I had the same problem and decided to forge ahead.  First I tried loosening a few wires to make the arch smooth, but that left me with a gap between panels that looked even stranger. After I wired and tack-welded the deck to the hull, it all fit pretty well.  It took a couple of straps to get the arch out during tack-welding.  The deck, however, did overhang the hull by up to a half inch in spots.  Since the overhang was symetrical. I accepted it and planed it off after everything dried.

I just happened to be in the CLC shop yesterday and saw  a Duck 10 under construction. Its deck assembly was laying out on a table and it too had an s-shaped just as you describe. So that's at least 3, including one being built by the Masters.  I'd say that it's not a problem, just an intermediate stage on the way to a good fit.

Laszlo

Laurie,  the wave did come out as I wired the deck to the hull.  Like you my deck overhngs the hull near the stern by as much as 1/2 inch.  Strangly it is worst at the deck and hull forms where it should have the best fit if the forms are sized correclty (it is as if the deck forms are too wide).  In either case did you plane off the extra once the deck as glued to the hull or before that step.  I think sizing it prior to gluing would make maintaing a clean arch very hard.  Thanks.

I waited until after the deck was glued to the hull before planing anything.  When you remove the forms before tack welding it together, the deck will tend to want to spring back to it's flatter shape move a little bit, so planing before that step could be a little risky.  Make sure that the overhang is symmetrical before planing or something could be out of alignment.

Thanks.  This should be a similar process to trimming the deck on the chesapeake 16 I built last year.  I never did get the stern perfectly aligned, but the little twist in the aft end should give it some character.