: Yes, Paul and the rest of you have been very bad influences!

Awwwww.

: These ribs appear remarkably strong, and took
: about 2 hours start to finish for this plodding novice to make.

At some point after you have these ribs atached to the sides and keel you'll probably want to head back and make some more ribs to fit between these. with the flexible materials you are using you may need added support and more ribs to tie things together. Attach the skin loosely or just wrap it around the frame temporarily, take the boat to a pond and put some load in the boat. Add weight slowly as you watch the thing bend. (get 2 or 3 bags of playground sand at the hardware store -- it is pretty cheap) If the bending is too extreme, then go back and reinforce things.

: . . . I've fastened the keel (1/2" PVC, of course) . . .

Personally, I would have gone with thicker PVC pipe for the keel. I think 3/4 is too flexible, too, but depending on the weight you expect to carry, 1, 1-1/4, or even 1-1/2 inch PVC pipe might be stiff enough. Don't look for lightweight drain pipe, look for "Schedule 40" pipe.

: and today need to mount vertical supports to which I'll attach the ribs
: I've made so I can start adding the stringers. The next step will be to
: make a C-beam with 1/4" plywood joining 2 PVC stringers. This will be
: in place of Putz' truss on the gunwales. Also, instead of building the
: boat upside down as per Putz, I'm building it right side up. Just seems to
: make more sense that way.

Well, three things here. Building upside down frequently gives you better access to more of the components and makes fastening a bit easier -- but -- boats built with ribs are frequently built right-side-up. Forget supports for the ribs. You won't need them. Attach your "c" beam sides to the bow and stern (loosely) and then just drop in the ribs. They will push out the sides to where they belong. Cable ties are great for making loose connections while you experiment with rib location. Once you get the boat shape right, drill holes and put in bolts or clevis pins to hold the components in place.

You may want NOT to make up the "c" beams first. If you start with two side pieces of PVC pipe and cable-tie them to a rib, you can jam an end of one of the pieces of the 1/4 inch ply right up to the rib, forcing it to be square with the side tubes. Then drill and bolt the plywood onto the tubes. Place a second piece of plywood immediately against the other side of the rib and your plywood will form a channel that will keep the rib nicely aligned.
A "U" bolt that bridges the two pieces of plywood can secure the rib in place without the need to drill holes in the tubes, which might weaken them at stress points. Since the plywood will be attached to the tubes so that it is on the inside of the boat ( the skin rubs on the tubes and never touches the plywood) you should be able to cut off any excess of the "u" bolts and your wing nuts will fit in the "shadow " of the tube -- in the 3/4 inch space between the plywood and the skin.

: My current plan is to join the stringers and the gunwale truss to the ribs
: with cable ties - they're cheap and easy to do. When I want to dismantle
: it, there's just some snipping to do, and when I want to put it together,
: it means a dollar's worth of cable ties. After that, I'll need to attend
: to the floor of the cockpit.

This idea died already I think.

: I've got the PVC tarp from a billboard that I'm planning to use as a skin.
: I've discovered that I can heat weld it very effectively with a clothes
: iron.

A trick I learned for this: sandwich the seam between two sheets of aluminum foil before you iron it. the foil transfers the heat very well, and the iron does not get gummed up with molten plastic. it just glides along the foil. any molten plastic is trapped between the foil sheets, and when the weld cools, a gentle flex and the foil peels right off.

.
: 2. How about the coaming (bent PVC pipe, of course!)? I'm thinking about
: mating it permanently to the skin and then securing it to ribs fore and
: aft when I assemble the boat.

: 3. Will I need sponsons? If so, how have these been home made?
you may not need them, but if you can sucessfully heat seal your PVC skin you can make thes in the same manner. For a 4 inch diameter sponson that is 12 feet long cut a piece of material that is 14 inches wide and 13 feet long. I figure for other sizes you'll want material that is 3 times your diameter, plus 2 inches in width, and at least a foot longer than the finished size. Fold this so it is 7 inches by 13 feet and heat seal one end and the long side. You can make a neater tube if after you finish your sealing you turn the tube inside-out so the sealed edges are on the inside. Inflate through the unsealed open end and roll this end up with 2 or three folds. A strong clip can hold that closed, trapping the air inside.

: 4. I'll need to have pegs or a smaller diameter pipe inserted inside my
: 1/2" PVC stringers to stabilize the joints between stringer pieces

I use 5/8 inch dowel rods (wood) about 7 inches long when I make things of metal conduit. the same should work here, but why bother? You can buy off-the-shelf connectors which fit over the ends of the tubes. I'd solvent weld one side of a connector to the end of one pipe, and leave the other end unglued so your second length of pipe can jsut slip in. The connectors are cheap and very strong. If you object to the bulge they might make, you can reduce it considerably by sanding down the edges of the connectors so that they taper to meet the pipe. Just stick a connector (unglued) on the end of a short length of pipe and hold it against a spinning sanding disk. A few turns and the edge sands down smoothly.

: 5. What am I missing (other than the obvious critical absence of grey matter
: between my ears)?

I think you may be needing to reinforce the bracing of the keel, which has a lot more pressure on it than the sides, and I hope you are considering adding 2 or 4 more tubes along the bottom, that would be one or two on each side of the keel, between the keel and the bottom of your side. These you could actually hold in with cable ties, as all they do is add more support to the skin. Or, you could connect themn with plywood pieces, like the sides, and use those plywood pieces as your floorboards, or footrests. Use 1/2 inch or at least 3/8 th inch plywood under your seating area and it should nicely reinforce the area under the cockpit (where you wont't be able to put a rib to keep the keel from bending)

Send along some pictures as this goes. Sounds like you are having fun.

PGJ