Nice pictures. My Shearwater 17 has that sort of curvature in the bow and the stern.  I think the problem is not the inward curvature, but the fact that it is not curverd that way on both sides. I think the curvature is caused by the large amount of twist in the panel. So I would either want to induce that in the other panel or eliminate it in this one. 

Water is not going to work. Plywood is great stuff because it is dimensionally stable.

I would try another bulkhead or even just a "stick" of wood, but to change the shape I I think you are going to need to relax your copper stiches, especially along the bottom keel chine. In fact re stiching could solve the problem entirely. Although, as I say that the pictures look like you have already epoxy tacked that keel.

If you are going to paint the hull - do nothing now and change the shape with some thinkened epoxy will a lite weight filler like micro balloons later.

I have use the wire across the inside of the boat to induce reverse curves in the panels (I think they are very cool and fast looking) - this might work if you would like the other panel to also have a reverse curve. I would back the wire with a piece of 1/2"x 1/2" x X would on the outside to spread the pressure.

Ok thats my 2 cents, three different ideas - maybe I have created more confusion?

Joel

You can also try cutting the tacks closest to the deformation and see if the wood relaxes back into the fair shape naturally. If it does, or if you can push it into place and have it stay, then just tack it back into place. Otherwise go with the sticks and wires.

Good luck,

Laszlo

   Thanks guys! I will try to fix it. That place is very stiff. Even if I install an extra bulkhead I will not be able to compensate the curvature completely.

i had to do some 'un-tacking' and re-aligning on my recent shearwater build.  

if you want to cut a tack, i would recommend doing it in conjunction with a heat gun.  heat the area to make the epoxy soft (don't burn the wood - work from the inside if possible) and you can then use a sharp knife to cut through the tack which will become like taffy when heated.  if you are going to do it....take lots of the adjacent tacks off.

if the ply was not obviously warped prior to starting...i bet you will be able to work it out using some of the techniques suggested above.

that said, all of these boats will have slight imperfections...so make sure to focus on the 'big' issues and not get caught up in minor things that nobody will notice and won't impact strength and performance.   fwiw...the big things for me are hull 'keel-line' being straight (easy to see), no twist in the hull (harder to see)  no signficant asymetries in the hull form right-side vs left side is also on my list (hardest to see).   

if you can't easily see the problem, you are probably fine.  that said, based on your description, your problem is an assymetry.  with regard to root causes, asymetries are typically linked to the following  - the bulkheads are assymetrical or more likely, the panels were not symetrical before stitching or the panels were not stitched sysmetrically, or the materials have different bending properties left vs right side (not typically likely with marine grade ply).

once you confirmed your bulkhead is not the problem, re-stitching can address most of the other problems.  the focus in restitching should be on getting the hull form in the best shape...not getting the tightest fit between panels.

howard

As was suggested here I have un-tacked and slightly realigned the problematick part. I have not achieved the desired result... The question is whether or not this imperfection will be reflected on the overall perofrmance of the kayak? Will it be steady on course etc. ?

if your keel line is basically straight, there will be no discernable impact of your performance.  :)

Even if the keel line is off, it has to be quite alot. On my dad's wood duck 12, our first build, we were slightly off when we started wiring up the bottom panels. We didn't catch our mistake until the whole boat was filleted, taped and glassed on the inside. When we flipped it over to do the bottom, we noticed that the stern was slightly off one way and the bow the other. Maybe 3/16 off from the center line. No noticeable performance issues. Only me and my dad, and now all of you know about it. Nobody notices it and still paddles way better than a commercial boat. good luck, JRC