Unveiling Bullseye Strops for low friction rings

[Seaworthy Lass]

Originally Posted by Jim Cate

SWL, if you don't want to use "diamond" in the name, you could substitute "rhomboid"...

Jim

Good suggestion, but I am smitten by "Bullseye" and won't budge .

SWL

[Seaworthy Lass]

Gosh, I am struggling with load issues. I keep thinking the light has dawned, then I change my mind .
Evans, thank for your patience.

Effect of bend ratios is good now at least .

Next issue: Effect of throat angles

From everything written lately in the two threads on loop strops retaining low friction rings, I understood the minimum throat angle we are aiming for is 2:1 in a loop strop system in order to make sure system strength was not compromised by the throat.

We have frequently thrown about this 2:1 criteria and in Dockhead's thread I specifically listed 2:1 min throat angle in the list of features desirable in any design for retaining a LF ring in a loop strop. No one corrected me.

This is actually incorrect for a loop strop. Same issue as with the 1:1 bend ratio being OK in your tests when a loop is passing around a curved object. Both sets of tests were performed with eye splices, not a continuous loop.

Load testing data is incomplete when it comes to deciding what doesn't limit strength for a loop system, so we can't say much about the effect of throat ratios in loops other than:

1. A 1:1 throat will give around 75% of line tensile strength instead of 200% with no throat. This reduces the possible maximum loop system strength by 63%. This is FAR worse than the figures of 15-20% loss we have been discussing, a whopping loss in fact! Dockheads instincts of wanting next to no throat angle are perfectly justified.

2. For anything other than 1:1 we can make no conclusions other than if it is 2:1 then loop system strength is at least 50% of its potential maximum. We have no idea if 2:1 is actually "OK" for a loop.

This contradicts what we have been discussing in both recent loop strop threads.

Am I correct with all this?

SWL

[Seaworthy Lass]

Effect of throat angle continued:

If I am correct in the previous post, then this new interpretation of system strength losses with throat angle will have no effect on the Bullseye loop strop, as each of the two throat angles is only on one leg of the strop, so full system strength is retained.

The effect of throat angle on the Bullseye soft shackle strop (which has two legs per throat) depends on the answer to my next question.

Evans, in your load tests was the throat angle strength limited by the strands being stressed by whatever was holding them together at the throat, which was the bury point in a splice in your tests, but in other splices they could also be stressed by clamping or whipping, stitching etc?

I would think this is the case. If so, none of your throat data applies to the Bullseye strops, as the throat is caused by kinking of the line due to deflection, not by a splice or clamp.

I had been working on achieving at least 2:1 throat angle in the design, but maybe the weave can be jammed right up against the LF ring with minimal loss of system strength for all the Bullseye strops. This would just mean the LF ring could be super snuggly retained in case of flogging of the strop.
I am just playing with proportions .

SWL

[estarzinger]

SL I honestly don't understand your speculations in post #137.

But I can answer your question in #138 - no you have the fundamental mechanism wrong - the throat angle "problem" is actually about simple trigonometry. It is mostly not about strand stress at the constriction point (edit - there is in fact a "peeling load" issue there but it is not the primary driver of the throat angle concern).

Let's take a different application for a minute - jack lines - if a jack line is very tight, when you pull on it, there is huge leverage (force multiplier) on the end attachment points. While if the jack line is slack, the leverage, and force on the end points is much lower. This is leverage due to trigonometry - force vectors. It is why you can tighten a halyard by pulling out (away from mast) on it at the mast, when it is too loaded to pull down on.

The same apply some to loops. As the angle of the loop as it goes to the ring gets high, the leverage and force multiplier increases. this applies similarily to end loops as to continuous loops with throat constraints. The trigonometry creates higher load on the rope all around the ring, not just at the throat. They both break "in the loop" with very high throat angles, and thus their system strengths are the same despite the continuous loop being "stronger" outside the throat angle effect.

So, does that help you reframe your question?

[Seaworthy Lass]

Originally Posted by estarzinger

SL I honestly don't understand your speculations in post #137.

But I can answer your question in #138 - no you have the fundamental mechanism wrong - the throat angle "problem" is actually about simple trigonometry. It is mostly not about strand stress at the constriction point (edit - there is in fact a "peeling load" issue there but it is not the primary driver of the throat angle concern).

Let's take a different application for a minute - jack lines - if a jack line is very tight, when you pull on it, there is huge leverage (force multiplier) on the end attachment points. While if the jack line is slack, the leverage, and force on the end points is much lower. This is leverage due to trigonometry - force vectors. It is why you can tighten a halyard by pulling out (away from mast) on it at the mast, when it is too loaded to pull down on.

The same apply some to loops. As the angle of the loop as it goes to the ring gets high, the leverage and force multiplier increases. this applies similarily to end loops as to continuous loops with throat constraints. The trigonometry creates higher load on the rope all around the ring, not just at the throat. They both break "in the loop" with very high throat angles, and thus their system strengths are the same despite the continuous loop being "stronger" outside the throat angle effect.

So, does that help you reframe your question?

Thanks. That answers my question well in post 138. In your load testing doc you discuss "peeling load" and I incorrectly thought that may refer to the load at the end of the throat.

-----

I will rephrase my main question in post 137 more simply:

If you have a closed loop (perfectly end to end spliced) and that loop has a throat at one end what is the throat ratio need to maintain full system strength?

Rough diagram:

(PS Just going for a dive)

[estarzinger]

here is a picture - may be helpful



So we have a rope leading to a post. the external load on the rope is 2 units (lets say kgs). The throat angle of the rope before the post is constrained - it does not matter if this is done in an end loop splice or a whipping on a continuous loop, or simply a tightish continuous loop. And, in this particular set of angles, the load on the rope as it goes around the post is 2.8 units (kgs) - or 40% higher than the base load. That is the load on the rope all the way around, not just at the constriction.

That is the most basic mechanism at play here. In 'the real world' there are some others - peeling and frictional effects - which complicate it a bit. The peeling effect does depend on the specific throat constriction approach and the fiber type.

The cordage industry standard says this throat angle should be minimum 3, preferably 5 (for commercial applications). Those (have seemed to me to be) are quite conservative.

[Dockhead]

Originally Posted by estarzinger

SL I honestly don't understand your speculations in post #137.

But I can answer your question in #138 - no you have the fundamental mechanism wrong - the throat angle "problem" is actually about simple trigonometry. It is mostly not about strand stress at the constriction point (edit - there is in fact a "peeling load" issue there but it is not the primary driver of the throat angle concern).

Let's take a different application for a minute - jack lines - if a jack line is very tight, when you pull on it, there is huge leverage (force multiplier) on the end attachment points. While if the jack line is slack, the leverage, and force on the end points is much lower. This is leverage due to trigonometry - force vectors. It is why you can tighten a halyard by pulling out (away from mast) on it at the mast, when it is too loaded to pull down on.

The same apply some to loops. As the angle of the loop as it goes to the ring gets high, the leverage and force multiplier increases. this applies similarily to end loops as to continuous loops with throat constraints. The trigonometry creates higher load on the rope all around the ring, not just at the throat. They both break "in the loop" with very high throat angles, and thus their system strengths are the same despite the continuous loop being "stronger" outside the throat angle effect.

So, does that help you reframe your question?

I wish we had a smiley for a light bulb going off in your head.

Thank you for this extremely important information! It never occurred to me, and suddenly I understand this whole thing a lot better.

Where did you learn all this stuff? I guess this must be a whole branch of engineering -- ropes, cables, knots, etc. The engineering aspects of this are not covered in the rigging books I've read (The Rigger's Apprentice being my favorite). Maybe you should write a book on it?

[estarzinger]

Originally Posted by Dockhead

Maybe you should write a book on it?

Thanks - I do not understand this as well as I wish I did - SL's questions are good - make me further my own understanding.

As I told SL, I have really moved away from all this rope stuff - moved onto another quite different (set of) project(s), so I am really dredging this stuff up from two years ago when I was studying this. I often screw up and am sloppy in that memory.

[estarzinger]

Originally Posted by Seaworthy Lass

Thanks. That answers my question well in post 138. In your load testing doc you discuss "peeling load" and I incorrectly thought that may refer to the load at the end of the throat.

-----

I will rephrase my main question in post 137 more simply:

If you have a closed loop (perfectly end to end spliced) and that loop has a throat at one end what is the throat ratio need to maintain full system strength?

Rough diagram:

(PS Just going for a dive)

Simple answer - any throat angle at all will decrease that closed loop system strength. If you constrain the closed loop at all then you create trigonometry which imposes higher loads on the rope than the 'pulling load'.

The difference with an end loop system is that the body of the system is starting weaker than the loop. So in that case you can have a throat angle which decreases loop strength but still maintains system strength.

Yea . . . peeling loads - honestly I do not understand them well. Samson mentions them, and so I did. I know they are still an effect of the trigonometry, but pulling outward on the constriction point. But beyond that I dont really know much about their dynamics.

[Dockhead]

Originally Posted by estarzinger

. . . I often screw up and am sloppy in that memory.

Well if that's so, then I think we can safely say that you forgot more about it yesterday, than we'll ever know


It's a discipline which is really relevant to our sport, and I am amazed and humiliated over and over again by how little I know about. Every time you poke your nose into one small aspect of it, you see whole worlds of other things you never even knew existed. It was just last year (or the year before) that I realized that I didn't really understand how pulleys work (reeving to advantage, etc.)

I think that there would surely be a market for a concise guide to the engineering of cordage and tackle, for yachtsmen.

Concerning rigging in general, I sure love Toss' book, which I've read and re-read several times, but it doesn't cover this stuff in any depth.

[estarzinger]

one further thought on peeling loads - related to a prior discussion we have had, they impose 'ring loading' on the constriction point. As we saw previously with the grog/brummel, ring loading can effect rope constructions in a quite different way than end loading. If you are expecting ring or peeling loads then you need to design the construction quite differently than if the primary load will be end loading - and the best ring/peeling design will almost always be a bit weaker in end loading. So, I think the industry basically says, make the throat angle such that the ring/peeling load is minimized and then use the best end loading construction.

What I do not know, is if you are making a design that will have peeling loads, what are the best designs. In a couple days I am going to take apart an antal ring/strop and see what they actually did.

[Seaworthy Lass]

Originally Posted by Dockhead

I wish we had a smiley for a light bulb going off in your head.

Me too LOL. And that feels good, as I hate groping around in the dark.
Evans has been brilliant with clearing things up.

Originally Posted by estarzinger

Simple answer - any throat angle at all will decrease that closed loop system strength. If you constrain the closed loop at all then you create trigonometry which imposes higher loads on the rope than the 'pulling load'.

Oh, of course . Yes, I was in a rush and did not think out my simplified question .

The correct simple question is: "What loss in strength will you get with a closed loop system and various throat angles at one end?"

We only have limited end loop information in your load testing document.

Originally Posted by estarzinger

The difference with an end loop system is that the body of the system is starting weaker than the loop. So in that case you can have a throat angle which decreases loop strength but still maintains system strength.

Yes, precisely! That is exactly what I was rambling on about in post #137.
A throat of 2:1 did not reduce system strength in a end loop system.
BUT we have been discussing closed loop systems and continuing to say 2:1 is OK.
I think we have no basis for doing this. I think all that your load test data on end loops shows is that when the throat ratio is 2:1 in a closed loop system, then we have at least 50% of system strength.

Your load data does, however, give 1:1 throat information for a closed loop I think. This is 75% of line tensile strength, reduced from a potential maximum of 200%. This is a huge loss. Bigger than we have been discussing. The figure of 15-20% loss has been repeated frequently for a 1:1 throat in a closed loop system. It is actually a 63% loss - down from 200% of line tensile strength to merely 75%.

Does that make post #137 any clearer?

Originally Posted by estarzinger

I do not understand this as well as I wish I did - SL's questions are good - make me further my own understanding.

I am not scared to ask questions and look stupid if it helps me understand .
Particularly the last posts have crystallised things well. You will be pleased to hear that is it for now when we finish with the above. Everything else is clear.

SWL

[estarzinger]

^^ do you understand that the way 1:1 throat ratio is defined that would be a closed loop that is completely tight all the way around the ring? I guess I am still puzzled about your #137 assertions. I am not sure that any of my test data is relevant to the closed loops 1:1 or 2:2 throat ratio question - what specific data are you referring to? Where are you getting the 75% and 50% numbers from?

As to the "what loss at what throat ratios" for closed loops - I do not know - the minimum loss would be the simple trigonometric load increase. That is easy to calculate (as I did in the drawing above). Then on top of that you would have to add these peeling load losses, but they are much more complicated and depend on the construction (design and material) of the constriction mechanism (eg does that now become a failure point or not - it might or might not in the same load case/same throat ratio depending on how it was constructed - Dock's whipping peeled, but the Antal one does not).

[Seaworthy Lass]

Originally Posted by estarzinger

^^ do you understand that the way 1:1 throat ratio is defined that would be a closed loop that is completely tight all the way around the ring?

Yes, got that (see my super rough diagram I scrawled above, for my understanding of ratios).

Originally Posted by estarzinger

I guess I am still puzzled about your #137 assertions. I am not sure that any of my test data is relevant to the closed loops 1:1 or 2:2 throat ratio question - what specific data are you referring to? Where are you getting the 75% and 50% numbers from?

I will draw pics and explain in a sec.

[estarzinger]

On ratios, usually they are shown as the inside length to width of the loop . . . But if the central thing is exactly circular (as a friction ring is) then yea your drawing is correct.