Just a short update this time. Had to have a sit on the stool and think about this one for a good while. I did NOT do the inner keel right when I initially set it up. It should have been beveled so that the planking would come up over the top of it and meet in the middle. Time for another hackjob on the bottom. Remember again: Paint.
The solution I came up with was to dig out the rabbet planes and cut back the planking so that an inch of inner keel would be exposed all the way along the spine of the boat. Planking on either side of this inch-wide gap was cut at approximate 45deg angles so that the outer keel and skeg would sit and align nicely in the trough while still maintaining a good width.
It's a good idea to have a template for this type of work so you aren't constantly measuring (Figure 82). I roughed the trough out with a framing chisel and then used my Stanley 78 rabbet plane to smooth out the rough job on both sides. The result was not fantastically pretty, but I think it will work. My template now slides nicely from the transom all the way to about 24 inches from the bow, where I don't really have enough keel to continue with the inch width. Here I'll just eyeball it and then custom cut the outer stem to fit the narrower trough width as necessary.
Figure 82: Template for cutting the trough
Figure 83: Closeup of the finished trough. Cedar strips on the sides and exposed inner ash keel underneath
Up at the stem, none of this trough business is strictly necessary. The original design called for an outer stem that sits flush with the inner stem across the end grain of the planks. Fair enough. Got out my little block plane and shaved it down to the face of the inner stem. Transition to the trough happens at about 12" back from the bow (or thereabouts).
Figure 84: No trough required up front where the outer stem goes.