Corrosion Quiz: Question 2 Answer

The zincs on a boat are lasting only two months.  With a new set of zincs installed, the hull potential is -986mV.  When the radar unit is turned on it reads -761mV.  What is the problem?

A)  Radar is set to wrong range.
B)  House battery is weak.
C)  Galvanic isolator has failed.
D)  Hull zinc is too small.
E)  Radar unit is miss-wired.

The correct answer is E.


This was an actual problem and unfortunately one that's fairly common with improperly installed electronics.  The boat's ground return for the radar unit was attached to the boat’s bonding system – not the boat’s DC power ground – creating a voltage drop (IR) on the boat’s bonding wires.  This +225mV shift in the boat’s hull potential caused the zinc anodes to experience accelerated corrosion. The solution was to remove the radar’s power ground wire from returning to DC ground via the boat’s bonding system.  The important lesson: Never use your boat’s bonding system as a DC power return to ground. Otherwise, serious electrolytic corrosion to your underwater metals can result.


DC Electrical Wiring -- Precautions Against Corrosion

Every vessel device powered by DC should use two power wires, a positive (supply) and a negative (return). The negative must not run through the frame of any device, through the hull, or through the vessel’s bonding system.  It is preferable that engine alternators, starter motors and other DC powered motors (e.g., freshwater pump, windlass motor) have an insulated negative terminal rather than a metal casing connected to the battery negative. This ensures that high amperage DC currents cannot pass through the vessel’s bonding system. In vessels that have a negative ground engine system in which the starter motor, starter solenoid, and alternator are single pole devices using the engine block as the DC ground return conductor, it is extremely important to connect the engine block to the battery negative with a heavy battery cable and connectors to ensure no DC voltage drop occurs.  


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