North West Model Boat Club had a member who loved making paddle steamers (largest was reportedly 25 feet long and was operator controlled - in fact at the ellesmere show, one of the other members sat in it and sailed it up and down to the top lock, apparently they had to take his front window out to remove it from his front room), anyway all his boats wre mainly constructed frrom uhu glue and cereal boxes!.
In hearing this i buit a castle class corvette based cardboard boat hull as a test project which i will try and post a picture of, without priming the outside and no ballast it sat in the water by about 15 to 20mm, it has a double bottom keel i.e. flat inside equipment deck with a lattice section tapering from the keel to the sides and then covered with cardboard keel that is made from the type you find on the back of A4 writing pads, so there is an air gap between.
Your guess is as good as mine if this can be used to support the running gear with capsizing.