PS Advisor December 2007 Issue

Well Grounded

Bonding is better alternative than zinc fishing.

We recently installed an aluminum fuel tank on our 1984 Endeavour 40. During installation, we took several steps to avoid galvanic corrosion, including fixing gaskets at contact points, providing airspace around the tank, and connecting it to the boat’s grounding system. We considered connecting one of those "zinc fish" to the grounding system, hung over the side at the dock. Could I fasten a small zinc to the tank’s grounding tab? Or attach zincs to each tank and through-hull?

Peter Verne
Delancey, Endeavour 40
New York, N.Y.

Diesel Fuel Tanks
Even diesel tanks should have deck-fill fittings bonded to the tank.
Bonding the tank is an American Boat and Yacht Council requirement for several reasons, including electrocution prevention for shore-power-equipped vessels, lightning damage mitigation, and side-flash prevention. According to the ABYC, the boat’s bonding system, the DC negative system (which includes the engine block and battery negative) and the AC safety ground all should be connected and remain at the same potential. The resistance between any two components in this system should not exceed 1 Ohm. (It’s important to note that any bonding wire attached to the engine block must be sized to safely carry full engine cranking amperage.) Bonding the tank minimizes the likelihood of damage caused by stray current corrosion, and it prevents static electricity build-up on or in the tank, which could lead to a spark and explosion (admittedly unlikely on diesel installations). If the tank is bonded and the bonding system is properly attached to an underwater hull zinc anode, then this anode may provide some corrosion protection to the tank, if the tank were immersed in water, or if it had water in it. Grounding the tank, however, is unlikely to prevent other corrosion that attacks aluminum, the most notable of which is poultice corrosion. This occurs when aluminum is exposed to oxygen-depleted, stagnant water—like that found in bilges—or when water is held against a tank through contact with wet surfaces such as wood, or insulation (see "The Ideal Fuel Tank," May 2007).

The zinc fish falls into the "it can’t hurt" category. The fish’s connecting wire is typically stainless steel, a comparatively poor conductor, and connecting it to the boat’s bonding system with less than 1 ohm of resistance is challenging. You are much better served by a permanently installed hull anode that’s well connected to the bonding system using properly terminated, high-quality, tinned 8-gauge bonding wire.

Attaching zinc anodes to the tank would be ineffective unless the tank and the attached zincs were submerged. You could give each piece of underwater metal hardware its own underwater zinc, but a properly installed bonding system that utilizes a central zinc anode is easier. Just be sure the connections and wiring are sound and corrosion-free. Formulas for determining just how much zinc anode surface is required for any given vessel are available from the ABYC or naval architect Dave Gerr’s book, "The Elements of Boat Strength." For more bonding system details, see Charlie Wing’s "Boatowner’s Illustrated Electrical Handbook."

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