Dealing with chainplate corrosion

From Ocean Navigator #107
July/August 2000
When we acquired the current Clairebuoyant, a 1981 Pearson 424 sloop, we knew that she needed attention. A friend who owns a sistership urged us to pay special attention to the port and starboard chainplates. It was advice we were glad we followed.

The author found considerable corrosion when he removed the chainplates from his Pearson 424. Here, one of the units had failed completely.
   Image Credit: Quentin Kinderman

The standing rigging on the Pearson 424 is typical of boats of her size, age, and price range. The forestay and backstay are of 3/8 1 x 19 stainless steel wire cable; port and starboard shrouds are of 5/16 similar cable. They consist of forward and aft lower stays and single upper stays through a single set of spreaders. Chainplates port and starboard are 3/8 by 1 1/2 inch stainless bar stock. They are bolted to fiberglass knees bonded to the hull. These chainplates penetrate the side deck inboard of the caprail. Bronze toggles and turnbuckles connect chainplates to mechanical eyes on the stay cables.

Superficially, the standing rigging looked to be in good shape. Except for some obviously inadequate chainplate covers that were aluminum, badly corroded, and covered with gobs of silicone caulk, nothing was obviously wrong.

Belowdecks, the chainplates were concealed behind teak plywood cabinetry. The first indication of trouble was the discovery that brass screws holding the plywood covers in place were severely corroded. Underneath, there was more corrosion. The chainplate area was damp and salt stained. Salt water had entered at the deck and run down the chainplates and knees. Foam-backed vinyl glued to the hull had soaked up the seawater like a sponge. The inside of the teak plywood covers was salty and delaminated. The chainplates were rust-stained. Obviously, they needed to come out for inspection. Still, I wasn’t too concerned. Even with four 3/8 holes cut in them, the chainplates were hefty pieces of metal bar. I didn’t think superficial rust would be a problem.

We had a surprise, however, after we unbolted the chainplates and pulled them through the deck. When I pulled on the portside aft lower stay chainplate, only the top half came up in my hand. It had appeared rusted but intact when I unbolted the cap screws, but actually it was broken in half through the second bolt hole from the top. Clearly, this was a matter of concern. Chainplates should be as strong or stronger than the stay they anchor. In this case, that is 5/16 stainless 1 x 19-wire cable, which has strength of 12,500 pounds. The smallest cross section of the chainplate is at the 5/8-clevis pin hole at the top. This still leaves two sections of 7/16 by 3/8 inch to support the load. Assuming tensile strength of 80,000 pounds per square inch of cross section, this chainplate should hold a load of 26,000 pounds at its weakest point. Moreover, although we were to discover additional problems at the mast end of this stay, there were no other obvious indications that the chainplate had been overstressed. The cap screws were intact, and the holes in the chainplate were not elongated. Unlike other chainplates, this one was not bent. Why then had this chainplate failed completely, and why had it done so across the second bolt hole from the top? It is apparent that this wouldn’t be the point of maximum stress. Thankfully, this left one cap screw to carry the load of the stay. Our first reaction, and I suspect the advice of most marine surveyors, was to replace the chainplates and address the problem of salt water entering through the deck. New chainplates were fabricated from 316-alloy stainless steel, thought to be more corrosion resistant and definitely more expensive than other alloys. Surprisingly, although the port and starboard chainplates had failed to keep out the water, the deck core was still in decent shape. Probably this was because the water could so easily enter the boat once it was through the deck skin. After the standard epoxy treatment at the holes, we installed the new chainplates, with new 316-alloy cap screws, washers, and nuts after cleaning out all the old hull liner and salt residue.

The alignment between the chainplates and the lower stays is not perfect. In order to seal out the water, the chainplate cover must attach to the chainplate but be able to cope with sidewise pressure that the stay imposes on the top of the chainplate. We addressed this problem by mounting our new stainless steel chainplate covers on fiberglass plates. The screws attaching the covers to the plates penetrate through the plates and extend about 1/8 inch. This holds the plates slightly off the deck and they can “float” on a layer of highly adhesive polyurethane caulk.

Quentin Kinderman, retired from the federal government, lives near Annapolis, Md. He has restored several boats and his current project is Clairebuoyant.

By Ocean Navigator