| The only time this can be done is when the finished seam angle will be a straight line.

Why can it not also be a simple curve?


As you point out yourself these are from box moulds,
hence the hull halves can be anything you can achieve with the box mould.
The ends BTW are usually carved (or otherwise) found forms.

Do you mean to suggest that a proa does not ride well for its size?

Its a good question, field evidence has it that longer hulls pitch less, and here we seem to be talking longer in terms of 33%. But also the bow offset of proa hulls is likely to reduce that further. Besides there are hydrodynamic fixes for pitch and pitch oscillation, just because catamaran builders rarely make the effort to control pitch doesnt mean it cant be done.