Originally Posted by
nanok
yes, that is the "static" model of what i'm describing, and it's very nicely illustrated, thank you for that.
when you add stretch on a taut ridgeline, what happens basically is that the rigging angle which is annalyzed in the pic above will change as the line stretches when you apply load to it perpendicularly, the pic you shared helps to grasp this in discrete steps, except that it is only in the final position where things settle to equilibrium where the load sustained by the rope is the full weight of the object, before that, as the line stretches, it does so because it is still incapable of withstanding the full load, and this is why the object will keep moving down and cause the stretch and the sag (we're talking statically for now, imagine very gently resting a heavy object on a ridgeline, with no kinetic energy, and without allowing gravity to accelerate it, so just a controlled lowering, for the time being, not free fall)
this does not fully describe what happens on impact (so when there's kinetic energy at play too), but it's a good start to begin to understand what can be expected. and again, it becomes clear: with tautly rigged ridgeline and no stretch, you can expect extreme tension in the rope and anchor points, and breakage under loads much lower than you might expect (in the pic above, a line rated for 500lbs will fail with just a 100lbs load, and that's already 5 degrees, so there's some angle already, some dyneema ridgelines as they are rigged might not reach that angle and break first)
it can easily be calculated what elasticity module (amount of stretch) a line would require to for instance fail when loaded like this at the same load as the line is rated for, if one would find it amusing. however, the simple conclusion for practical purposes is very straightforward: rig at 30 degrees or more, just as with the hammock, and stop worrying.
edit:
ok, you made me do it :P
staying within typical hammock rigging limitations, means the originally taut ridgeline can't go beyond the sag the hammock was rigged at (30degrees), otherwise it's pointless for this discussion (of course, it depends how low it was rigged taut, so often even less than 30 degrees, but trying to keep it simple)
a line which will stretch enough to achieve the 30 degree sag, would be stretching by a factor of .154 or so (so 15% stretch), at that particular load, if it is at the lines breaking point, the load it is supporting then is the lines rated strength (so for a 500lbs line, that would be 15% at 500lbs). the lash-it in 1.75mm rated for 500lbs, mentioned above, would reach 4% at breaking point (if we assume we can extrapolate linearly, which should be close enough), which means an angle of about 16 degrees, which works out to a best case scenario of withstanding 55% of the line strength, or 275lbs.
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