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  1. #1
    Mouseskowitz's Avatar
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    Amsteel blue as structural ridge line?

    Hi, this is my first post. I've been hanging around the forum for a while and have learned a lot. However, I have some questions I'm not finding answers to.
    I'm in the process of building my first DIY hammock and am planning on using Amsteel blue for a structural ridge line. My first thought was to do a continual using something like a clove hitch or butterfly to tie off on the ends of the hammock. In researching what type of knot would work best I ran into some info that is making me wonder if Amsteel is the best thing out there for this type of application.
    In my research I found that as a rule of thumb a knot will reduce the strength of a rope by 50% but specific knots may be more efficient maintaining upwards of 70% strength. The caveat is that these tests are generally done in nylon kernmantal ropes and it would seem that material and construction significantly effects toleration of knots. Some of the data I saw indicated that Amsteel may break as low as 30% with knots.
    I'm thinking this weakening seams to come from the difference in the bend radius between required with different ropes. It states here http://www.sailingservices.com/catal...02009%2029.pdf that nylon rope needs a radius 3 times the diameter for the bend with 4 being ideal. However when you look here http://www.samsonrope.com/site_files...Retirement.pdf it would seem you need an 8:1 bend. The one thing I'm not sure about though is that is listed as a rotating sheave block and only 3:1 for a fixed pin termination.
    So, the question is where is the correlation between the theoretical data and real life application. What is the breading strength of a larks head on the end of a woopie? Is it only 30% of the 1600 lb of 7/64 giving me 480 lb, which is only about twice my weight which isn't much wiggle room.
    The next question would be, if bend radius is an issue would putting a nylon cover on the knot portion of the rope make a difference? This would take it from a 2:1 to a 3or4:1.
    I hope I've been clear in what I am trying to figure out. Also does it even makes a difference?

  2. #2
    Knotty's Avatar
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    It's hard to find definitive answers but the general rule of thumb is no knots in Amsteel (aka Dyneema). Two main reasons. 1 - amsteel is very slick and knots can slip and/or change form at relatively low loads. 2 - while extremely strong in tension, dynmeema is weak in compression and knots are all about compression.

    Splices spread out the load over a large area, overcoming the inherent problems of slickness and weakness in compression.

    As is we already tend to use our amsteel suspensions in ways that create a bending radius that's smaller than those used in rating the rope. I try to avoid adding more problem areas.

    Regarding the larks head at the fixed end of the whoopie, well that's usually applied to the gather at the end of the hammock so the bending radius is pretty large and the loop overlaps the line at a point where there's a bury so that helps spread the load. I would expect a whoopie larks headed to a hammock would still break at the usual whoopie weak point, the exit of the adjustable bury.
    Last edited by Knotty; 03-28-2012 at 11:10.
    Knotty
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  3. #3

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    yes but. You are starting with a line rated over 2000 lbs so you derate it to 1000 with a knot. How much does one need for a structural ridge line? From what I see used 500 lb is probably more than sufficient.
    What are you calling the ends of the hammock? Could be the line attached or the bundle of cloth. The latter would be more than big enough but not a good position. If you are doing the loop of line larks head to the hammock bundle then you have two diameters in the bundle. If you are then running amsteel between the larks head loops you are close to your 3x diameter but can stand a 70% deration. If you go to smaller line then you get 3x or more for your SRL.

  4. #4
    adkphoto's Avatar
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    I really don't know anything about the scientific data behind breaking strengths of Amsteel. I rely on the huge amount of information on this site showing reliable real-world application of it. Along those lines, I can tell you for a fact that Zing-it is more than strong enough for my structural ridge lines and its rated strength is a fraction of Amsteel. I would stay away from trying to tie a knot with Amsteel. Like Knotty said, the stuff is super slick. Just splice it. There are awesome tutorials on this site and it's easy to do with stuff you can find around your house. After I did it once, I was addicted.

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    David

  5. #5
    Bubba's Avatar
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    Sounds like the OP is thinking of doing a Single Line Suspension so it is not solely a question of ridgeline strength. For a SLS you'd definitely need amsteel IMO. I have read some people have had success using alpine butterlfy loops in amsteel. If I were to do a SLS I'd use descender rings on the hammock ends to make the ridgeline portion adjustable and the bend around the rings are less than in the alpine butterfly. To the OP, search the forums for Single Line Suspension and you'll see what I mean regarding using descender rings.
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  6. #6
    Knotty's Avatar
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    May be a misread on my part. Like Bubba, I was assuming the OP is asking about a form of SLS.

    For just a structural ridgeline, do what you want because the loads are less and failure doesn't put you at risk.
    Knotty
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  7. #7
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    Oh, sorry if I misinterpreted. Definitely need something as strong as Amsteel for SLS. Good advice Bubba.

    David

  8. #8
    Mouseskowitz's Avatar
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    Different forms of ridge lines be it SLS or something else is what triggered this thought process. What I'm not understanding is how to avoid a tight bend somewhere in the system.
    With the larks head I'm not seeing how to avoid a tight bend (tight being anything less than 4:1 which is still half the recommended) at the point where it crosses the standing part. I believe I know what you're talking about with the descender rings but the wraps still seem tight for a high modulus rope such as Amsteel. I may be misunderstanding something so please clarify if we don't seem to be on the same page.
    northermark, your post makes me think (I may be wrong) that you're not understanding that the deterioration of Amsteel strength due to knots is not the same as a nylon rope. Some of the information I read put the efficiency or % of original strength maintained in the 30% range and some anecdotes had it lower.
    Let me attempt to clarify my overall question this way. I understand that something in the splice family is the ideal way to deal with this rope as that is the way the strength of the rope is rated. However, in my understanding there is no way to completely avoid at least one tight bend somewhere in the system. The information I have been able to find does not seem clear on the behavior or the rope under these conditions. Thus what is the margin of safety we are working with? Going back to the math I used. 1600 lb rope with a 30% knot gives 480 lb. Since I am around 240 if I end up pitching at 20 degrees instead of 30 I can easily meet or exceed that level of stress.
    Does this help to clarify my question and train of thought?

  9. #9
    Knotty's Avatar
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    OK. That clarifies things Mousekowitz. Unfortunately there probably is no definitive answer to your concern about the small bending radius problem. All part of why I feel safest using at least 7/64" amsteel for suspension. The 1000# rated Dynaglide is just cutting it too close for too little gain IMHO. However, field experience of those using Dynaglide does not support my concerns.
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  10. #10

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    Quote Originally Posted by Mouseskowitz View Post
    Different forms of ridge lines be it SLS or something else is what triggered this thought process. What I'm not understanding is how to avoid a tight bend somewhere in the system.
    With the larks head I'm not seeing how to avoid a tight bend (tight being anything less than 4:1 which is still half the recommended) at the point where it crosses the standing part. I believe I know what you're talking about with the descender rings but the wraps still seem tight for a high modulus rope such as Amsteel. I may be misunderstanding something so please clarify if we don't seem to be on the same page.


    northermark, your post makes me think (I may be wrong) that you're not understanding that the deterioration of Amsteel strength due to knots is not the same as a nylon rope. Some of the information I read put the efficiency or % of original strength maintained in the 30% range and some anecdotes had it lower."
    Answer 1
    Actually I learned the problem with monofilament fish line. Same problem. I think Amsteel blue is dyneema with a proprietary coating. That makes it an Ultra-High Molecular Weight Polyethylene. Translate that to no cold flow problems, high impact resistance, and almost no stretch. Nylon stretches and cold flows under load. That is part of the failure mechanism. It is part of the limiting factor of bend radius. Essentially the bigger the bend radius in a knot or over a post the larger the support area so the lower unit load. Dyneema (Amsteel) is less sensitive so should be able to do a tighter bend. OTOH bends are the failure point so they should be minimized.

    I did goof on line strength as I was quoting numbers for 1/8 and you may be looking at 7/64. Either way the point was use a line size that can stand the derating. If you have a 500 lb load and are taking an 80% hit use 1/8 in amsteel with a 2600 lb breaking strength.



    Quote Originally Posted by Mouseskowitz View Post
    Let me attempt to clarify my overall question this way. I understand that something in the splice family is the ideal way to deal with this rope as that is the way the strength of the rope is rated. However, in my understanding there is no way to completely avoid at least one tight bend somewhere in the system. The information I have been able to find does not seem clear on the behavior or the rope under these conditions. Thus what is the margin of safety we are working with? Going back to the math I used. 1600 lb rope with a 30% knot gives 480 lb. Since I am around 240 if I end up pitching at 20 degrees instead of 30 I can easily meet or exceed that level of stress.
    Does this help to clarify my question and train of thought?
    answer 2

    So go to !/8 in amsteel as one easy option.

    The other option is to look at how the system is designed. Most folks use a separate line to lash the ends of the hammock. This provides a relatively large diameter bundle of cloth for the eye spliced loop to larkshead around. No radius problem. On the other end it goes around the knot, not the toggle. large radius. The moveable part is in the sleeve, different forces.

    The structural ridge line has a low load, often slack at the optimum hang angle so it can be a smaller diameter line so if you design things well the smaller line will be tied around a double diameter part of the loop splice and be relatively close to to 4 or 5 to 1 bend radius as well as being under less load.

    As I said, you can always go bigger. I made my whoopies out of 1/8 both for my weight and for ease of working on the line. At your weight I'd go 1/8 to be safer. YMMV. I'm heavier than you. Complaining, not bragging. ;-)

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