: That's your first mistake :)

: That's your second mistake.

: Since 6061 and 6063 aluminum tubing used in kayak building have a
: tensile strength of about 40,000 psi , I assume that by
: "soft" you mean flexing
: along it's length, and most of the flex you encountered has to do
: with the insert connections between the 4 tubes. In actual
: usage, the aluminum
: tubes are under stress, thereby negating the "softness"
: you describe. Add in a minimum of 6 full length stringers,
: tightly snap connected to
: several HDPE cross sections, hard stem connections, and a tight
: skin, and you have the basis of the boat's unique strength /
: rigidity / durability.

: Your "string" experiment has no basis in reality. What
: could 4 aluminum tubes sections suspended with strings possibly
: have to do with anything ?
: A folding kayak's strength and rigidity is the sum of all it's
: parts, not a single tube hanging from strings.

: Feathercraft, the leading manufacturer of high end aluminum
: stringer folding kayaks uses 3/4" thin wall tubing on most
: of it's folders, and on some they use
: 5/8" tubing. These boats have a legendary reputation for
: strength and durability. There's a reason why Feathercraft uses
: 3/4" thin wall tubing. Can you guess why ?

: Tom

Hi Tom, Marc, oldie, Bill & Brian
Thanks for the input.
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.
Before I posted I did take a look at the keel from one end while putting some load on it .. to see if it would kink at the joints .. but it doesn't, the bending is evenly distributed along all tubes. I've been pretty meticulous about the jointing because it's not the first time that I'm around that particular problem.
I get you about tightening everything up should add more rigidity than the sum of the individual parts. My intent was to wire stern and aft (my maritime language leaves something ..) together along the length of the boat .. to provide the nice curve and a well defined and stable vertical and perpendicular plan that separates left and right, up and down. I thought that the load on the keel would be 'exponential' to provide a growing arc, but once it's bend it doesn't take much extra force to expand the bending .. I cannot really prevent that the weight of my body will produce a bending. Maybe I should skip that wire, it's a mess when it passes the coaming anyway.
I've buckled/bend the stern and the rear .. and cemented a 10mm 10cm bent steal screw with 'metal-cement' inside the inner tubing to provide an absolute solid joint. Once I tighten the above wire the two joints will be under a lot of stress, so I'm about to add a support-beam too. The hanging test of the keel was to see if it's joints needed the inner cementing, but I think they'll go free.
I've been around the pictures to have a look at what I perceive as an extra inner support-beam (the inner gunwale on the Sea Ranger photos) .. I've seen it somewhere that it be locked to the chine or keel and stand as an arc. In my mind it would provide the uppersit support of what I seem to need (it will add extra downward pressure on the center of the boat). Maybe it makes sense if I take my wire out of the equation.
About tightening everything up: none of the longitudinal tubes can be locked together .. if for no other reason, the chines and gunwales are carbon-fiber not well suited for such action .. (if I forgot why I added the wire, I'm reminded ..)
Tom, there's a lot of things to be had from the drawings even though I don't follow the prescription .. but I admit that changing some basic principles is a risk or new challenge that may not turn out happily. I've got bamboo for the inner segmentations, and some (a lot of) welding with durafix in front of me. I'm either walking or bicycle-riding .. I simply need it light.

regards
Carsten