: Hi All
: After visiting the yostwerks pages on building a folding-kayak I
: decided to build my own. As it is, I'm usually not following the
: prescription on anything, and so too as to a transportable
: kayak. My experience in the 'wild' stresses weight as a dominant
: and primary factor .. anyway, short story: I bought 20 1m*10mm
: carbon-fiber tubes to act as gunwale and chines with a set of 5
: 1m 30mm alu-tubes to act as a keel. I've been fiddeling with
: some of the minor details, but today I got 4 of the 5m alu-keel
: up and hanging in strings attached to each end. This setup does
: not reflect how it will respond in water, but .. gee, it's SOFT.
: Awful soft. My instinct tells me that it's too soft to act as
: the central and only 'rigid' structure to 'hang' everything else
: up on.
: Has any of you any idea weather I'm out on a too soft limb here?
: The alu-tubes are 30mm ø-outside 26mm ø-inside. They are attached
: by inlined tubes 24mm ø-outside 22mm ø-inside - the 2mm
: discrepancy covered with tape, but the bending of the long
: assembled tube is not particularly focussed here, it appears to
: be the overall elasticity of the collective tube.

These are my thoughts but I haven’t yet built a SOF especially a folder so this is all theory . . .

George Dyson’s Al-framed Baidarkas are intended to be flexible. Much of the stiffness is provided by the skin. They are based on 0.75" OD x 0.075" wall (19 x 1.9 mm) tubing, the alloy has 40,000 psi yield strength but all Al alloys have more-or-less the same stiffness. Even though your tubes have thinner walls they should be at least as stiff due to the larger diameter.

If you just have the keel without chines, stringers and gunnels it doesn’t sound like a fair test. With most traditional boat designs the keel is a significant support member but the modern trend is for strength to be distributed. PVC skin is less rigid than many other materials unless cloth-backed so the frame must be somewhat stiffer than for a typical SOF. The most efficient frame design would have the largest longitudinal members at the gunnels and keel - i.e., well-spaced - with many lightweight frames plus diagonal stiffening such as the Kevlar roving used in Platt-Monfort’s Aerolite designs. Without roving - which is time-consuming - the best source of added stiffness would be rigid joints between the longitudinal members and the frames or alloy stems with diagonal elements. A long pointy stem is also a help.

Stiffness of a tube is proportional to the square of the diameter directly proportional to wall thickness, and directly proportional to material stiffness. Although carbon is much stiffer than Al the tubes are smaller and may well be thinner.

Hope this helps a bit . . .