In praise of Lawson Glowire

[nanok]

okay, so you _want_ to go into the physics. we might get banned, you realize

your calculations are very interesting. (btw, we're talking 980 N*m)

(this is regarding 1)
the problem here is that trigonometry is the real *****: if the crl is horizontal, and taut, at the moment of impact the force in the line will be infinite to maintain that no-sag attitude, this is why stretch is important: the line will stretch, and will get help from trigonometry to reduce the multiplication factor by adding more sag. how much it will stretch for how much tension determines the maximum force the line will withstand (even statically) without breaking, in that configuration: if the line is very strong, but doesn't stretch enough to create some sag, it will break sooner then a weaker, more stretchy line. the shock absorbiion effect of it comes after in importance i think, but it is a very interesting property as well. now, if you setup the ridgeline with the same ammount of sag as the hammock let's say, from the beginning, then everything changes, as you will start at a point where the force on the ridgeline is translated to the same force on each side (instead of infinite), so much better position to absorb any shock even with very little stretch. this is one reason i advocate rigging the ridgeline to the hammock treestraps, not under them: the rigging angle makes a huge difference in the cost of withstanding both winds and eventual falling "enemies"

- (2) hmm, bowline depends which in what material. bowline in dyneema, and "simple bowline" is somewhere around 10-20% according to the testing i've seen and read about (bowline simply does not work in dyneema, except when you don't need the strength); the water bowline is known to not slip in dyneema, and retains about 50% of line strength (the water bowline is the one which has a clove hitch as a nipping loop). the bowline in "honest" fibers like nylon or such was tested (from the top of my head) to withstand in the range of 70-80% of line strength.

- (3) if it's dyneema, you might be surprised (just because the bowline is so terrible, i don't have numbers for the truckers hitch). if it's glowire or some other honest rope, i think both will be reasonably strong (in the range of 80% to be conservative), depending also on how it's tied etc.

- (5) if they do not, they will break. but your calculations need to take into account this pesky trigonometry, because as i said: at contact, the force in the line would be infinite (so it would break), so it stretches; the effect is that we start at contact, with the line starting to stretch, and applying basically zero force against the falling object, as it stretches and starts to sag around the object, the force applied increases gradually, so the rope starts to actually do some work to stop the object. in other words, the first 10cm of travel don't do the same work as the next 10cm and so on. yeah, i know, it's a pain

[cmoulder]

Thanks for 'math-ing' that out a bit. I'm not equipped to validate or refute but at least there's some substance to it.

I'm more of a 'Kentucky windage' kinda feller who relies more on trial-and-error, hoping never to be overwhelmed by error.

[slugbait]

Where is Grizz (our math whiz) when you need him?

[packman9000]

So we're talking about impact strength versus simple forces at play with tight ridgelines?

You've all likely seen this but perhaps it'll help the discussion at hand...or not. I'm not a maths expert

Forces at work.jpg

[nanok]

So we're talking about impact strength versus simple forces at play with tight ridgelines?

You've all likely seen this but perhaps it'll help the discussion at hand...or not. I'm not a maths expert

Forces at work.jpg

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.

[nanok]

now, when it comes to impact, it comes down to absorbing the kinetic energy, and what determines the impact force, aside from the mass of the object and the speed, is the decellarration (so how fast it stops), without going into calculating the actual impact forces for different objects, it is evident in our case that, before breakage occurs, the decellerration is determined by the elasticity modulus, which piles on top of the above static calculation, to make matters even worse for static (non stretchy) lines: for two lines of the same break strength, if one stretches by 4% at that same limit, while the other by 16%, and assuming the stretch is elastic (linear), the impact withstood by the stretchy line will be a quarter of that of the more static line, assuming the same object with the same speed is hitting them. or in other words, as the energy is proportional with the mass linearly, the stretchy line will stop 4 times the mass at the same speed, compared with the static line, and if we assume they were both rigged taut, the calculation above might bring the real figure closer to 1/8. that's... a bit worse than even i expected. can somebody verify my back of the napkin "science", please?

[uninjured]

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.

Hmm, this changes everything... I guess my original model would need to be further generalized to reflect the change in tension over the duration of the object's impact path given the change in angle. I'm no mathematician but I suspect it would involve something like integrating over the change in tension as a function of the change in angle.

[nanok]

Hmm, this changes everything... I guess my original model would need to be further generalized to reflect the change in tension over the duration of the object's impact path given the change in angle. I'm no mathematician but I suspect it would involve something like integrating over the change in tension as a function of the change in angle.

that would be the typical math tool to "fully map" the problem, but for our purposes it's easy to cheat (see my post above): if we work with energy, and we understand the model of how the line stretch decellarrates the moving projectile, and what we want to do is compare two different "barriers" (in our case ridgelines), it's easy to isolate the parameters which will determine the comparison, and see how they influence the comparison, without having to solve the problem entirely (we're doing a ratio of similar things, so most of the complicated variable stuff on both sides of the ratio reduces to numbers). what we're left with doesn't bode well for dyneema, and that's not surprising: it's nothing new that static (rigid) materials are not good at taking shocks and endup absorbing energy (and not a lot of it) by disintegrating. this is why modern cars are designed to deform so spectacularly on impact, and why rope designed for climbing is designed to be very stretchy.

[rmcrow2]

that would be the typical math tool to "fully map" the problem, but for our purposes it's easy to cheat (see my post above): if we work with energy, and we understand the model of how the line stretch decellarrates the moving projectile, and what we want to do is compare two different "barriers" (in our case ridgelines), it's easy to isolate the parameters which will determine the comparison, and see how they influence the comparison, without having to solve the problem entirely (we're doing a ratio of similar things, so most of the complicated variable stuff on both sides of the ratio reduces to numbers). what we're left with doesn't bode well for dyneema, and that's not surprising: it's nothing new that static (rigid) materials are not good at taking shocks and endup absorbing energy (and not a lot of it) by disintegrating. this is why modern cars are designed to deform so spectacularly on impact, and why rope designed for climbing is designed to be very stretchy.

I believe that you all have gone so off into the weeds chasing the physics of this situation that you had forgotten to remember Murphy's contribution to mathematics. And empirical evidence as to how probability operates.

No matter how much energy absorption potential there is in the Ridgeline of your tarp. And the Ridgeline of your hammock with suspension.

I am quite aware that if a branch falls from above me and happens to strike my Ridgeline. It WILL be deflected so that it stabs me Point first.

I can prove it mathematically but you'll have to buy me a round to get to hear the supporting documentation. I'll sing a couple verses of "Murphy is my Guardian Angel" for free.


Sent from my SM-G991U using Tapatalk

[nanok]

I believe that you all have gone so off into the weeds chasing the physics of this situation that you had forgotten to remember Murphy's contribution to mathematics. And empirical evidence as to how probability operates.

No matter how much energy absorption potential there is in the Ridgeline of your tarp. And the Ridgeline of your hammock with suspension.

I am quite aware that if a branch falls from above me and happens to strike my Ridgeline. It WILL be deflected so that it stabs me Point first.

I can prove it mathematically but you'll have to buy me a round to get to hear the supporting documentation. I'll sing a couple verses of "Murphy is my Guardian Angel" for free.


Sent from my SM-G991U using Tapatalk

oh, you underestimate us, sir. i mentioned/we agreed (i think) a while ago that the probability of any branch falling not touching the ridgeline at all is quite large. we calmly discarded that fact as "yeah, but where's the fun in that. now, about that integral..."

be that as it may, i'd buy you that round gladly. and be takin' notes. always keen on exchanging supporting documentation and evidence with a fellow murphy devotee.