Rotten Bulkheads to Bulletproof Chainplates| youngbarnacles

Building Carbon Fibre Chainplates: Part Two — How It Actually Stays In

Part One showed the build. This one answers the question that came in from almost everyone who watched it: how does that black carbon strap actually stay attached to the boat? It’s a fair question, and the answer is worth understanding properly, because it’s really a lesson in how composites work more broadly, not just this one part.

The old system was a stainless steel strap chainplate held on with bolts — several separate hard points, each one transferring load individually into the fibreglass and plywood behind it. The new system does away with all of that. It’s a single integrated piece: carbon fibre unidirectional, laminated directly into the structure rather than bolted to it.

The way it stays attached comes down to one core composite principle — distributing load away from a hard point and spreading it over the largest possible bonded area. At the top, near the stainless steel toggle and pin, everything is narrow and the load is concentrated. As the carbon fibre runs down into the bulkhead, it widens out, and each individual fibre picks up a share of that load. That widening creates a large bonded surface area, and it’s that area — not any single point — that actually holds the chainplate in.

The carbon isn’t just glued onto bare foam, either. The bulkhead first got a layer of E-glass double bias laid over the foam core, creating a proper bonding bed. The carbon strap was then bonded to that E-glass layer — not directly to the foam, which wouldn’t give a strong enough bond on its own. Once the carbon was in place, a second layer of E-glass double bias went back over the top of it, sandwiching the carbon between two layers of glass. That sandwich effectively doubles the surface area actually gripping the carbon, since now both the layer underneath and the layer on top are sharing the bond, rather than relying on just one side.

If you want to see this same bonding principle applied in a very different context, our post on the inside of a modern carbon race boat covers a chainplate bonded directly to the hull rather than a bulkhead — same load-distribution logic, different structural application.

Compare that to the old bolted arrangement. Seven or eight individual bolts, each one a hard point trying to transfer load into fibreglass and plywood that isn’t particularly good in bearing — meaning the material around each bolt has to resist being crushed or torn without a lot of margin. It’s not an elegant way to move load, because each bolt is fighting its own small, concentrated battle. The new bonded system instead relies on the glue’s shear strength spread across a large area — and the bigger that area gets, the less load any single part of the bond line has to carry.

The foam core inside the new bulkhead section does no structural work at all when it comes to the chainplate — that’s worth being clear about, since it might look like the whole assembly depends on it. The old plywood core mattered because it had to bear directly against the bolts. The new carbon system doesn’t need that at all; the load goes from pin, into carbon, into the fibreglass skin — which is the genuinely strong part — and from there out into the surrounding hull structure: the topsides, the deck, the whole shell working together to absorb it. The foam’s real job here is a completely separate one — stabilizing the panel on the side of the boat so it doesn’t flex, which is the bulkhead’s other essential role beyond carrying the chainplate.

Splicing the new bulkhead section into the old one mattered just as much as the lamination itself. The foam core was simply butt-jointed — edge to edge, no fancy tapering — because foam carries almost no structural load here beyond a small amount of shear resistance between the two skins, stopping the bulkhead from wobbling front to back. Twenty millimetres of foam handles that easily. The skins are the part doing real work, effectively forming a shear web — and ideally that shape would run unidirectional fibre the full length of the cutout, turning the whole thing into something like an I-beam. The plus/minus 45 fibreglass orientation used here isn’t the strongest option for that kind of longitudinal tension, but it’s stiff enough for the loads this bulkhead actually sees.

What mattered most in the splice was making sure the fibreglass skin from the new section properly overlapped and transitioned into the old bulkhead — not just butted against it, genuinely bonded across a shared overlap, so load could transfer cleanly from the new laminate into the old structure. Getting that bond right meant real attention to preparing the old bulkhead surface before laminating. The fibreglass also wraps around the corner into the hull topsides, transferring load sideways as well, and continues up into the deck itself. That’s a deliberate mechanical safeguard: the deck physically prevents the bulkhead from ever being pulled out the top, but the lamination still needed to carry that same load path structurally, not just rely on the deck being in the way.

One question that came up a lot: why not build the whole replacement section out of carbon, since carbon was already being used for the chainplate itself? The honest answer is that it wasn’t worth it. The weight saving from making the surrounding bulkhead structure carbon rather than E-glass would have been marginal, and to actually take advantage of it, the whole bulkhead’s skins would have needed to go lighter too — which wasn’t the goal here. There also wasn’t enough carbon on hand to do the whole job without buying more, purely to save a small amount of weight in a spot that wasn’t asking for it. Not every part of a build benefits from carbon just because carbon’s available elsewhere on the job.

On safety margin — a question that came up just as often — this chainplate was designed to a 10:1 safety factor as standard practice, deliberately over-strong so that if anything in the system ever did fail, it wouldn’t be this. In practice, the actual margin ended up considerably higher than that, partly because the wall thickness was sized to match the visual proportions of the stainless steel toggle at the top rather than being trimmed down to the structural minimum — a case where aesthetics and structural design happened to align, rather than one compromising the other.

The real proof came from actual sailing, not just the calculation. A hard sail not long before filming had the hull skimming the water with the rig fully loaded — genuine full working load on the chainplate, not a theoretical case. It held without the slightest concern. That’s the kind of validation that matters more than any number on paper: knowing, from lived experience under real load, that the part does exactly what it was designed to do.

There’s a broader point worth naming here too — one reason this whole project was possible at all is that this bulkhead, and the areas around it, are fully accessible: no boat liner, no built-in furniture hiding the structure away. That access is exactly why the choice was made to paint the interior rather than replace liners during this refit — a boat you can actually get to and properly service, long-term, is worth more than one that looks finished but can’t be reached when something like this needs attention.

Part Three covers the starboard side repeat of this same process — including what changes second time around once you already know exactly what you’re doing.

In This 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.