: Tom, If I understand you right I'm somewhat off the hook for now ..
: that the slobby keel may not be a problem if I do every else
: right.

You were never on the hook ! My boats and the Feathercraft's they emulate ( albeit simlified to help insure homebuilder success)
have a long and successful history as is ... no reinventing of the wheel is required.

If you don't trust the well tested aluminum stringers, why not go with all CF ?, though the fact that aluminum snaps onto HDPE makes this all work.
CF tubes, wood, etc have their own unique qualities, but homebuilder simplicity is not among them. And from my experience, simplicity is the goal !

It's easier to design a complex folding kayak than to design a simple one :) The Falco and Sonnet folders are my attempts to simplify the process even more
vs the more complex Feathercraft-like folders. The Falco's 4 minute effortless assembly is a sight to behold !

Keep us updated on your progress and above all, don't overthink this. Keep in mind that speed of assembly is "the" critical element, and it goes hand in hand with simplicty of design. The more complex you make it, the slower the assembly, and the less likely you are to use the boat when frequency of assembly is required.

Here's several ways to increase hull rigidy ...

1. Use sponsons between the gunwale and chines like the Feathercrafts and others. When inflated, these add rigidity in addition to allowing for a looser fitting skin during assembly. After assembly, the inflated sponsons tighten up the skin. A looser skin initially means you can assemble the frame inside the skin, rather than using a skin that fits around an already assembled frame. The downside is cost and complexity.

2. A Feathercraft (and others) technique is to use stretcher bar inserts at the center of the stringers (coaming opening area). These stiffen the hull lengthwise for greater rigidity. The downside is slower assembly due to struggling to assemble a frame inside of a skin, plus the awkwardness of using the 5 stretcher bars.

3. Small riveted aluminum plates greatly stiffen the gunwales and chines to form what amounts to a rigid side panel ( like a very wide gunwale). Possible downside ... Insure that the 2 insets in each section are aligned properly so the sections can slide together easily.

4. Inwales or multiple chines. Downside ... More weight and slower assembly.

Of note is the fact that Aleut Baidarka's had deep stiff gunwales and flexible / sectioned keels. The gunwales kept the boat rigid overall, but the sectional / lashed keel allowed the lower hull to flex in the waves for greater control, stability, and speed. Flex is good !

And that's all I have to say about that :)

Tom