Unveiling Bullseye Strops for low friction rings

[Seaworthy Lass]

Originally Posted by 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.

Yes, the central thing was an artistic impression of circular .

As the line is snug on this, the outer edge of the ring is the same as the inside edge of the line.

[estarzinger]

I did just go back and look at my throat ratio tests.

The (near) 1:1 test all broke at the throat - which suggests the throat design was wrong, could be improved . . . and implies there is little to be learned about a continuous loop at 1:1 other than the whipping needs to be strong. Also . . . one of my tests had an odd/low result. I kept it in the average, but it makes that average result suspect. And a true 1:1 is essentially impossible to achieve in actual practice. The loop will always be pulled at l;east a bit off the ring in actual practice. So, this is a "near 1:1" rather than a true 1:1

On the 2:1 tests, they all broke at the end of the splice taper - that is at the weak point in the single strand of the system and not in the loop. Yes, I guess I agree that means that a 2:1 in a continuous loop will definitely be stronger than 50% of system strength (stronger than 100% line strength). I think the trigonometry would give us a more useful result than that.

[Seaworthy Lass]

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?

This is directly taken from your load testing document:

"What impact these 'peeling loads' with have on splice strength will depend in part on the specific splice and line.* But to gauge rough impact on strength I built three samples with 1:1 bend radius and 3 with 2:1 radius. The 2:1 bend radius samples all broke at the splice taper end near the line rated strength. While the 1:1 bend radius lines broke at the throat at an average of 75% of line strength (note: one of these pulls broke at what seemed an anomalous low value, but I left it in the average because I could not see anything 'wrong' with it). *So, for this splice (simple bury) I would say that 1:1 loses some (15-20%) strength but 2:1 is near full strength."

Taking the 2:1 ratio first:

All three samples all broke at the taper.
Therefore a 2:1 system has at least 100% line tensile strength.
In an end loop situation this gives 100% system strength.

So, in a closed loop situation, doesn't that give at least 50% of system strength? See diagram for more notes.

That is where my figure of 50% came from.

I will explain the 75% one next.

[estarzinger]

Sorry, I jumped ahead of you see post #152 above - yes I agree "at least 50%" - but suggest we could get a better answer than that from the trigonometry.

[Seaworthy Lass]

Originally Posted by estarzinger

Sorry, I jumped ahead of you see post #152 above - yes I agree "at least 50%" - but suggest we could get a better answer than that from the trigonometry.

Yes, agreed, I will work it out.

[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? 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?

This is where my 75% comes from:

Quote from your load test doc referring to the three 1:1 samples:

"the 1:1 bend radius lines broke at the throat at an average of 75% of line strength"

The three samples broke at an average of 75% of line strength.
In an end loop system the system strength is 75%.


In a closed loop system it is still 75% of line tensile strength, but system strength should be 200%.
This gives 37% of system strength.
This is a 63% reduction.

For some reason, have been discussing 10-15% losses with a 1:1 throat angle in a closed loop situation, not 63%.

This was the issue I was trying to point out in post #137.

[estarzinger]

^^ I disagree with that conclusion. These samples broke at the throat. Which suggests the throat design was poor for handling the peeling/ring loading. It was a simple bury splice with no whipping. I don't think you can conclude that a near 1:1 continuous loop with say a proper whipping would break at that strength - because the original test weak point is changed completely.

I believe all it says is not to use a simple bury in a very tight loop. I do not know what would be the best construction there or how much stronger the best could be.

[estarzinger]

Fwiw. . . . The antal ring with 6mm dyneema line has a rated swl of 45% of the rated line tensile strength. The swl rate a 6mm shackle with hard toggle at 63%.

Let's say the breaking strength of that soft shackle with toggle is 220% of line strength. That implies, if they are using the same swl factor, the ring/strop is 157% of line strength (or 78% of system strength).

We do not know if the strop or the whipping then breaks at that load.

It is whipped quite close. It is not 1:1, because nothing really is, but it looks closer than 2:1 ( I will know exactly in a couple days).

[Seaworthy Lass]

Originally Posted by estarzinger

^^ I disagree with that conclusion. These samples broke at the throat. Which suggests the throat design was poor for handling the peeling/ring loading. It was a simple bury splice with no whipping. I don't think you can conclude that a near 1:1 continuous loop with say a proper whipping would break at that strength - because the original test weak point is changed completely.

LOL!

That is why I asked about this in post #138:

Originally Posted by Seaworthy Lass

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.

And you replied:

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).

So I think I was correct with my initial hypothesis.

Evans, what I think is important is not the theoretical calculations, but what occurs in practice using dyneema strops with low friction rings restrained using whatever methods we have at our disposal.

A throat angle of a bit more than 1:1 (the flange of the rings allows improvement of this ratio) can only be achieved by long bury spices (unlocked as you tested or locked), Brummel locks or some kind of mechanical binding such as whipping. I have my doubts whipping would be a lot better, but it would be great to test this out.

I doubt you have a poor bury technique for splices, in fact it would have to be far better than the average cruiser's. I imagine you would take care not to split the strands with the penetration. You would certainly not do a worse job than the average cruiser. If in fact whipping is better it would be nice to know this.

So I would have to disagree with you. Your results are not invalid at all in my opinion. In fact, they likely reflect precisely what is sometimes happening in the real world.

Originally Posted by estarzinger

I believe all it says is not to use a simple bury in a very tight loop. I do not know what would be the best construction there or how much stronger the best could be.

Possibly a Bullseye with no splice of any kind or clamping of any kind, just the line moved to that position by defection?

SWL

[estarzinger]

some drawings (I think I have done these correctly) - a 2:1 loop:



The loop load will only be 6% greater than the line load (the two red numbers), so system strength reduced by 6%.
The peeling/ring load on the compression point will be 35% of the line load (purple number).

1.5:1 loop:



The loop load will only be 15% greater than the line load (the two red numbers), so system strength reduced by 15%.
The peeling/ring load on the compression point will be 58% of the line load (purple number).

In a theoretical 1:1 loop (which can't happen in the real world), the loop and peeling loads will be infinite. So the above numbers rise up in a sharply rising curve as you get closer to 1:1.

[estarzinger]

Originally Posted by Seaworthy Lass

LOL!
That is why I asked about this in post #138:
And you replied:
So I think I was correct with my initial hypothesis.

I still disagree. And so does Antal.

My point was that the fundamental increased load that you cannot design around is the trigonometric load on the line. Yes there is peeling load, which yes increases with reduced throat angle BUT you can always design around that. Antal has obviously designed a whipping that withstands the peeling load.

I believe you are confusing design choices with fundamental engineering. My tests showed that I choose an incorrect design, NOT that peeling was an insurmountable problem.





I have my doubts whipping would be a lot better, but it would be great to test this out.

It turns out that clearly whipping is the design solution choose by essentially all the commercial products. You can always design a whipping to be stronger in peeling - you use stronger whipping line and more wraps.

We will disagree on this I guess. That is fine with me. But I hope you do now understand #1 the magnitude of the line loading and the peeling loading and (2) that there are possible design ways to deal with the peeling load but you are stuck with the line loading so that IS the fundamental constraint. and that Antal's SWL's seem to exactly confirm this (78% system strength in a design somewhat tighter than 1.5:1).

The bullseye may be a good design solution to capturing the ring while keeping throat angle low. As I said in a post somewhat above, what we dont know about the bullseye is whether there is any issue with the crossed strands. I guess not much, but we don't know and there are really no similar designs that I have seen tested. The bullseye shines in applications where you want to be able to move it frequently/easily, But, and I know you disagree, I prefer the two strand continuous loop better in many 'mostly fixed' applications. That is not worth debating because you have such pride in ownership in the bullseye (which is understandable - both Grog and Allen also took strong 'pride in ownership').

...........

[Seaworthy Lass]

Originally Posted by estarzinger

My point was that the fundamental increased load that you ca not design around is the trigonometric load on the line.

I don't disagree with this. I have fully accepted that the strength of the perfect restraint will be affected by the throat angle. I did that instantly.

Originally Posted by estarzinger

Yes there is peeling load, which yes increases with reduced throat angle BUT you can always design around that. Antal has obviously designed a whipping that withstands the peeling load.

I believe you are confusing design choices with fundamental engineering.


It turns out that clearly whipping is the design solution choose by essentially all the commercial products. You can always design a whipping to be stronger in peeling - you use stronger whipping line and more wraps.

We will disagree on this I guess. That is fine with me. But I hope you do now understand #1 the magnitude of the line loading and the peeling loading and (2) that there are possible design ways to deal with the peeling load but you are stuck with the line loading so that IS the fundamental constraint. and that Antal's SWL's seem to exactly confirm this (78% system strength in a design somewhat tighter than 1.5:1).

Well firstly, manufacturers are not necessarily known for selecting best designs. Take commercially available soft shackles for instance .

No, I am confusing design choices with fundamental engineering. I fully accept the issues of reduced strength with throat angle. I am not suggesting you can avoid this.

Given your comments, I also accept good whipping may be be better than a mechanism that allows the dyneema to pull apart internally. My above remarks above regarding whipping were only speculation on my part.

BUT! If whipping is a better solution, does it then not follow that entirely avoiding splicing or whipping as the restraining mechanism may not in fact possibly be better? One method of capturing the LF ring in fact does that at the moment .

SWL

[Seaworthy Lass]

Originally Posted by estarzinger

some drawings (I think I have done these correctly) - a 2:1 loop:

Attachment 128447

The loop load will only be 6% greater than the line load (the two red numbers), so system strength reduced by 6%.
The peeling/ring load on the compression point will be 35% of the line load (purple number).

1.5:1 loop:

Attachment 128448

The loop load will only be 15% greater than the line load (the two red numbers), so system strength reduced by 15%.
The peeling/ring load on the compression point will be 58% of the line load (purple number).

In a theoretical 1:1 loop (which can't happen in the real world), the loop and peeling loads will be infinite. So the above numbers rise up in a sharply rising curve as you get closer to 1:1.

Yes, you have done these correctly.

At least I agree with your maths 100% .

Edited to add: well, 99% at least. The reduction in strength in your last example is 13.4% not 15% I think.

Explanation:
I think the proportions to consider are 0.866 :1, not 1: 1.15
This is a 13.4 % loss, not 15%
Easy was to calculate the correct proportion is simply:
cos (half the throat angle) : 1


Thanks for the exchange. I enjoy thinking all this through and being challenged.

I will look at the throat angle when the Bullseye is tied as tightly as possible to see what reduction in strength is theoretically going to occur when the ring needs maximum possible restraint.
Given the Bulleye loop strop loses 50% system strength with a 1:1 line diam:bend diam attachment and the Bullseye soft shackle strop loses 54-64% system strength with the diamond knot, it is probably a good idea to reduce the throat angle to a touch less than 2:1 and really firmly restrain the LF ring.

SWL

[Seaworthy Lass]

Guys, guess what caught my eye today on our dive?

Anchors and octopuses barely received a second glance .

Dockhead, note they have given their twing a 4:1 purchase. I think they have attached the control line to their pad eye to do this and then passed it through the small lower rings once each and larger ring twice.

Check out the lacing on the lashing of the strops at at the base of the twing.



I took a few shots of the underwater profile if anyone is interested.

SWL

[Alan Mighty]

Originally Posted by estarzinger

My point was that the fundamental increased load that you cannot design around is the trigonometric load on the line. Yes there is peeling load, which yes increases with reduced throat angle BUT you can always design around that. Antal has obviously designed a whipping that withstands the peeling load.

Being a slow thinker, I took a while to remember my experience with what Evans and SWL are calling 'throat angle' and referring to in ratios.


It's an angle for which I cannot remember a dedicated name in wire rope rigging, but is effectively the exit angle of wire from a thimble or sheave (in the odd case when a wire is made temporarily around a sheave).



And the rule of thumb I was taught for the thimble exit angle is nothing smaller than 60 degrees. Since tan 30 is 0.57 then if my understanding is correct you should aim for a 'throat angle' never less than 1.14:1.


I took a while to find a reference on the net. See Figure 4 at: Crosby Wire Rope Clips - Warnings and Safe Installation Instructions


In wire rope work, we used 'throat angle' for a different animal, the internal angle of the groove of a sheave.