As with any boat with stainless steel chainplates extending through a cored deck, we worried about crevice corrosion where the ss stays wet in the deck.  We pulled the chainplates for an inspection and found tiny traces of crevice corrosion.  A machinist warned that crevice corrosion is sometimes like rot where it can be more extensive inside the metal than on the surface.  He said that if there is any surface indications of crevice corrosion to replace the chainplates.

After discussions with various naval architects, we decided to replace the chainplates with bronze.  We used Alloy 954 (aluminum bronze) which conveniently is available from McMaster Carr in exactly the same width as the chainplates.   Bronze is somewhat stronger than 316 stainless, and unlike ss, alloy 954 bronze is immune forever to wasting or crevice corrosion.  1/4 inch bronze plate is delivered slightly over 5/16 in thickness.  We just widened the deck slots to fit the new chainplates.  Alloy 954 bronze is hard, and requires a special process to bend.  Fortunately the chainplates on a Cal40 are flat bars and don’t require bends, just drilling of the holes for the bolts.

We reused the existing silicon bronze bolts after cleaning them by running them through a die when immersed in kerosene.  The existing bronze bolts cleaned up perfectly, and are valuable.  We made new backing plates out of G10.  We saturated the main bulkhead in epoxy when the plates were off and covered the bulkhead with a layer of 10 oz X-Mat with West Epoxy on both sides.

We asked our Naval Architect friends why SS is commonly used for chainplates when bronze is stronger, lasts forever, and the extra cost isn’t significant given the labor.  The answer was that sailors like things to be shiny.  We prefer never having to worry about crevice corrosion again.


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