In which case it would be possible to inflate the hulls in the un-obvious way. I have no evidence that Old Town actually does it that way; it just seems a possibility.
It doesn't seem possible to vacuum form such a hull in the obvious way, given the recurve at each end, and the relatively fine radius at the stem.
The vinyl/ABS/ABS foam/ABS/vinyl laminate would surely produce a much lighter hull than 10mm of HDPE.
The canoes are going to be made and used by a scout group, if it is a sucess other groups will be invited to use the mould.
so yes they have to be very rhobust and im not too concerned about the weight, (i worked it out to be about 29kg, 64lb in ye olde weight)
my main concerns are that the bow and stern are going to be stretched the most, therfor thinner wall thickness and weakest area that is going to take the most damage...
the gunwhale, im looking to bond and fasten strips of spare sheet, after the canoe is formed. What is the best method for bonding HDPE to HDPE?
ABS stacks up well against all your requirements except the last.
Under nonideal conditions, e.g. with scouts, you can't bond PE to PE, period.
If you can do without the recurved ends, i.e. everywhere concave up, and use a generous radius at the stems, you could use HDPE. It would look more like a skinny dory than a canoe. You could probably use sheet as thin as 3..4 mm if you flange the gunwales.
Or you could use HDPE, blanked to the shape of a birchbark canoe, with recurved ends, but each end would have a vertical stem seam that must be caulked or gasketed and have a mechanical retention means. Use a row of bolts and nuts, or lace it with wire or fishline.
Instead of making the canoes durable, you might consider making them disposable, or at least constructed from cast-off materials. They are using more stretched fabric these days, but billboards and signs used to be constructed of masonite or galvanized sheet steel. Maybe you can locate a source of used sheet ... something ... that could be adapted.
I have a poly canoe at home, I'm pretty sure the literature for these stated the (outer, poly) hull was roto-molded. The gunwales are mechanically fastened to the both inner and outer hull and seats.
Pat, not sure why you think a "Canadian" style canoe hull couldn't be rotomolded? Or are you saying you think the Canadian hull could be thermoformed? Like Mike has said, I can't see the prow of any canoe being thermo-formed and having (much) strength, due to the stretching/thinning of the material. Maybe the concept would work if the keel rib had provisions for adding strengthening material to the inside.
On the "other materials" topic, HDPE canoes are pretty nice, but heavy if they are to be durable. If I had the time, $$$, and wanted to build a class III capable canoe, I would find a good "fiberglass" mold for the style of canoe I wanted, and then read up on using either an epoxy/kevlar or a urethane/kevlar system instead of glass fiber. You can form some pretty complex shapes in an "open" hand-laid mold.
There are several local river boat races around where I'm from, which are usually won by people with home-built boats of the above materials. These races typically take place on class II and III rapids. Some of the winning hulls appear to be ~1/8 in. thick, extremely light, very supple (the hull can buckle over rocks/tree stumps without creasing/cracking) and strong. At least, that's what I could see from my old fiberglass tub, as the winners streaked past...
I really do like Pat's suggestion too, of a keel strake (or multiple strakes) of poly that are bolted or...dovetailed? on in some manner removably mounted onto the hull for abrasion resistance. Once they've started to get really scuffed up, you could strip it off and attach fresh grind rails...