Building a Lightweight Composite Door: A Complex-Geometry Lamination Story

We work on high-end race boats and large performance multihulls, and that work takes us all over the world. When we’re not on a client’s boat, this is home — sailing, exploring, and steadily upgrading our Catana 42. Being composite professionals means a lot of what we do to Paikea involves carbon fibre, but not everything does. The current push is simpler than that: heavy timber coming out, lightweight composite going back in, and an enormous amount of sanding and painting in between.

Rerouting Wires and General Progress

Part of this week’s work was rerunning the wiring in the main cabin — previously routed under the bed, now going through new holes cut across to run under the steps instead, freeing up that space properly. Elsewhere, prep work continued for paint: pulling contents out of lockers, removing hatches, taping edges ready for bogging. The usual rhythm of a refit week.

The Door That Became a Lesson in What Not to Do

The door earning the most attention this week — nicknamed the spaceship door, though it reads more like a barn door depending who you ask — started as a genuinely good design problem. The bulkhead it’s fitted to is 20mm thick, and the door needed a specific gap tolerance around its edge: tight enough that you can’t see or pass through it, without a door frame reveal that would leave thin, sharp edges exposed. The latch mechanism driving the decision was a simple gate-latch style fitting — one moving part, two sliding plates either side of the bulkhead — which needed 20mm of door thickness to work properly.

The door itself didn’t need to be 20mm thick everywhere, though — it’s a lightweight door whose only real job is blocking sightlines, so 10mm would have done the job structurally. The solution was building the core at 10mm and adding a second 10mm strip around just the edge, hitting 20mm exactly where the latch needed it while keeping the rest of the door as light as possible. Diagonal geometry got added for looks. All of which, as it turned out, made the build roughly ten times harder than it needed to be — for a saving of 350 grams, about 25% of the core’s weight.

Why the Lamination Was Genuinely Painful

The honest version of this story is worth telling properly, because it’s a good lesson in itself. All that added geometry — the diagonals, the stepped edge thickness — meant the fibreglass cloth had to bend and turn constantly to follow the shape, and every ridge it crossed shifted the fibre orientation a few degrees off where it needed to sit. Getting the cloth balanced from one side of the door to the other meant starting at one edge, seeing how the fibres settled, realising it wasn’t right, pulling it back up, and starting again from a different point — repeatedly, until it roughly balanced out.

Getting the cloth to lay properly into every corner without bridging — the resin-filling gaps that happen when fabric doesn’t sit flush into a tight corner — took a rolled piece of tape, worked into each corner by hand to press the fibre home. One layer of 300g cloth on this door took close to two hours, against a job that should have been straightforward.

The one thing that made it possible at all was spray tack — specifically a product from Easy Composites, applied to both surfaces. It holds the cloth in place well enough to work with, but with just enough release that repositioning it doesn’t destroy the fibre alignment the way an overly aggressive spray tack would. Peel ply came with its own version of the same problem — it doesn’t conform to complex geometry any more willingly than the cloth does — but with one real advantage: peel ply can be cut and joined in as many pieces as needed without consequence, where the structural cloth itself was kept as one continuous piece deliberately, since joins in a skin this thin (0.3mm) make fairing considerably harder than it already is.

Vacuum bagging the whole thing added a final layer of difficulty — getting the bag to conform to all that geometry, pushing the transfer medium into every ridge and edge without bridging, since any bridging in those corners fills with resin and adds weight exactly where the whole point was to save it.

The Comparison That Says It All

The flip side of this door was laminated flat on a bench — no geometry, no complexity — and took five minutes to sand ready for fairing. The complex side took nearly a day, and because the skin is so thin, sanding had to be done carefully enough to avoid going straight through the laminate at any edge — a mistake that would mean a repair, which means a lump, which makes fairing harder again. A compounding problem, and a clear verdict: this specific build approach isn’t one worth repeating.

Elsewhere: The Engine Room Door and Ayden’s Ensuite

The engine room’s composite door is fully installed now, with one small upgrade over the original hardware — the barrel bolt fitting that came with it rattled slightly in its housing, so a 3D-printed replacement went in instead, fitting more firmly with less movement over time. Nearby, a backing plate is being templated for the winch base, and the old jib sheet track location is being cleared out, likely to be replaced with a longitudinal reinforcement once the surrounding hatches are dealt with.

On the other side of the boat, Ayden’s room has come out entirely — 67kg of furniture and fittings removed — and is being planned as an ensuite bathroom: a new composite shower pan floor, fully watertight, with the holes through the half-bulkhead into the engine room getting properly fixed as part of the same pass.

And on the side project front, Oliver’s been putting his own composite skills to work on a pump foil board of his own, currently waiting on resin to cure before it can be sanded — a nice reminder of where all this hands-on family involvement in the refit eventually leads.

The Build Series

For the full build series for our interior engine room door visit Paikea’s Doors: Building Lightweight Composite Doors from Scratch or explore individual stories below:

For a look at door design from a different angle — how a production builder handles the same structural and jamming problems at scale — it’s worth reading Boat Door Design: What a Gunboat 68 Gets Right alongside this one.

Follow the rest of Paikea’s interior transformation on the Paikea’s Refit page.