Systems+Engines

Bronze Bondshells

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Jim Fischer of Ellicott City, Maryland, asks:

"All the bronze seacocks aboard my 1979 fiberglass 34ft sloop are connected to a keelbolt, along with the boat's lightning ground system, which is also connected to a keelbolt. Is this still the recommended arrangement, or do you think all the seacocks should be isolated from each other and not connected to a keelbolt? Can I assume that the lightning ground from the shrouds and mast should remain connected to the keelbolt?"

Nigel Calder replies:

Your question is a good one. Frankly, this is a thorny subject. From a lightning-protection point of view, it is generally accepted that the more underwater metal you connect together, the better. However, in my view the science on this point is not very strong. From the perspective of galvanic corrosion, on the other hand, it is clear that tying in those bronze through hulls will accelerate the zinc loss on your anodes. If the zinc gets used up, the bronze will start to prey on other metals, like your stainless steel propeller shaft.

If it were my boat, I would un-bond the through hulls, except the ones that are submerged in bilge water inside the boat. In those cases you should keep them bonded together because this can prevent another kind of corrosion - stray current corrosion - which can be even scarier than the accelerated zinc loss you might get by bonding the through hulls.

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