Are the panel edges supposed to be beveled? That would affect the panels fitting together.

Laszlo

It's a ballet; every stitch is carefully placed then tightend just enough to hold things in a position that's close to what will be final yet not so tight that it throws those on either side off... too much.

As a first-timer (to S&G building) I was put off at first 3 years ago as I started my first build. Questioning the designer of the kit I'd purchased, about how this all works, got me the reply that "1/8 is close enough..."  which I took to mean 'do the best you can but in the end don't fret the last 1/32"' 'cause it doesn't have to be that close to work as you get to the later stages, which proved true... eventually.

The inner plank edges are what you need to be focused on, getting them to touch one another. If your kit features 'lapped' planks, there's a rabbeted area on the lower edge of some planks that are milled that way so the outside edges of the lower planks can be fitted under the planks above in some places they're meant to overlap but not to double the effective plank thickness at those points.

Pretty much it's inside edge to inside edge everywhere else: the gaps that become evident will be filled with thickened epoxy before the hull shape is faired before finishing.

I'd concentrate on stitching the panels together first, then add the bulkheads. Take up the panel stitches 75% before adding bulkheads, then stitch those in place maybe 60% tight, then return to the panel stitches for adjustments as needed, taking them to 90% before taking the bulkhead stitches to 90%... a little here, a little there, having faith all the while that there have been many of these kits made and assembled before yours so there's little doubt the bugs have been worked out before your kit was cut.

There will be times when you have to drill more stitch holes, when there just isn't enough strength in the 18 ga. copper wire to pull things into alignment. That's no big deal, the same with taking whatever means you can contrive to 'move' planks into a close-eough fit by some other means like clamps that the stitches aren't doing all the tensioning work by themselves. Panel edges can take just so much before the ply will tear, so don't trust stitches alone to bring panel edges into proximity as those panels are being "stressed" into place.

The last stage of stitching is when you use a tool like a flat-bladed screwdrive or the jaws of a pair of pliers to push the inner section of each stich into contact with the insides of the panels. You don't want to do that before you're satisfied with the shape of your hull overall, but it serves to put a little more tension against the inner surface edges where the interior is meant to be essentially gap-free when it comes time to start filleting the joints. Twisting the outsides of stitches can be taken only so far before either the wire breaks or it rips out of the panel edges.

Take it slow. You'll get there. Trust the system.

Laszlo makes an important point: if the Shearwater Sport instructiions indicate the builder is supposed to bevel panel edges before stitching, you should take note.

I've bullt all of one S&G kit and its panels came square-edged, hence my comments about the inner edges should just touch as stitches are tensioned.

If you try again and again yet not end with a sensem of 'rightness' that allows you to proceed with confidence, by all means contact CLC's excellent tech support folks to help guide you to a solution.

This forum has a wide audience of builders yet some designs may have details that frustrate some builders while other designs are less troublesome. CLC's folks know them all better than any of us out here so they're best able to help you complete your kit with satisfaction.

Having only built one boat I probably can't add too much more wisdom, but given that my one boat was also a Shearwater Sport, and one that I recently completed, I can relate closely to where you are in the process. A couple of things I'd underscore include:

First, I think those panel bevels do matter quite a bit. I spent decent time trying to get these right based on the various angles in the instructions. I initially thought the bevels were to allow all the panel edges to mate flush across their whole 4mm thickness, but the bevel angles from the instructions turned-out being too aggressive for that. Each joint line ended up with a small "groove" made by the bevels, visible on the inside of the hull until filleting. Interestingly I think this allowed each panel to "bind" into alignment better with its mate during stitching, which I suspect is by-design. This little groove also later becomes a nice little channel to fill when tack-welding, increasing the surface-area contact for the epoxy.

Second, I needed to modify the temp bulkheads a bit to get a good fit, notably the one at the bow. No matter what I did the panels that form the keel line would not mate because the bottom tip of that temp bulkhead was getting in the way. I see something similar in your pictures. I rounded-off the bottom tip of that temp bulkhead to adjust for this and also needed to make a little rig with clamps to squeeze from the outside to close all the gaps right before tack-welding. I found you have to get pretty creative with clamps here-and-there, but once those tack-welds are in, everything holds really well. Epoxy is amazingly strong.