: This has got to have been posted before, but a search did not show it. Is
: there any reason why using PVC plumbing tubing would be bad for a folding
: kayak(canoe)?

I don't think so. Many others disagree with me.

Tom Yost has been pushing aluminum frames, and building some really nice boats with them. I suggest you read through his site, it is a wonderful resource. (www.yostwerks.com) His arguments against the use of PVC pipe seem to be echoed by most people. I've read his comments, and agree with them. But I also think there is a way (or several ways) to work around the worst flaws in this material, and make it useful. Problem is, I've just been playing with some of the materials, and haven't built any boats with PVC frames (yet) which could be actually tried. So what I have to offer you (right now) is just a bit past "theory", and barely getting into the "experimental" stage.

PVC is very flexible, and not exceptionally strong. Put some weight on a long length of PVC pipe and it flexes a lot more than you would like. However, the same can be said for thin wood strips, which are commonly used for stringers (or chines) on wood-framed SOF kayaks.

For decades Klepper has used thin ash strips for the chines of their folding kayaks. These are connected by panels of thin plywood. I believe that the same method can be used to reinforce PVC pipe to give a stiff, strong, lightweight frame for a kayak. But it won't be as lightweight as Klepper's ash frame, or Yost's aluminum frame. Check out the skeleton of the Klepper frame on their website: http://www.klepper.com/en/faltboote_aufbau_sicherheit.php

You can connect your pipes with the standard plumbing fittings, but those are not very strong. I would work with 1-inch-diameter, Schedule 40 PVC pipe as my main material, and join sections by inserting 8-inch-long sections of 3/4-inch Schedule 40 PVC pipe. The trick is to slit the 3/4 inch sections of tubing, then grip them in a large pliers, compressing the tube so that it is small enough to fit inside the 1-inch tube. You can use any common solvent glue to bond this in place. I'd personally glue half the insert (4 inches of it) into the end of a piece of 1-inch pipe, and leave the remaining 4 inches exposed as a stub. Bevel the end of it with a fle, or sand paper, and it will slide into the adjoining piece of 1-inch pipe as a nice, tight-fitting connector.

The plywood panels used for stiffening the tubing would be attached to the tubing with screws, or bolts. Stainless steel hardware, of course. if I went with screws I'd want to insert a piece of 3/4 inch tubing inside the 1-inch tube at the point where the screw went. This would give me a thicker plastic base to hold the screw threads. I'd probably cut 1- to 2-inch long pieces of 3/4 inch tubing, then ripeach of those either in half, or in thirds, to get the backing material. I"d place these where they were needed by poking them up the tube with a thin dowel rod. Once the screw was driven in they'd stay in place. I might try using some solvent glue on them, but that is an experiment I haven't done yet. Another option would be to buy or make a wooden dowel in a suitable diameter, and use short pieces of that to hold the screws in. Similarly, I would use wooden dowels at the ends of the tubes so that I could connect them to the bow and stern forms. Stainless steel bolts going through tubes, dowels and forms would be secure, and not crush the ends of the tubes.

If I use wood dowel parts they would be sealed with 2 coats of epoxy resin first, and probably epoxy-glued into their locations. That wouldn't be strong enough to keep them in place forever--you'll have the stainless steel bolts to do that--but this just keeps all the holes aligned when the bolts are removed.

The plywood panels should work fine for stiffening the sides of the boat. For stiffening the keel (or keelson, if you prefer) you have a couple of options. One is to make the floor from a fairly thick piece of plywood. you could use 1/2 inch, 5/8, or even 3/4 inch plywood. Even with the last option, you'd be using a piece that is roughly 14 inches wide and 4 to 5 feet long. That is not going to weigh a lot. Alternately, you could use a couple of wide boards, say 1x8s. this would give the greatest stiffness in the cockpit area, and beyond that the PVC would flex a little, giving a natural rocker.

Another idea is to use thicker, stiffer PVC tubing under the paddler's seat. A length of 1 1/2- or 2-inch Schedule 40 PVC pipe would flex less than the 1-inch pipe used elsewhere. There is also available a heavier Schedule 80 PVC pipe which has a thicker wall, and is stiffer.

And a third idea is to use more hull forms which are spaced closer together. Since you are just trying to give more support to the bottom tubes which form the keel, you do not need to make forms which go up to the deck. George Putz, in his book "Wood and Canvas Kayak Building" uses "floorboards" which are shaped like the bottoms of bulkhead forms. He uses about 8 or 10 spaced roughly 18 inches apart.

Your choice of technique for strengthening and stiffening the bottom of the hull is going to depend on the shape of the hull's bottom. You have essentially two choices: Flat, or "V" shaped. Klepper and the folding boat designs sold by Clarkcraft (www.clarkcraft.com) have essentially flat bottoms which are made of a pair of keel strips. A wood floor rests over these. Yost's designs, as well as most of the native kayaks from Alaska and Greenland, have a single strip along the centerline of the bottom of the boat.

Should you wish to bend your PVC, consider using hot sand. Boiling water is just barely hot enough to soften the pipe, and hot oil is too messy and dangerous. Heat the sand in a metal tray in a 325 to 375 degree F oven (about 150 to 180 degrees C). When it is hot, use a funnel to pour it into your pipe. This heats the pipe evenly, and prevents the pipe from collapsing as it is bent. Start at the lower end of the temperature range, and go up if you need to. Once the end is complete, pour out the sand and the pipe will rapidly cool, holding the bend. Boy's Life Magazine showed this method being used by a scout troop which was using PVC pipe for making snowshoe frames.

Current state of the art: There are no plans available, or any workable designs. Some people are using PVC as a direct replacement for wood chines and stringers in boats designed for wood construction. They get a boat which is a bit heavier, but they don't need to find long pieces of wood. Nor do they need to mess with scarfing or ripping woods strips. When combined with designs which use frames cut from plywood they can do most of their work with an inexpensive sabre saw.

If I was building a PVC framed kayak this week, I'd base it on the forms for Yost's Sea Ranger, simply because I have some already cut out of plywood, or Putz's Walrus, for nearly the same reason. I'd drill out the corners of the forms with a forstner bit to accept the PVC--assuming I could find one in the correct diameter. Failing that, I'd buy a spade bit which was a little too large, and grind it down to the coreect size, then sandwich the form between scrap wood so it wouldn't splinter or shatter as I drilled it. The forms are 1/2 inch birch. I'd use nominal 1/4 inch lauan (more like 5.5mm) as my reinforcement on the sidewalls, and cut pieces which would be spaced 1/2 inch apart at the location of the frames. These would then be held securely in place by the lauan pieces, and the skin. I'd cut a plywood bow and stern form, and attach the PVC to this with bolts going through the pipe. The ends of the pipe will be plugged with wood dowels, The ends cut off at a taper, and the bolts will be recessed into the wood, so it is all smooth. leaving the bolts a bit loose will allow the pieces of pipe attached to the bow and stern forms to fold in. When the forms are inserted they will be forced back out, into their proper position. I'd make the keel of the boat with 2-inch PVC under the cockpit, and join that to 1-inch PVC which would attach to the bow and stern forms. The joint would be a wood plug in the end of the 2-inch pipe, which woudl be drilled out (same forstner or spade bit)to allow the 1-inch pipe to be slid in. A bolt with a wing nut holds it all together. To make the boat collapsible for storage or transporting, I'd make the side panels a little under 4 feet long. There would be 2 panels on each side for the bow, and 2 for the stern. Adding on the length of the plywood ends I'd end up with a boat about 17 to 18 feet long. The skin would be made from clear vinyl from Wal-Mart, as I have some on hand.

If I was planning to make this sometime in the future I'd change quite a bit. I'd look into using a hull design program and work up a good design for a flat bottomed boat--closer to the Klepper shape, but narrower. Then I'd use 1-inch pipe for all of it, and make a floor from 1/2 inch birch plywood. I'd experiment with 1/8th-inch or 3mm marine meranti, lauan, or aircraft birch for the sidewall supports. If that was strong enough I'd drill that out to lighten it further. If it wasn't strong enough I'd try 4mm marine plywood. I'd use the more durable "coverlight" material that Yost uses for a skin.

Hope this helps

If you decide to go this route, keep us informed.

PGJ