I'm still not sure based on the pictures you are posting, but the last one looks strangely familiar...on my demo, I got down to something that looked very similar and symmetrical, just like what I'm seeing in this pic. I used a cutting wheel and carefully cut a 6" section out, like a rectangular window. It was completely hollow underneath, unless you count the wet mulched wood. These, place equidistant on each side of the keel, were 1" x 3" 's layed down flat, glassed over as some sort of stringer???? I took them both out and got to a flat surface/hull where I could later glass my standing upright stringers. I may be way off base, but just a thought...pictures aren't helping me.
