Compound curvature in plywood

Conventional wisdom says that plywood can't take compound curvature. That it can only be bent into conically developable sections.

That's not entirely true. The Starwind 860's amas have a bit of compound curvature, particularly in the forefoot. Here's how we create it.

With the hull panels stitched together at the keel, we start by clamping, wedging and otherwise forcing the plywood into a rough approximation of the curvature it needs in the sharply curved direction. Before going any further, we'll fillet and tape the keel joint. The rigidity provided by this joint is essential; the wire stitching alone is not enough to resist the weird twists and bulges that the plywood will try to create along the keel edges as we force the curvature into it later. We also rely heavily on the A-frames over the jig to provide the necessary pressure to keep the panels in place.

To torture compound curvature into plywood, you need clamps. A lot of clamps. The wood's going to fight back every step of the way, bulging out into odd shapes you'd never imagine possible, unless you force it to bend right where you want it.

The plywood can only take a very small amount of compound curvature. Here, the radius of curvature is 1 to 2 metres in the sharply curved direction, but closer to 15 metres in the gently curved direction. This is pretty close to the limit for 6 mm plywood, at least if you aren't using a steam box.

We pull it in with the clamps, a little at a time, using lots of screws into the hull's thick laminated gunwales to hold everything in place.

It's a surprisingly quick process, and with the (temporary) screws holding the panels in place, the hull can now be lifted clear of the jig.

The resulting hull is almost perfectly fair, needing only some slight smoothing on a few seams where our fibreglass work was less than perfect.



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