: Howdy all,

: In the last episode, Paul Jacobson challenged me to keep playing with conduit
: PVC. I've done so, and have two things to report and some questions to
: ask.

So if things go wrong I guess it is all my fault.

: 1. I've successfully bent conduit PVC. My method was to fill it with hot sand
: (heated to ~300 degrees F in our oven), then use a heat gun to make it
: pliable in just the right place.

Try kicking the sand up to 350 or even 400 degrees. It will cool quickly to a lower temp as you pour it into the pipe, but this may keep you from needing to use the heat gun. With enough sand on hand you could knock out ribs in record time.

: I hammered some nails about 2.5"
: apart on a piece of plywood in a rough 12" diameter circle. Then I
: slowly bent the PVC around the circle. Result? A very sturdy and primitive
: looking proto-rib! A 45 degree bend joint added to the ends and presto,
: Chester, I've got a complete rib. Time involved to do it was roughly 20
: min., but with increased efficiency, I think a rib could be made this way
: in 5-10 min. easy.

You can make more complicated shapes by using a template cut from 3/4 plywood or particle board. Since this would be a bit stronger than the nails you might be able to work at a lower temperature with the pipe and sand. Good gloves seem to be required.

: Okay, so I can make sturdy ribs now. What about longitudinal rigidity?

Before we get to that, consider some form of bracing for the sides that also locks the ribs to the sides or bottom. Somehow you have got to attach the ribs to the rest of the boat. Might as well do it with a component installed at such an angle that it increases the rigidity of the sides.

: 2. As you may recall, Paul encouraged me to look at Putz' boat, and to think
: about how to build the longitudinal "trusses" that would provide
: longitudinal rigidity. Since I'd prefer not to do lots of cutting and
: gluing of the triangular pieces (name?) in a classic truss, I played with
: the suggestion that I use a sheet of more rigid material secured to the
: PVC pipe, in effect creating a kind of I-beam. PVC sheet stock was
: available locally, but I remember that I've used it before in lattice form
: on a fence, and the stuff really bends. So I bought some 1/4 luan plywood
: instead (quite a bit cheaper, too!)

: Paul suggested that I might fasten the plywood to the PVC with screws, which
: remains an untested option.

If you test this option you may find that your next question does not need to be answered. Again, I look to the Klepper frame for inspiration, as it has a history of many years of durable service. They mount the plywood stiffeners to the inside of the wood stringers. I suggest the same idea. If you use thin material, like the 1/4 inch lauan, then use flathead bolts. Insert then from the outside edge of the stringer into the inside of the boat. REam the outer hole so the head of the screw fits inside the profile of the pipe. That way the skin will not rub on it. Nuts and washers on the inside complete the process of attaching the braces. This should work well for development, too. If you find too much flex, you can unbolt a brace and install a longer one, which should give greater stiffness.

Let's suppose your kayak design calls for a side height of 7 inches. You construct your side pieces so the bracing holds the PVC pipes 7 inches apart (measured to the outside edges of your tubing). Lets say you want to have your bracing pieces be 7 inches (side height) by about 12 inches. You cut a bunch of plywood into 7 inch strips and then hack them to length. lay the tubing on the brace piece, drill through everything, drop in the bolt and tighten away. My first thought is to leave about a foot of space between each brace, but on reconsidering, I'm tempeted to mount pairs of braces just far enough part so that a rib would fit snugly inside the gap between them. A slot in the plywood would allow you to secure the ribs with some strap-type stainless steel hose clamps, or you could use nylon cable ties if you didn't mind cutting them off and buying new ones eeach time you assembled the thing. U-bolts are another possibile hardware item for fastening the ribs in place.

: Instead, I made a roughish lengthwise cut in
: the PVC pipe with a drill saw. This was kind of hard to hold straight, so
: my cut wasn't at all square, but wandered some. Nevertheless, I've
: successfully made and assembled a 4'X 8" I-beam with 8" wide
: plywood jammed into PVC pipe at the top and bottom edges. This appears to
: be quite strong enough and rigid enough to do the job.

OK, you are making the equivalent to an "I" beam. The way I propose would make the equivalent of a "C" beam. While "I" beams are great for strength when the weight or pressure is from directly above, I'm guessing the "C" beam cross section may be good for the forces applied from the side. Have to get an opinion from an engineer on this, though.

: Now some questions: 1. How to secure the longerons (right word?) to the ribs?
: I've thought about cutting a larger piece of PVC in half lengthwise (so
: far I'm using 1/2" pvc, so let's say here I'd use 1". Then take
: a 1" piece of this and gluing it horizontally to the outside of my
: vertical rib. This would provide a place for the longeron to nestle, and
: support the joint. Then I could drill through the longeron and the rib to
: have a hole into which to place a clevis pin. Thoughts on this? A better,
: stronger way come to mind?

See above:)

: 2. Is there a better way to construct my PVC pipe/plywood "I-beam"?
: I want it strong first and then secondarily easier to build. I haven't yet
: glued my prototype. Any recommendations of a glue to use to join PVC and
: wood (the PVC solvent did nothing but give me a mess to clean up).

Again, see above. No glue needed

: 3. My thought is to get Putz' book and create real size plans on paper, then
: place them on a piece of plywood and use nails around the perimeter about
: every 1.5" as a guide for making my ribs. Anything more elegant come
: to mind?

Build directly from his forms. Lay the keel strip and heat bend a bow and stern from PVC, or cut the shape from 3/4" plywood. Cut ends of tube at an angle and bolt them to the bow and stern. Clevis pins would work. so would bolts and nuts. make a cap of cardboard or paper to cover any rough edges, and then cover that with a layer of fiberglass cloth and resin on both sides. No need to do a fancy job of filling the weave on this, it is just a buffer to keep the PVC from rubbing through the skin. Alternately you could use a piece of aluminum roof flashing with any sharp edges covered with duct tape.

After mounting the side pieces to the building forms and adding the bracing, lay out a rib alongside each building form. you can use the building forms as a guide.
IF you really think you need more ribs, add them as needed. Instead of using many ribs you might be able to substitue for alternate ones by just using short sections of straight PVC connected across the boat with "T" connectors, kind of like the thwarts on a canoe, but you'll need an upper and a lower one for the top and bottom rails of the sides. If you want ot cross brace these (top left rail to bottom right rail) this might work, too.

: 4. I want the kayak to be able to carry about 250 lbs. (Me + an easy week's
: worth of food and gear). Could I reasonably expect the Putz boat to do
: this?

: 5. Are there other designs I should consider?

Sure, but time constraints prevent me from listing them all, so I won't even start

Most skin boats could be redesigned to use these materials. Once you have the basic components you can shorten them with a saw to make a shorter boat. A wider or narrower boat is only a matter of making ribs that are just that: Wider or narrower. For a boat that holds more weight, make the side height greater. For less wind resistance, make the sides narrow to 3 or 4 inches at bow and stern while keeping them at full width in the center. Of course your cocpit layout and coaming is totally up to you as for shape and outfitting.

: Could a reasonably sturdy and cheap folder really be possible?
Absolutely. But with a decent size PVC tube it might not be vary light. Anything under 50 or 60 pounds should be fine, though, as you can package it in two or three bags for transport.

Happy building

PGJ