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How To Check Your Boat's Bonding System

Bilge Networking

Ensure your bonding system is doing its job.

Bonding Primer

For the most part, all metals touching sea water should be bonded, including seawater pumps, strainers, and plumbing. Moreover, all electrical equipment and large metal objects should be grounded to prevent electric shock and increase lightning safety.

Fuel tanks and fills should be linked with the main DC negative bus or a battery terminal. Bronze through-hull fittings are an exception. ABYC leans toward bonding, but also allows through-hulls to be isolated. As noted above, among the issues that can arise is wood damage—the tiny amounts of current passing through bonding wires can deteriorate nearby wood, including the wooden backing blocks that are often installed beneath the flanges of through-hull fittings.

Immerse a bronze propeller and a stainless steel shaft in sea water (an electrolyte), and you’ve just created a battery. The same is true of any other combination of two or more dissimilar metals. Even if it’s just a quarter of a volt higher, the metal with the positive charge will erode. In the example above, the stainless steel will actually pit or maybe even crack.

By comparison with the basic metals used on boats, zinc acquires a more positive charge than anything else, thereby protecting all other underwater hardware. (Magnesium typically replaces zinc in fresh water.) But zinc only protects metal that is part of what’s called a bonding system—a network of wires that interconnects rudders, shafts, trim tabs, and struts, as well as interior components, to a transom zinc or zincs. But as we all know, wire connections can go bad. So how can you be sure your bonding system is really working? The following four tests will help:

See a video of how to test for continuity, resistance, voltage potential, and stray current flow in a boat’s bonding system here

This article originally appeared in the June 2013 issue of Power & Motoryacht magazine.