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Discovering Chainplate Movement: Bolt Failure and Bulkhead Rot

Some problems on a boat announce themselves. Others hide in plain sight for years, waiting for the right conditions to become visible. This one falls firmly into the second category — and it only came to light because of an unrelated job Shayne had done not long before.

He’d recently sealed the chainplate at deck level with Sikaflex, closing up a gap that had always been there. With no clear reference point before, there was no way to tell if the chainplate was moving up or down relative to the deck. Once it was sealed, that changed completely. Any movement would now show clearly against the sealant line — and sure enough, it did. One chainplate had shifted. The other hadn’t yet, but the writing was on the wall.

The cause turned out to be a bolt problem that’s more common than it should be on production boats. The chainplate bolts were fully threaded rather than shanked — meaning instead of a smooth, solid shaft bearing against the composite bulkhead, the thread itself was in direct contact with it. Under the boat’s normal cyclic rig loading, that thread doesn’t just sit there. It behaves like a saw, slowly chewing into the bulkhead material with every load cycle. The boat had come with A80 grade bolts, which was the right call on material strength — but being fully threaded undid a lot of that advantage. It’s the kind of shortcut a production yard makes without necessarily understanding the consequence: they’d specified the right grade, just not the right bolt geometry.

The fix wasn’t as simple as just swapping to shanked bolts of the same grade, though. A fully threaded bolt has a smaller effective diameter at the point that actually matters in shear — the thread cuts the cross-section down from 10mm to something closer to 8mm at its narrowest. So before ordering anything, it was worth running the numbers properly. A fully threaded A80 bolt works out to a shear capacity of around 1.966 tons. Switch to a fully shanked bolt — even dropping down a grade to A70 — and that capacity jumps to 2.66 tons. The increase in cross-sectional area from having a full, solid shank more than makes up for the drop in material grade. In other words: geometry beats grade here. A shanked A70 bolt outperforms a threaded A80 by roughly 35%, purely because it isn’t sawing its own diameter down.

Fully shanked A80 bolts were the ideal target, and worth chasing down if available — a genuine upgrade over the original spec, which was very likely intended to be shanked from the start, before a production shortcut swapped it out. Getting the shank length right isn’t a five-minute swap either. Threaded stock bolts always come with too much thread and not enough shank, meaning every single bolt needs to be custom-cut to length. And there’s a right and wrong way to do that cutting: never cut a bolt to length with a four-inch angle grinder while the nut’s still threaded on the other side — the heat generated melts the nylon insert in a nylock nut, rendering it completely useless as a locking feature. The right approach is to cut with a hacksaw or file, then dress the thread back into shape with a sanding disc so it runs cleanly, before the nylock ever goes near it.

Before pulling any bolts, the rig needed to come off tension first — nobody wants a chainplate trying to rip itself out mid-swap. Loosening the turnbuckles, with clevis pins instead of split pins (split pins bend, scratch, and fall out — clevis pins are the cleaner, reusable option), and taking a careful reference measurement first meant the rig tension could be restored to exactly where it started once the bolts were back in.

Replacing the bolts one at a time, keeping the chainplate loaded and located throughout, is what led to the bigger discovery: delamination in the bulkhead itself, behind the stainless steel backing plate. Looking closely at the bulkhead, there was a visible line — dust and dirt that had worked its way in between the glass skin and the foam core over time, a clear sign the laminate had separated in that area. The question wasn’t just whether delamination existed, but how far it extended — whether it stopped at the backing plate or ran all the way across to the chainplate itself.

One piece of good news: there’s a Kevlar patch built into the layup at the chainplate itself, and Kevlar is genuinely excellent in shear tear resistance. It’s not impossible for a chainplate to tear out through it, but it’s very difficult — that patch was doing real, meaningful work here. The bigger concern wasn’t the chainplate tearing free outright; it was keeping the laminate skins properly in column. Once skins go out of column relative to each other, problems escalate fast.

The bolt holes themselves told the rest of the story. Where a bolt had chewed through the bulkhead over time, the original hole edge was still visible next to the now-elongated one — in one case, nearly a full bolt diameter’s worth of movement, around 10mm. That’s not something that happened recently. That’s years of slow, cyclic sawing, only made visible now because of the sealed reference point at deck level.

Faced with a real structural question — repair here, in Brittany, or push on and deal with it in the Canaries, or even the Caribbean — there wasn’t a single easy answer. What made the immediate decision possible was that the plywood core in the bulkhead had rotted away over time, but the fibreglass skins themselves were still fundamentally intact, and intact skins are the part that actually matters structurally. With the bolts upgraded to shanked hardware, the sawing action that caused the original damage was eliminated, which meant the situation could be stabilized without an emergency haul-out.

That’s not a green light to ignore rot in a chainplate bulkhead — quite the opposite. The reason it was reasonable to continue in this case came down to actually understanding the failure mode, not hoping for the best. Before making any final call, the plan was to go out and sail hard that same afternoon, loading the rig up more aggressively than the passage itself would demand. If the chainplate moved under that test, the trip was off. If it held, that was real evidence the interim fix was doing its job.

This is the beginning of a longer story — what started as a bolt swap turned into the first chapter of a full chainplate rebuild, which we cover properly in the carbon fibre chainplate series that follows.

Part of a series

This post is part of our chainplates deep dive. Read the full story here: Paikea’s Chainplates – The Full Story

Or explore the rest of the series:

In the Members Library

We’ve put together the complete photo build log for this one — from the first bolt that told us something was wrong through to the finished, painted chainplate. See the full build on the members page.

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