Dave, I'd start tightening your stitches from the center backward and forward, keeping an eye on how things align along the way.  Could be the 90 degree twist you refer to that's pulling things askew.  If you get 3/4's of the boat stitched up and still have a noticeable issue, then I'd take some pictures and come back for more help.

Good luck,

Larry

Dave --

I think Larry's advice is exactly right.

I would supplement it by suggesting that you don't draw the stitches at all tight until you have them all in place, and then start tightening them little by little in stages, again working from midships to the ends, taking care to adjusting stitch tension as you go to correct any tendencies away from fair and plumb. Do it carefully and slowly, tweaking the stitch tension here and there as you go, and I think you'll find that you'll resolve the problem as you proceed.

  -- Jim

Larry,  Jim :

Many thanks for the tips.  I'll get going later today and report back.

Dave

Larry and Jim

I eventually unstitched the sternmost four feet of the boat.  I then planed a more pronounced chamfer along the inside contact edges of the bottom panels about 18 inches from the stern (the area giving all the trouble).

After this I used some brute force, instead of the usual 18 gge copper wire I had some 14 gge (1.62 mm dia.) Using this I re-stitched at selected points whilst the stern was clamped with two three inch G-cramps.  I was able to put considerable pressure on the thicker wire, without fear of it snapping or tearing through the ply.

It worked.

Dave

Dave, most excellent.  Time to mix some thick epoxy and start "welding" that boat together!

Larry