The reason I asked this question is I was sceptical about the practicality of it and still am. As a design proposal what you put forth is so long and complex and so full of unaswered questions that getting a boat out of it looks like a life's work.
One thing is certain. Jumping thru' all the hoops that PVC construction would require is the best argument for not bothering with it.
Just because something can be done is not a good rational for doing it. There has to be a payoff. What would a PVC boat give you that you couldn't get using the common materials we already use?

: I said early on that this was still in the "experimental" stage.
: They should hold just fine, but the only proof of that is to actually
: build a few boats like this and beat them to pieces just to see how much
: abuse they can take.

: Nice thing about this is that you'll only need about $25 worth of PVC pipe,
: so breakage and experimentation are not going to be too expensive. Frames
: and floor should only use 1/2 sheet of plywood (about $10) and one sheet
: of lauan (again under $10) should do for all the reinforcing of the PVC
: tubes. Add on 6 yards of heavy vinyl at $3.50 a yard and the whole thing
: should come to well under $80.

: While the illustration only shows 4 screws, that was just for simplicity in
: drawing. Each screw is going to hold with a certain amount of force. For a
: stronger bond of wood to the pvc, you just use more screws and space them
: closer together. That kind of redundancy also ensures that one or two
: loose screws won't compromise the integrity of the pieces.

: It is also possible to use machine screws or bolts and secure them with nuts
: which are inside the tube. Holding the nut while you tighten the bolt is
: going to be a problem, but not an impossibility. A "T" nut
: inserted into a dowel rod (making a barrel nut) is one possible way to do
: it.

: Or, you can put a wood plug in the tube, drill through it and insert a bolt.
: The head of the bolt can be recessed into the wall of the tube and the
: hole smoothed over with bondo, epoxy or even chewing gum. The threads of
: that bolt would then protrude into the interior of the kayak, as studs.
: You would put the plywood over those studs, and secure the pieces with
: nuts, washers and lockwashers on the inside of the boat.

: The more complicated the process the more it will cost, and the more time it
: will take to make. At the point where it gets too involved, the advantages
: of using PVC are outweighed by the disadvantanges of the other steps.

: PGJ