Aluminum Hull Used as a Bilge-Pump Return
A 28 ft aluminum boat has a 12 V system with a console negative bus. A bilge pump was added: its positive lead goes to a 5 A breaker, but its negative lead is terminated with a stainless self-tapping screw into a hull stringer. With the pump running, a clamp meter reads 3.2 A on the pump negative lead and 0.19 V between the hull and the battery negative post. The owner reports pitting around the screw and rapid zinc loss; the zinc anodes last about three months. Shore power is disconnected. Reference-electrode readings (Ag/AgCl) are: hull -0.62 V, zinc anode -0.95 V, stainless trim tab -0.15 V. The trim tab is not bonded to the hull. The boat has a bonded through-hull system.
1. Explain the stray-current path and the pitting. What is the correct repair?
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The pump negative is using the aluminum hull as a return conductor. Current leaves the battery negative, travels through the hull stringer to the pump, and returns through the hull to the engine or battery negative. The 0.19 V drop at 3.2 A shows the hull is part of the circuit. At the screw and at any bond to a more noble metal such as the stainless trim tab, current leaves the aluminum into the water, making those spots anodic; that causes pitting and rapid zinc consumption. Correct it by running a dedicated negative conductor from the pump to the console negative bus, removing the stainless screw, cleaning and inspecting the hull, and repairing any corrosion. Do not use the hull as a conductor. Then check all other accessories for hull-return wiring.
2. Interpret the reference-electrode readings. What does the hull-to-trim-tab difference mean, and how should bonding be handled?
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Hull -0.62 V is about 0.47 V more negative than the stainless trim tab at -0.15 V, so the aluminum hull is anodic relative to the trim tab and can sacrifice itself to it. The zinc at -0.95 V is protecting the hull, but the trim tab is a nobler, cathodic surface. Bonding the trim tab to the hull will bring them to one potential and reduce the localized galvanic difference, but it also increases the cathode area and zinc demand. First eliminate the stray current, then bond the trim tab to the hull's bonding system and verify zinc capacity and hull potential. Monitor for overprotection or rapid zinc loss. Do not bond as a substitute for fixing the hull-return fault.
3. How would you verify the repair and monitor the system afterward?
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After the repair, run the pump and clamp the new negative conductor to confirm about 3.2 A and no current in the hull path. Measure hull-to-battery-negative voltage under load; it should be near zero, not 0.19 V. With the shore power disconnected, take Ag/AgCl readings at the same points: the hull should be in the protected range and the trim tab should be close to the hull after bonding. Inspect the screw area for bright metal or white corrosion and clean it. Finally, track zinc consumption over several months; a sudden return of rapid loss means another stray path or a failed bond.