A window catch that holds the same squeeze all year
The frame swells in summer and shrinks in winter, so a fixed-throw catch crushes the seal flat in July and does not compress it at all in February. This one has a preload that stays nearly constant across a couple of millimetres of movement — and a cover that reads as a casting.
The problem
Opening windows on a wooden boat are held shut by a cam or toggle catch with a fixed throw. But the frame swells in July and shrinks in February, two or three millimetres across a coachroof window. The catch that compressed the seal correctly in May either crushes it flat in August or fails to touch it in October, and then the window leaks. Owners have accepted for a century that every catch needs adjusting every season, and most of them do not. The water goes into the carline underneath.
The fix
The preload sits neither in a coil spring nor in an adjusting screw, but in a pair of shallow arched beams in PA-CF — 34 mm span, 2.6 mm rise, 6 × 1.4 mm section — working past the point where they have buckled. A beam like that has a nearly flat force-deflection curve in that region: across 4.5 mm of stroke the force stays within about ±7 % of 210 N. So the cam delivers essentially the same squeeze on the seal whether the frame has swelled two millimetres or shrunk two. PA-CF rather than PETG, precisely because the plateau is a low-stress state where PETG would still creep out of it over a season. The beams print flat with the perimeters running along the span so the fibres carry the bending, and the whole mechanism prints in one piece at 0.35 mm clearance. The toggle goes over centre into a printed detent, so vibration cannot open it.
The cover is a separate part in ASA with a sand-cast surface applied as a displacement in CAD rather than as a slicer texture, printed at 0.28 mm layers with the layer direction laid along the draw direction a casting would have had. After filler, primer, a bronze base and a dark patina wiped back, the part reads as cast. The mechanism inside is modern and hidden. It is filler and a brush, not equipment.
And then the thing that has to be said plainly: industrially this is not an unsolved problem. Southco and Dirak sell self-adjusting compression latches that hold gasket compression constant with no screw to turn. What they do not do is look right on a 1938 wheelhouse, and they are not tuned to the seal section actually fitted to that particular boat. That last part is the product: the plateau is dimensioned against a measured compression curve for that profile. Creep across a full season is the open risk, and the only way to settle it is to leave a catch clamped up over a winter.
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