Daggerboard Load Paths: Why Bulkhead Position Matters More Than Size
Two genuinely different problems came up working around Paikea’s daggerboard case this week: how to support a floor over geometry you can’t physically reach to laminate, and why a bulkhead’s position matters more than how much material is in it.
The Problem: A Floor With Nothing to Land On
Where the new floor meets the daggerboard case, there’s currently nothing underneath it to bond to — no flange, no ledge, nothing but open space. One option is a timber cleat glued to the side, giving the floor a simple edge to rest on. The better option, and the one used here, is a return flange: essentially building the underfloor support as its own separate piece, laminated where it’s still accessible, then installing it as a finished unit before the floor goes in permanently.
The reasoning is straightforward once you’ve tried the alternative — running tape and coving directly under a floor, reached only through a small hatch, is genuinely difficult. A return flange sidesteps that entirely by doing all the awkward lamination work while everything’s still open, then presenting the floor with a clean, solid surface to glue straight onto.
How a Return Flange Actually Gets Built
The build starts with an offcut from an earlier piece of floor, which conveniently already has the correct flat geometry at the right level, sitting on the same side cleats the floor itself will use. That offcut becomes a mould, not a functional part.
Getting it to match the daggerboard case’s actual shape meant working through some genuinely rough factory geometry — an original cutout that didn’t follow the case’s profile well, requiring foam blocks packed in to fill the gaps. A release tape went onto the daggerboard case itself, then the whole cavity got backfilled with bog, taking a direct moulded impression of the case’s actual shape. Once that cured and released, the result was a custom-fitted mould, ready for lamination.
From there, it’s straightforward composite work: coving into the corners, taping the mould to the case shape while it’s still fully accessible, then releasing the finished flange once cured. What’s left is a strong, pre-built ledge — in this case laminated with three layers of 1,200g glass on each face where it needs real strength, dropping to two layers higher up where it’s carrying nothing but the floor itself. The finished flange came in at roughly 2.5mm thick and 65–70mm wide, considerably stronger than it strictly needs to be for a floor load — deliberately so, since buckling isn’t a real risk here given the short span and the fact that the flange gets glued directly to the floor once installed, locking it in column either way.
Why This Isn’t Just About the Flange
The bigger engineering question underneath all of this is where a daggerboard’s actual load goes. The daggerboard case itself mostly just keeps water out — the real load transfers into the hull at the point where the case meets the bottom of the hull, plus a bending moment that needs somewhere sensible to go from there.
There are two broad approaches to handling that bending moment. One is a large bearing block at the base, spreading the load over a wide surface — something like 150mm deep. The other, used here, is transferring the load into the surrounding structural frames, provided those frames actually connect to the right part of the daggerboard.
That “right part” matters more than it sounds. A daggerboard, like any foil, is thickest at roughly 30–40% of its chord — the distance from leading edge to trailing edge — not at the halfway point most people would guess. That thickest section is where the daggerboard carries its own internal structure and shear webs, and it’s the deepest, stiffest part of the foil. Tying the surrounding hull structure into that specific section, rather than anywhere else along the case, is what actually lines the load path up correctly.
Fixing a Bulkhead That Missed the Point
Checking both sides of the boat against this principle turned up a real discrepancy. On one side, a single stringer connects correctly into the bulkhead sitting right at the daggerboard’s thickest section — exactly where it should. On the other side, two smaller half-bulkheads had been positioned either side of that same high-load section, missing it entirely rather than tying into it.
The fix was straightforward once the problem was identified: remove both half-bulkheads, and replace them with a single new bulkhead copying the correctly positioned side — laminated in with two layers of 600g tape plus two layers of 1,200g, transferring load properly into both the bulkhead and the inner hull skin. Deliberately built stronger than strictly necessary, given there’s real load moving from the daggerboard case into the surrounding hull structure through this connection.
Where the Weight Numbers Stand
Checking in on the ongoing weight tally: Ayden’s room accounted for roughly 28kg on its own — the floor plus an old plastic cable tray — with a couple of small half-bulkheads still to weigh on top of that. The running total sits at 346kg removed so far. Averaging roughly 100kg out of each room across the boat puts the eventual total in the region of 1,000kg, with the galley and its solid timber table still ahead and expected to be the single biggest contributor yet.
In This Series
See the associated post on Daggerboard Crash Box: Building a Watertight Safety Compartment. This crash box is one piece of the daggerboard work on Paikea. For the full picture — how the case is engineered, the load paths, and the rest of the build — head to the Daggerboards field guide.
Follow the rest of Paikea’s interior transformation on the Paikea’s Refit page.