Chainplate crevice corrosion: how surveyors inspect chainplates and when to replace them

Chainplates carry the rig's loads from the shrouds into the hull. Most are stainless steel, and stainless corrodes where it's starved of oxygen and sitting in salt water: in a crevice. On a chainplate that crevice is the slot through the deck, under sealant that has long since failed. Capt. Wayne Canning, AMS, writing in Blue Water Sailing, calls chainplate failure one of the leading causes of dismasting on older boats.

How a surveyor inspects them

On deck, the surveyor looks at each chainplate where it enters the deck: rust stains, cracked or missing sealant, and whether the deck round it is soft. Below deck, the chainplate, its bolts and the knee or bulkhead it's fastened to, with a light and a magnifier. Canning lists the signs as rust stains at holes and welds, cracks, elongated bolt holes and leaks.

None of that shows the section inside the deck. The Rigging Company is plain about it: the only fix is to remove the chainplate, clean it and inspect it, with dye penetrant or x-ray where the eye can't settle it. A pre-purchase survey rarely includes pulling chainplates, so the finding says what was seen and when removal is due.

How often

The Rigging Company advises lifting, cleaning and resealing cover plates every two to three years. Canning advises inspecting annually and pulling chainplates every 5 years once a boat is over 10 years old. Glassed-in chainplates are the hardest to inspect or replace, and worth saying so in the report.

What replacement costs

Wet core round a chainplate slot is common, and it turns a rigging job into a deck repair. Check the deck with a meter at the same time. See wet deck core.

Photos to take

Each chainplate at the deck, a close-up of any rust or crack with a ruler, the below-deck fastening, and the bolt holes if any look elongated. With Axsail, name the chainplate as you photograph it and the photo goes on its finding.

Writing it up

Name the chainplate by side and shroud, say what you saw at the deck and below, whether it moves under load, and its age if known. If you couldn't see the section through the deck, say so in the finding, so the reader knows the limit of the inspection.