Chasing Down Corrosion

The bronze underwater components on my boat — rudders, struts, prop — are showing major corrosion. It looks like the bonding system is also attached to the motor block. Is that OK or a no-no?
H.T., via

While the terms are often used interchangeably, there are differences between grounding and bonding systems, both in their installation requirements and the goals they’re trying to address. Bonding is an attempt to prevent corrosion; grounding is an attempt to prevent electrocution and fire. The short version of all this hubbub is that each of the various systems — DC negative, AC safety ground and the bonding system — are required to be connected as one in order to be ABYC compliant. The disagreements arise, however, from exactly what components should be bonded or connected together.

When a boat owner says he has a bonding system, that generally is understood to mean that all of the underwater metal components — through-hulls, struts, rudders, propeller shafts — are interconnected to the grounding and bonding system, which normally includes the engine.

The first component I’d ask about are your sacrificial anodes (aka zincs). Are they installed and maintained? In other words, do you have them checked regularly to ensure the bronze components are always protected? If you installed zincs during a haulout three years ago and now find they are gone and corrosion is evident, that shouldn’t come as much of a surprise. As the name implies, sacrificial anodes waste away to protect a boat’s precious metals. It’s what they do.

You should have some idea of what the normal rate of wastage is for your vessel’s zincs and plan to renew them accordingly. The goal is to replace them before they are used up and no longer provide protection. Their rate of wastage will depend on a number of factors, not only with your boat but its surroundings as well. Issues such as faulty wiring on a neighboring boat or even the dock itself can greatly accelerate the rate of zinc wastage.

If you are unsure how long your zincs last, I’d recommend having new installations checked every 30, 60 and 90 days, to gauge the rate of wastage. This is also good advice if you move into a new slip or marina. Keep in mind that this rate can change if situations around your boat change (e.g., a new boat with a wiring issue moves nearby).

If you are burning through zincs at an unusually high rate, contact a marine surveyor to have a corrosion survey conducted on your boat. This will help determine where the problem originates — your boat, a neighbor, something else.

Finally, there is a lot of debate on the benefits of bonding in general. My personal opinion is to protect with anodes as required but not to bond together throughhulls, props, shafts or any other immersed metal that can be electrically isolated. — FL


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