corrosion.

The all too familiar bolt-on zinc anode is necessary for protection against galvanic corrosion.

Protection against galvanic corrosion is a bit simpler and yet comes with its own set of rules. “With fiberglass boats, Mil Spec A-18001K Zinc alloy sacrificial anodes are most suitable for the saltwater environment if the underwater metal you are trying to protect is bronze, stainless, and Nibral,” said Bob Olsen, President of “With brackish water, aluminum anodes (Mil Spec A-24779), which also provide saltwater protection as well, work best. “If you use your boat only in freshwater, magnesium anodes (Mil Spec A-21412) is what you will need.” If you run a stern drive or outboard boat, use aluminum protection. It’s best to check with your manufacturer for any special needs. In addition, with inboard engines, always make sure to check your pencil zincs twice a year.

The size of the anode is a function of how many square feet of metal is being protectedmil-spec and if you are re-zincing, make sure you confer with your yard or marina service manager. “And but for a few situations, you cannot over protect your boat,” Olson added so feel free to collar your shafts as well as long as your boat is properly bonded. Any questions on this, call in the right technician to have a look.

Making sure your boat’s metal fittings are properly protected against both galvanic and stray current corrosion is one of the most important preventive maintenance regimens aboard your boat and one you should always be aware of.

For more information, please contact

1 Comment

Posted by on May 1, 2014 in Maintenance


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One response to “Maintenance

  1. Tommie

    January 11, 2015 at 4:52 pm

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