Thanks for the responses.
My math says for an equilateral suspension triangle, the pole only sees ~60lbs for a 200lb occupant. Each side of the hammock gets half the weight (100lbs), and the lateral force applied to each end of the rod is 100lbs * tan(30d) = 58 lbs. Please correct me if I've got this wrong.
At 6" diameter, the end of the cylinder is 28sq.in. 58lbs over that area is ~2psi. At 8" diameter, that drops to 1.15psi. A cursory web search says human lung power can be as high as 2psi, and Jeff-oh quoted 1.3psi. 2psi in a 6" cylinder generates a tension around the circumference of just 6 lb/in - well under the strength of 1osy nylon; ditto the 8lbs for an 8" cylinder. This would be a great application for 0.5oz DCF - very low stretch equates to rigidity, and the small area means affordable - one yard would do it.
Either way, I figured the pressure would rise with the load, just like an air mattress. And varying the diameter (football shaped) did cross my mind.
HandyRandy - I was thinking permanently attached, basically as partial end caps.
Grizz - not long after posting (as it so often happens) I saw the problem you describe w/ relative tension on a lower suspension/spreader. With out any lower suspension, the lower spreader isn't really good for much at all. It seems a fixed ridge line should let you fix the upper/lower suspension lengths, no?
Regardless, the original motivation was weight savings, not changing how the structure works.
I suppose you could control the buckling by putting a vertical crease in the middle of the tube so it only wants to deform horizontally (towards/away from your head, not the ground), and then tethering the crease to the suspension and cat curves to stabilize it in that plane. That effectively cuts the spread span in two, which makes each half of the cylinder much more resistant to buckling. Harder to do w/ an inflation bladder than heavier heat sealable fabric, but still doable.
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