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Old 08-15-2001
Mark Matthews Mark Matthews is offline
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Thru-hull Corrosion

In searching your site for information regarding bronze thru-hull maintenance I found a very interesting article by Tom Wood. However, it did not answer my specific questions. We have a 1988 Sabre that lived in freshwater until late 1999 when we moved it to the coast. After inspecting the bronze thru-hulls, we found that they function fine, however, there is a green corrosive substance at the base of them. What is the cause of this and what product could be safely used to remove it? Also, is this an indication of a problem, if so, what, and what needs to be done to correct the problem?

Mark Matthews responds:
Thanks for the question. The green substance at the base of your thru-hull is a byproduct of corrosion. What is happening is that slowly, but steadily, the thru-hull is dissolving, leaving a green-blue powder in its wake. Salt water is much more demanding on the many fittings aboard a sailboat than freshwater, especially those below the waterline. Any time two different metals are immersed in the same body of water, corrosion can occur, but salt water is particular adept at removing ions and compromising fittings.

There are a number of potential culprits here. AC shore power and dubious wiring on the dock where your boat is kept (as well as wiring on board neighboring boats) can prompt current to travel through the boat’s system and eventually degrade thru-hulls, prop shafts, and other expensive and difficult to replace items.

Most marinas are electrically active environments. In my own marina, the owner of a large powerboat recently decided to run the boat's generator and shorepower at the same time—the end result, for whatever reason—was that this dissolved the zincs of his neighbors! I have also experienced the disconcerting sensation of cleaning the bottom of my boat and accidentally scraping the outside flange of a thru-hull clean off after leaving my boat in a somewhat funky marina in Costa Rica.

The best safeguard against these and other corrosive anomalies is to ensure that every metal fitting under the water has a sacrificial zinc nearby and that these are regularly inspected and replaced when needed. Corrosion is essentially impossible to stop completely, but by having zinc anodes on the hull, propeller shaft, and other parts of the boat—including the engine—you can be sure that these will corrode first before other areas do.

Zincs should be replaced when they have been reduced to about a half of their original size. If you have a diver periodically cleaning the bottom of the boat, he should be able to replace these, or you can do it yourself during a haulout. Generally speaking, if the zincs need to be replaced more than once a year, there is a stray electrical current—either coming from your boat or another source—that needs to be found.

To resolve your current problem, wash off the area with freshwater; you may have to scrub it a bit to get the green look to go away—if large flakes of metal start coming off the fitting or it looks pink instead of bronze in color, the thru-hull will have to be replaced when the boat is hauled. If you’re interested in more information, I’d suggest joining the Sabre e-mail discussion list here at SailNet. Once you're logged on to that you can query other owners about their experience with the boat. By the way, here are a couple of articles we've published where you can find out more about these topics: Electrical Work Rules by Don Casey, and Bonding Thru Hulls by Kathy Barron.