Thanks for all the inputs. Not enough time to respond to all but here are some specifics.

Charlie: good point about staying in the mold until the ribs are in. Should be possible to do that with an open mold.

Dave: I think wider spaced ribs would have to be thicker and therefore steamed; I was hoping to avoid that. The point about wood movement is a good one though: I think it is going to be like plywood - provided the expansion is distributed evenly shouldn’t be a problem. After all, the riveted/nailed canoes lasted for years.

Bill (and others): I have no plan to use nails! No way! All those “old-timers” would have killed for epoxy! My 12 footer weighed 22 lb, I was disappointed as I was aiming at below 20 but I forgot to allow for the weight of the seat.

Glen: thanks for link to a nice boat: that is more or less the result I am aiming for although I would want to lose all the nails! The images looked like ½ sq softwood ribs on about 3" centers or 1/6 of length. If stiffness is the issue that suggests for the the same I would need ½ * cuberoot (1/6) or about 1/4" over the entire inside surface. For the same cantilever strength it would be ½ * squareroot (1/6) or about 0.2" over the entire inside surface. For the same simple tensile strength ½* (1/6) or 0.083" (2 mm) over the entire inside surface. That is much more than I expected even with the hardwood ribs I was planning for; also more than the regular core veneer provides in plywood boats which work OK. I can probably go thinner or less ribs but I think I would be breaking new ground with all the risks entailed.

Brian: you are correct, but I am exploring what it would take to make a boat that is “all-wood” as far as is practical.

Etienne: may be the way to go, I would not use cedar for a cold-molded hull of course, something harder except for the core. Not sure I want to go to that much trouble though.