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Last week our friends at Performance metals sent along the following press release discussing the why of on board bonding systems. Its a short but informative read so I thought I’d share it with you all:
Bonding, in its simplest form, is internally connecting all the underwater metal parts of a boat such as thru-hulls, rudder stocks and shaft struts with copper wire. Along with the use of high-quality anodes, it’s an essential part of safeguarding a vessel against galvanic corrosion. Performance Metals Products, the manufacturer of Navalloy®, the state-of-the-art anode material, provides best practices for building or overhauling a bonding system.
When two or more dissimilar metals are connected together in water, either through direct contact or by wire, galvanic corrosion ensues. This is an electrical current that eats away at the least noble material such as aluminum, brass and stainless steel. Even two fittings of the same alloy can develop slightly different voltages and begin to oxidize. By bonding all the parts together, it ensures the anode becomes the sole sacrificial metal.
Anodes like Navalloy are what are referred to as “more active.” They’ll take on all the galvanic corrosion, being slowly eaten away while protecting the valuable fitting. It’s simply a matter of replacing the anode when it’s almost completely gone.
When designing a bonding system, the main conductor should run well above the bilge level to the boat’s common ground point at the engine block and negative post on the battery or main DC negative bus. At a minimum, #8 AWG insulated stranded wire should be used to connect all the underwater fittings. Internal electrical equipment through which water passes—engines, strainers, pumps and air conditioners—and metal fuel tanks and pumps should also be wired-in. The system typically terminates at a sacrificial anode mounted on the transom with through bolts.
It’s important not to daisy chain from one fitting to the next or use bonding wires as current-carrying conductors. Also, don’t connect through a part, e.g., into and out of two different installation bolts on one fitting.
Trim tabs should have their own anodes, rather than wired into the bonding system; likewise with shafts and propellers. The electrical connection through the engine and transmission is very poor because it passes through gears and bearings that are immersed in non-conductive oil. Shaft brushes, usually made of graphite, iolite bronze or copper, are also poor conductors and provide unreliable results. A slip ring assembly with silver brushes, however, can be effective, albeit expensive.
With steel or aluminum boats, the hull itself becomes part of the bonding system. Fittings made from metals different from the hull material should be insulated to prevent local galvanic corrosion.
Choosing an anode is just as important as a quality bonding system. Navalloy is a lightweight material that lasts 30% longer than zinc and up to four times that of magnesium, making it ideal for use in fresh, brackish and saltwater. Most include Performance Metals’ patented Red Spot Wear Indicator, an easy-to-see visual cue that it’s time to replace.
Bonding makes a safer boat. It protects underwater fittings that, if they corroded enough, could fail and create a serious leak. Since the bonding is connected to the common ground point, it provides a path for harmful charges from malfunctioning AC or DC appliances to be bled-off to the supply ground. It also protects against the internal collection of stray currents in a wet bilge that could cause an electric shock. Additionally, it provides lightning protection and radio noise suppression.
Contact Performance Metals, 2 Washington St., Cambridge, MD 21613. 410-228-2400. contact@performancemetals.com; www.performancemetals.com.

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