VHF antenna mounts and Electrolysis

[JVannice]

As it happens I recently delt with a vessel with a major corrosion issue related to the VHF ant bracket... The vessel's keel coatings had a history of failure all the way back to delivery from the dealer. It is my understanding that the keel was striped and re-BC/bottom painted once by the dealer's agent and twice by an indepentant company (at owners great expence). I was on hand when this vesel most resently hauled out and much to the frustration of those involved...the BC was blistering off the keel. I noted that the lead in the blister locations was bright and shinny an indication that the keel was part of an active galvanic cell. I had the vessel relaunched and perfored a corrosion survey of the vessel. The vessel was set up with a factory lightning suppresion system; mast, shroud chainplates, stay chainplates, and arch all bonded to the keel. The rest of the vessel was unbonded ie zincs on shaft and no electrical connction to any other underwater hardware suposidly. In conversations with the factory electrical enginer he confermed that this is how the vessel was ment to be set-up: electricaly isolated lightning bonding system and engine/prop shaft/ DC neg bonding. Unfortunatly no one told the riggers... The VHF bracket was rivited to the mast, the ant was attacted to the bracket using the supplied nut on the grounded connector, and the cable screwed onto the ant. This created an electrcal path from the keel, to the mast, to the bracket, to the ant, to the cable shield, to the VHF radio, to the DC negative, to the engine block, to the prop shaft. this in turn allowed a paralell galvanic cell to form causing the keel to become cathodic in relation to the shaft zinc and leading to the repeated disbondment of the keel coatings.
After the keel was repared and the vessel in the water again I went aloft and installed plastic isolators to the ant. When I resurveyed the keel for galvanic activity I found the keel to be effectivly isolated and now "inactive." I hope you all can appreciate the irony of such a small item, as far from the keel as you can get causing such an ordeal.