That was exactly what I was thinking. Smaller scantlings or White pine (if I can find it). Sometimes you will find some mixed in the yellow pine boards. I love to work with it, but I don't use much pine except for construction.

Yellow pine is heavy but I can't imagine it adds that much weight. It would still be an extremely light weight boat. Main thing is just how it would hold up. I was thinking of spraying the frames with a good sealer. Not sure what yet. Poly probably. Of course I know that pin holes happen and water will get in sooner or latter. But I also think it wouldn't be too hard to replace a section of frame either. These boats seem easiest to repair to me if needed.

As for scarfing, I was thinking or making a jig to cut the scarf. But it still will take a fair amount of time to rip, trip and scarf. Even with a all the right tools.