Thank you, they are steamed white oak mortised into pine/fir gunwales, lashed together with a synthetic sinew (basically very heavy dental floss) and a heavy ballistic nylon shell coated with a 2-part epoxy to make it waterproof. It’s surprisingly tough to get really fragile materials to end up in the shape you want; the current guru that I know of is a guy named Brian Schulz, who runs Cape Falcon Kayaks somewhere in Oregon.
I haven’t made any tandems, I like smaller solo boats, but my pack boats (12.5ft or so) weight 30-32 pounds, my longer boats (14.5ft or so) are around forty. I’ve never actually weighed the longer ones, just made sure i could carry them over my head. I’d rather add five pounds and have a boat I can bash into a rock, if I need to, rather than keep the weight off and have to babysit it. I paddled one of the bigger boats above 220 miles down the James river one time; it ran into a rock or twelve on that trip and I patched a few drip-leaks but am very confident in the materials now.
I’ve seen +/-30lb full-size skin-on-frames, and talked to people who’ve built them, and I get the feeling that they just don’t behave in a very canoe-y way.