Bonding, Grounding, Cathodic Protection Interfaces, and Lightning Protection
Core concepts
Bonding, grounding, cathodic protection, and lightning protection are related but not identical. The AC safety ground provides a fault path; the DC negative is the return path; the bonding system ties non-current-carrying metal-engines, tanks, through-hulls, struts, rudders-to a common point so stray currents and potential differences cannot drive corrosion. Underwater metals are protected by sacrificial anodes (zinc, aluminum, or magnesium) selected for the water type; aluminum anodes are common in mixed salt and brackish water, magnesium in freshwater only. A galvanic isolator blocks low DC voltage on the shore ground while passing AC fault current; an isolation transformer breaks the shore ground connection entirely. Lightning protection uses a masthead air terminal, a low-impedance down conductor (typically 8 AWG or larger), a ground plate, and bonding of metal fittings to reduce sideflash.
Traps
Do not confuse bonding with grounding; do not connect the bonding system to shore ground through a low-resistance path; do not use magnesium anodes in salt water. A common exam trap is thinking anodes stop stray-current corrosion-they do not; stray current is a wiring fault. Another is assuming a lightning protection system guarantees protection. Also, more anodes are not always better; overprotection can damage aluminum.
Worked example
A bronze propeller shows pitting and rapid anode loss in salt water. Check for a missing bonding jumper, a failed galvanic isolator, and a DC leak to ground. Rule of thumb: bonding conductors for underwater hardware are at least 8 AWG, and each anode should be sized to protect the wetted surface it serves, not simply copied from the previous owner.