: A variant would be a classic Feathercraft-Yost structure with alu stringers,
: only the chines would have more stringers (14mm tubes could do) and would
: be fitted with thin plastic sheets turned over the stringers all along the
: middle part of the hull which is where you'd needed a round shape, or even
: further fore and aft, depending on the desired hull shape.

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Correction: not so thin plastic sheets...as they must of course retain some stiffness so as to curve around the bunch of stringers...like a batten over a series of points. Bendable ply or veneer could offer an alternative.