: What if I made a cheap knockoff of the Kahuna by feathercraft?

Don't. There are plenty of plans and lines available for free in books and magazines. You'll get better numbers to work from with any of them, and you won't be cheating someone by copying their protected, creative ideas. Besides, their hull calls for only 5 longitudinal members. You can't hope to make that work with off-the-shelf PVC. With the flex in the PVC material you'll need more longitudinal members (not more ribs) to get back some stiffness.

Consider the lowly cedar strip. Take a 10 foot length and hold it horizontally at one end. the other end sags several feet under its own weight. Yet, when combined with other equally flexible strips it can make a very rigid canoe or kayak. When properly braced PVC pipe is very flexible, but it can span a 20 foot distance (length of a kayak) without significant sag. The trick is to get the bracing right. Take a look at the design of the Walrus in George Putz's book. The pair of longitudinal pieces that make up each side are connected with angled bracing pieces, or trestling, which serves to tie the two pieces together into a beam which is very rigid. By nthe way, you can buy the book and use that design. It is a good one. I have a webpage that translates his drawing into useful coordinates for making building forms with a minimum of wood.

Go to Klepper's website and you'll see that they do the same thing by tying together the upper and lower longitudinal member on each side with rectangles of thin baltic birch plywood. (www.klepper.com)

With PVC pipe you need to add similar crossbracing. Here is what I have considered. Make a jig that will SECURELY hold a 45 degree angle PVC fitting. With the fitting in this jig you should be able to safely run it through a table saw so that you can cut open one leg of the fitting. Make two for each brace. Shove the untouched ends of the fittings on a short piece of tubing. This will be your brace. The ends you have cut open you should be able to glue onto the sides of the longitudinal pieces.

The only real trick is to get the length of the short piece of tubing to be exactly right so the brace can fit at exactly 45 degrees where it is installed. Try fitting each one and adjusting the length a bit at a time. When you have it right, slosh on the solvent and cement the thing solidly together. What fits on the left side should be exactly the same for the right side, so cut one and fit it, then cut the one for the other side to exactly the same length. Install both before going on to the next pair of braces, which should be close in size. Whatever the length of one brace is, the one next to it should be about the same size. Probably no more than 1/8th inch difference between adjacent braces.

There should be plenty of remaining plastic on the fitting to make a strong bond with the longitudinal member, but there may be a ridge or bump on the inside of the fitting that will need to be removed by aggressive sanding, filing, or maybe the use of a router. You would definitely want to secure your fitting in a secure jig if you tried to remove the excess material with powertools.

If you want to emulate the Klepper idea, find a supplier of PVC sheet stock and cut rectangles from 1/8th inch stock or thicker. Use the cement to solvent bond these to the inside of the longitudinals (so they don't rub on the skin). You can drill holes in the panels to make them lighter. YOU'll have to try a couple of tests to get the size of the PVC panels and the spacing between them to be what you want for the stiffness you need. The cost of the materials is not too great, and the time to make them is not very great. Play with this a bit.

You can use plywood panels for bracing by ataching them with brass, bronze or stainless steel bolts. 2 light coats of epoxy coating on the wood will waterproof it wonderfully, but 3 to 6 coats of a polyurethane finish should be less expensive and still be sufficient for many years of service.

: . . . I'd need properly sized ribs made of plywood instead of their "injection molded polycarbonate",

Plywood is a good option, but not the only one. You can use 1x4 pine, spruce, cedar or fir stock (whatever is cheap) and assemble the pieces by overlapping them at the appropriate angles. Before you glue and screw them together you can cut away some of the excess material between the corners, bringing the thickness down to 1 1/2 to 2 inches. This lightens them. Since you are using round tubing, drill out the corners of the forms to provide a rounded place to seat the pipes. Use a dril bit that matches or is jsut a slight bit larger than the outside diameter of the tube you use. If you do this before you cut them out of your boards or plywood you'll have ample areas to insert your sabresaw's blade.

You could also make ribs from your PVC pipe. It bends easily when heated. You would need some sort of form to work from, and I suspect there would be a lot of waste, but the material is relatively cheap, so even with three or 4 bad tries to get one good part you'll be doing fine. If it so happens that you make the peak of your deck coincide with the angle and shape of a 45 degree connector then it would be a good thing. Bend the rest and pop the ends into the connector with a bit of cement.

Somethings to think about. You mention nesting the ribs so the packing is smaller. Sounds good. Make your largest rib first then use the material you cut out fo making another rib somewhere else on the boat. And what you cut out from that . . . and so on. The standard orientation of ribs is vertical, or perpendicular to the horizon. I don't see why this needs to be the case here. While such an orientation gives ribs that resist the crushing action of the water as the boat is immersed, and the use little material, they give little or no resistance to twisting and bending motions. Consider making ribs that go at an angle. Look at a skeleton of a kayak frame and draw an imaginary line at 45 degrees through the hull. (YOu could put the thing at 30 degrees or 60 degrees jsut as easily) If you make a rib that follows the lines you've just imagined, it would be long and thin, but it would add to the bracing of the boat, and help offset the flexibility of the PVC. Think of the legs on a folding TV tray, or tea table. They nest inside one another when folded, but unfold to become an X-shaped trestle for holding the tray. Consider making a pair of ribs like that.

: Then I could make a skin of the recently mentioned PVC material used on
: billboards, or scraps from a truck tarping company.

And there are many more covering options out there. When assembling the cover, consider sewing one end into a pouch that you slide the frame into. The open fabric is brought up on the other end and folded over the deck, then the loose ends ( leave enoguh excess mnaterial) are folded over three times to keep out water, and the deck is then laced tight like a shoe, with the folded in material acting like the tongue of the shoe. Tightening the laces forces the boat deeper into the pouch end, drawing the fabric tight on that end, while it draws the fabric tight on its own end, too. If you lace the bow end of the boat you can keep an eye on the laces while you are paddling, and fix things should the skin or lace stretch and allow the fabric to become slack.

Hope this helps. Hapy building

PGJ