: 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?

Wonderful! To boldly go where none have gone before..... I salute your pioneering spirit!

Something to consider when knotching and drilling PVC - PVC is sensitive to stress raisers and so sharp edges, square knotches and so on should be avoided where possible, particularly in high stressed or flexing members.

My thought on your problem above:

Take a block of wood slightly thinner than your rib and groove or slot one edge to take the rib. Use a saddle clamp or similar (3 large zip ties?) to hold the rib in the groove. As an additional security a screw or glue could be used.

Now, at 90 degrees drill a hole for your stringer. Cut the wood back so that there is about 2/3rds of the hole remaining, this way the PVC will 'pop' in and out. Once the boat is assembled the skin tension should keep it in place. If this is not so, another saddle clamp, webbing strip or something like that could be used to hold it in place.

For the block, perhaps the 'Text CAD' picture will give the idea.

_______________ _ _
I_ _ _ _ _ _ _ I I \_/ I
I _ I I _ _ I
I / \ I I I
I_____\ /______I I_____I

Good luck!