Building a Carbon Fibre Seagull Striker and Martingale
By this point in the front end build the beam and longeron were in, the centreboard and rudders were painted, the saildrives were out being serviced. The next job was the seagull striker and martingale — the compression strut and the tension strap that together stop the longeron from deflecting downward under forestay load.
The question Shayne had been turning over was whether to bother. The beam was theoretically strong enough without them. The answer came down to where the load was actually going. The weakest point in the whole assembly isn’t the beam itself — it’s the junction between the beam and the longeron, because the longeron doesn’t run continuously through. There’s a discontinuity there, an angle, two sections joined rather than one piece running through, and that junction sees a lot of load. To put enough carbon into that junction alone to make it strong enough would have taken more material than the entire seagull striker and martingale assembly. Add the truss and you get a stiffer beam, a lighter beam, and the junction is no longer the weak link. It was more work but it was the right answer.
The seagull striker went on first — glued to the top of the longeron, set up carefully so it sat at the correct angle in all three directions before the glue went off, because once it’s cooked you’re not moving it. Then the martingale.
The original martingale was 12mm stainless steel wire, just over five kilos. The replacement is a solid unidirectional carbon strap — just over one kilo. Twenty layers of 300 gram uni, fibres all running the same direction along the length of the strap to maximise tensile strength. Shayne laid it up against an aluminium frame and clamped it rather than vacuum bagging — if you can clamp it tightly enough to consolidate the laminate properly you don’t need the bag, and the frame gives you a straight edge to work to. Six millimetres thick, twenty millimetres wide, trimmed and radiused after cure.
It’s quite flexible out of the mould — long and thin, it wibbles. That’s fine. Once it’s slotted through the beam, bonded at top and bottom, glued to the underside where it exits below and to the top where it exits above, it becomes a structural part of the whole assembly. The load path runs from the forestay attachment, through the longeron, down through the seagull striker in compression and the martingale strap in tension, out to the ends of the beam and into the hulls. A proper truss.
The foam wedge sections where the strap passes over the spreader tip were shaped and glassed separately — double bias over foam, slot cut after cure to drop over the striker. Shayne had been piecing together enough uni from what was left in the loft — some of the material set aside for the job had gone walkabout — but there was enough.
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The full front end build is documented in the series — from the first beam sections through to sea trial. If you want the detail behind the engineering decisions, the members’ technical library is where we go deep. Join here.