Through-Hull Fittings And Canaries In Coal Mines

A trawler owner recently contacted us to schedule a routine haul-out and bottom-paint job.

- Steve Zimmerman

He told us that he had some concerns about past discoloration of the bottom paint around through-hull fittings (see photo, page 24). As it turns out, the paint discoloration pointed toward a much more serious condition, a life threatening one at that. This example also serves as a reminder to pay attention to your through-hull fittings each time you haul your boat.

Through-hull fittings get no respect. Hidden below the waterline, slathered inside and out with bottom paint, and often fouled with marine growth, these fittings do their jobs in obscurity. When you haul your boat, the through-hulls usually receive little more than some scraping and more bottom paint. But bronze through hull fittings offer a surprising and important window into the condition of your boat’s electrical system. Like the canary in the coal mine, through-hulls can warn you about potentially life-threatening AC electrical leaks, damaging stray current corrosion, bonding issues, and sacrificial anode problems.

In this particular case the problem and the solution involved multiple issues. Before we dive into the dangerous condition that discolored the bottom paint, we need a bit of background information.

MAKING THE CONNECTION

You might be wondering what an underwater pipe fitting has to do with your electrical system. On most fiberglass powerboats all underwater hardware connects to your electrical system through the bonding circuit. Bonding refers to connecting all underwater hardware with a low-resistance, reliable electrical connection. Bonding achieves two goals: first, it minimizes corrosion due to dissimilar metals (galvanic corrosion); second, it reduces the risk of an electrical shock from a faulty shore power system.