This is where, as you have probably anticipated, the plot thickens. I hadn't given it much thought but the gap in the scarf was in the middle of the joint, not the ends as you might expect if you were to force the pieces back and forth. Photo 1:
Yes, the stopwater hole is aligned. Unfortunately that's about the only thing. Well no - the bearding line (where the inside of the planking meets the stem, above in illustration 2). The rabbet is completely off - like nearly 3/8" off. I should also note that I had to cut off a chunk of the piece on the left - go back and see photo 1, you can see my saw cut (I put the piece back for the photo). On photo 2, the gap is clear - that big black wedge shaped void. And as my mind wandered, suddenly I saw it. The rabbet is not off. Somebody MADE it that way. Look again. Photo 3, a little closer view. A red arrow points to the smoking gun.SOMEONE and I'm not pointing fingers (at the idiots and assholes who may have repaired - or thought they were repairing this boat before me) but the rabbet was cut back to match the misaligned pieces! The photo doesn't do it justice - its more than apparent in the flesh. Wood flesh. Real life. Whatever. Bastards.
Okay. I'm onto something here. Clearly I've found the original orientation, and I'm getting farther afield from my index point - remember that's the stopwater hole - but the inner and outer curvature still eluded me. Look again at the parts aligned on the stopwater, from a distance. Photo 4:
Okay. Stepping back and looking at it, I realized that the answer was remarkably simple. Go back to Photo 1, and look at that checking in the wood of the lower part of the scarf. I'll wait.
The checking forced the wood to expand, forcing the lower right portion of the scarf to push to the right and throw the scarf out of alignment. The maintainers unknowingly just kept shovelling putty into the gap. When somebody got around to repairing the resulting sprung garboards (which explains the condition of those parts quite well) they had to chisel back the rabbet to make it fit. If I were to just carry the curve from the right piece onto the part of the left piece that isn't checked. And surprise surprise - its a fair curve. Look at Photo 4 again. It seems so obvious to me now. Elementary, my dear readers. Yeah right! Took me about three hours of heaving and hawing, adjusting, readjusting and playing with the batten.
Now I can proceed tomorrow with finishing up the pattern. Why didn't I do it tonight? I dunno - because I've been too busy BLOGGING about it. Oh well. Small victories.