Author Topic: Important Double Sheet Bend query  (Read 19366 times)

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #30 on: June 20, 2014, 06:50:10 AM »
It is still too early in the morning to go test anything as the sheet winches are right above our cabin and my better half is asleep.

So, I have been playing with the single and double lapp bends in the same diameter line. I am extremely impressed. I will explain why I think these bends may be brilliant (they need proper load testing beyond my scope to check security, but I am very optimistic).

The big reason is that when pre-tightened up by hand as much as possible (so that the working tail ends up perpendicular to the two standing ends), when tension is put on the two standing ends everything just pulls together tighter. So when the tension is released, the knot is still snuggly set. On the other hand, the sheet bend, having had pressure put on to open up the apex of the bend slips a bit to set, and then has a big gap created in the apex, making it very easy to undo when the tension is released, but also making it extremely prone to coming undone on its own. Just holding a standing end and flapping it around a few times can undo the sheet bend.

Secondly, the lapp bend is quicker to tie than the sheet bend, as the tail never needs to be fed under any component, just through the bight, which is a much easier alternative. For those with clumsy or cold numb fingers, this is important.

Looking at it, I can see absolutely no drawbacks with it compared to the sheet bend, except perhaps that a bit more care may be needed to pre-tighten it so that the working tail ends up perpendicular ie it needs to be snugged well (only takes a couple of seconds, so no big deal). The sheet bend has no such problem as it naturally slips anyway when load is put on it.

So, if the above is correct, other than when you want to untie the bend very quickly after load has been applied, when would it be more appropriate using the sheet bend? Why is the lapp bend not currently commonly in use? No sailors I know have heard of this bend. Is seems better, so what is the catch? (Other than needing a diagram to learn how to tie it :))

PS You have successfully distracted me from the burning question on hand of why 1434 is presented as a DSB rather than 488. But my current question still stands :).
« Last Edit: June 20, 2014, 06:58:06 AM by Seaworthy »

Dan_Lehman

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Re: Important Double Sheet Bend query
« Reply #31 on: June 20, 2014, 06:56:35 AM »
[...]
Of course, the zeppelin is woefully awkward with much
line-diameter difference, [...]
With much line diameter difference, any bend is going to look awkward.  Did you have some elegant exception in mind outside of a hitching solution?
"The sheet bend family" doesn't look so awkward,
and goes some way.  But, yes, it does really become
such a joining that one might prefer to see it as
hitching --indeed, I like to refer to a special
class of rope-2-ROPE joints as "bight hitches",
with the bigger rope pretty much playing an
*object* rather than doing much participation
in the *knotting*.  (It's a slippery slope, though,
as to what forms one might see as being such
"objects" being hitched to.  The bight (open or
closed --i.e., an eye) seems apt for the treatment.)

AND I think --w/o good knowledge-- that one
marine application would be messenger lines
--the hauling of ever larger lines by another
(of which I've read of breakage, so it mustn't
be entirely trivial!).


--dl*
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Dan_Lehman

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Re: Important Double Sheet Bend query
« Reply #32 on: June 20, 2014, 07:12:52 AM »
Double lapp jamming and non jamming versions attached.
Before I go test it, is this finally correct?

If I just followed Dan's instructions it would have made it easier :).
Yep, reading "in slow motion" as it were would
be a help to many, here (who whine about photos)!   :D

And I might've noted the relation of #488 reversed,
though I was focused on the non-jamming version.
WHICH, incidentally, can --again-- be guarded against
that jamming : i.e., while the wrap-all wraps prevent
one from the forcible loosening method of pulling
apart the bight legs to prise in some hitching SPart,
one might build in resistance to jamming 'a la
sheet bend at the bight tip where the hitching
line enters (as I also suggested, above).

NB : I find it helpful to treat even eyes as though
they are "open" bights --i.e., a bight with one leg slack--,
as the focible loosening method just noted will work
best if the draw of the hitching SPart has pulled
that "slack" (tail, if "open" bight) leg a little over
the other leg, and reversing that movement in
forcibly untying I think gives one a little extra
movement (potential).

--dl*
====

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #33 on: June 20, 2014, 07:31:13 AM »
Double lapp jamming and non jamming versions attached.
Before I go test it, is this finally correct?

If I just followed Dan's instructions it would have made it easier :).
Yep, reading "in slow motion" as it were would
be a help to many, here (who whine about photos)!   :D

Suitably chastised :).
Blame Grog for my inability to read instructions :).

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #34 on: June 20, 2014, 09:17:23 AM »
I will move this discussion onto an old lapp thread. Any suggestions which one is most appropriate?

I have just trialled the jamming version on our sheet winches and results will be buried in this thread.
Will do non jamming version later. On the move shortly

enhaut

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Re: Important Double Sheet Bend query
« Reply #35 on: June 20, 2014, 05:50:38 PM »
Hi Seaworthy,
That was an interesting thread.
Maybe you already know Asher's Simple Simon Under.
Here it is ; a loose an a dressed version.
Are you able (time, interest) to test it with your gear?
I would love to know it's behavior vs the Double sheet bend. (slipping, jamming)
ths.

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #36 on: June 20, 2014, 08:06:00 PM »
Hi Seaworthy,
That was an interesting thread.
Maybe you already know Asher's Simple Simon Under.
Here it is ; a loose an a dressed version.
Are you able (time, interest) to test it with your gear?
I would love to know it's behavior vs the Double sheet bend. (slipping, jamming)
ths.

Very happy to do that. My gear is very limited though, but a double sheet bend slips if tied as 1434 (16 & 10 mm double braid polyester was tested), so it has its uses.

I am using a 'Lewmar 55 two speed sheet winch' to winch the heavier line and tying a midline bowline on the other and looping it around an adjacent winch. I am winching by hand, maximum amount comfortably. It is not even vaguely near the breaking strength of the line. Wondering if I can put a load cell in the set up, or if more room between the winches is needed for this, but that would need to wait until I had a delivery address at the end of the year. I measured the amount of stretch in the line over 10 cm this morning using a ruler (gives a rough idea of relative load for each trial). I will make it 20 cm next time.

I had only had a chance today to do one quick trial on a double lapp jammable version. 5-10 trials of each would be good to make results more reliable. Slippage can just be measured with calipers by looking at tail length before and after the trial.

Before I set up proper trials I will have a carful read of Evans Starzinger's pdf to see exactly what test procedure he used. I also need to get hold of some new line (lots of old stuff only on board).

Anyway, I am not familiar with the Asher's Simple Simon Under, but will test it out with interest. Roughly what diameter line would you like tested? It is most likely it will perform differently for different sizes.


« Last Edit: June 20, 2014, 08:25:47 PM by Seaworthy »

enhaut

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Re: Important Double Sheet Bend query
« Reply #37 on: June 20, 2014, 09:13:48 PM »
Quote
Roughly what diameter line would you like tested? It is most likely it will perform differently for different sizes.
The rope's ration  in my pictures is around 2/1.
Anyway pitch in anything you have at hand!
It's the slippage that is of interest for me.
Thanks
jr.
Ps Harry Asher also devised the Simple Simon Over and the Simple Simon Double.

enhaut

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Re: Important Double Sheet Bend query
« Reply #38 on: June 20, 2014, 09:53:58 PM »
Forgot the images ;D

xarax

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Re: Important Double Sheet Bend query
« Reply #39 on: June 21, 2014, 12:33:12 AM »
  5-10 trials of each would be good to make results more reliable.

  20 -25 results would be much better, IMHO. If you want to study their statistics, I recommend the Weibull distribution. Evans is a statistician, so he may help you on this, too.
This is not a knot.

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #40 on: June 21, 2014, 05:31:10 AM »
  5-10 trials of each would be good to make results more reliable.

  20 -25 results would be much better, IMHO. If you want to study their statistics, I recommend the Weibull distribution. Evans is a statistician, so he may help you on this, too.

It is ages since I analysed anything statistically (30 odd years) so I am extremely rusty, but number of trials required also depends on spread of results. Any statisticians here able to lend me a hand? There is nothing commercial in any of this. I do not even have a blog, do not participate in Facebook, do not have a website. Results will be freely shared. I will report any conclusions on the cruising forum as well.

The problem on board is that apart from equipment, I am extremely limited by materials. I currently have old jib sheets and halyards I can use - precious as testing on old line is, I feel, vital. Old line has been stretched, UV exposed and salt impregnated. Unlike climbers who would replace line frequently after big loads, sailors continue using line for years. Tests in new line are great (and I will do these too), but unfortunately don't replicate 'real world' performance. Simply storing large quantities of line in different diameters is difficult.

The line (and particularly the knot) shouldn't be reused for trials as the trauma to the line may affect the subsequent result. Over a metre of line (x2) is used each time. So imagine how much line I am going to need if 25 trials are done per knot (per diameter, as thicker line performs very differently: the thicker the line, the bigger the gap when it bends). Just way beyond my scope.

I think perhaps all I can do is get a rough idea what knots look promising and need to be investigated further.

xarax

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Re: Important Double Sheet Bend query
« Reply #41 on: June 21, 2014, 06:25:13 AM »
  the number of trials required also depends on spread of results.

  And the  spread of the results depends on the number of trials !  :) The more tests you perform and the more results you acquire, the spread gets bigger. What will tell you the required size of sample you need , is the required reliability : of course, we do not need 5 sigma in knotting, as in Particle physics !   :) ( = one chance in two million, to be wrong ! )

Over a metre of line (x2) is used each time. So imagine how much line I am going to need if 25 trials are done per knot

Now you know one of the many reasons I prefer OPT !  :) ( Other People s Tests ).

thicker line performs very differently: the thicker the line, the bigger the gap when it bends).

   No, that is irrelevant : the relative geometry of all the elements of a knot does not change with size.
   What does not scale uniformly, is the size of the knot ( which changes ) in relation to the size of the individual fibres ( which remains the same ).

I think perhaps all I can do is get a rough idea what knots look promising and need to be investigated further.

  A "rough idea" is a much much much better thing than "no idea whatsoever" ( = the black box we were talking about the other day...). Go on !
« Last Edit: June 21, 2014, 06:26:54 AM by xarax »
This is not a knot.

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #42 on: June 21, 2014, 07:01:21 AM »
thicker line performs very differently: the thicker the line, the bigger the gap when it bends).

   No, that is irrelevant : the relative geometry of all the elements of a knot does not change with size.
   What does not scale uniformly, is the size of the knot ( which changes ) in relation to the size of the individual fibres ( which remains the same ).

No, it is not irrelevant at all. It is critical. Wider diameter lines do not bend as easily, they become stiffer as the diameter increases. Imagine a 10 cm thick line. It can simply not be bent back well on itself - there is too much difference in the distance the outer part of the bend needs to travel compared to the inner (the inner portion can simply not compress enough to accommodate this). Let you mind go further and consider a metre wide line. How well will this bend? Then consider a thread. A very sharp turn is possible.

One unusual characteristic of unsheathed Dyneema is that it can be bent back at acute angles. Hugely in its favour. Imagine how much worse it would perform if this were not the case.

I think perhaps all I can do is get a rough idea what knots look promising and need to be investigated further.

  A "rough idea" is a much much much better thing than "no idea whatsoever" ( = the black box we were talking about the other day...). Go on !

Thanks for the encouragement. The work I do now will just be ground work. It will perhaps give me something interesting to work on when age or ill health make me a 'landsman' again.

xarax

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Re: Important Double Sheet Bend query
« Reply #43 on: June 21, 2014, 07:39:51 AM »
No, it is not irrelevant at all.

It is !  :)

Wider diameter lines do not bend as easily, they become stiffer as the diameter increases.

 "Stiffer", is a measure of the force you need to bend them. The wider the diameter, the bigger the size, the greater the force you need to apply, to bend a rope. I claim that, all other things remaining the same ( the braiding of the individual fibres, for example ), the size and the stiffness scale up uniformly. 

  Imagine a 10 cm thick line. It can simply not be bent back well on itself - there is too much difference in the distance the outer part of the bend needs to travel compared to the inner (the inner portion can simply not compress enough to accommodate this) .

  You are still talking about the force required to bend a rope. This force is relative to the size of the rope, of course. The difference in the length of the paths of the inner and outer threads scale up in exactly the same way as the size of the rope.

One unusual characteristic of unsheathed Dyneema is that it can be bent back at acute angles. Hugely in its favour. Imagine how much worse it would perform if this were not the case.

   Now you shift the goalpost again !  :) You mention a property of the material, not of the geometry...Dyneema does the same thing in small sizes and in large sizes, so the size of the line is irrelevant. I made a comment on your post, where you said that :

 
thicker line performs very differently: the thicker the line, the bigger the gap when it bends).

 

  Thanks for the encouragement. The work I do now will just be ground work. It will perhaps give me something interesting to work on when age or ill health make me a 'landsman' again.

   Actually, I believe that, on a boat, it may be easier to perform tests of knots than on land - where you have so many other much more seductive/important things to distract your attention and consume your time !  :) All you need is a long ram hydraulic bottle jack ( I do not think that the mechanical winches are suitable for strength tests...), your old lines, or old lines of other boats you will encounter in your journey, ( which may be weaker, but, if they are not worn locally, they will generally behave like just ropes of a smaller size...), and patience:)   
This is not a knot.

Seaworthy

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Re: Important Double Sheet Bend query
« Reply #44 on: June 21, 2014, 07:50:15 AM »
thicker line performs very differently: the thicker the line, the bigger the gap when it bends).

   No, that is irrelevant : the relative geometry of all the elements of a knot does not change with size.
   What does not scale uniformly, is the size of the knot ( which changes ) in relation to the size of the individual fibres ( which remains the same ).

Just coming back to this issue, I tested the single lapp bend using the same single line yesterday. I literally burst out laughing at the ease in which it slipped on the winch (I never imagined knot testing could be so funny)- it slipped like a hot knife through butter. I think has I tested this in thin line (still keeping the same ratio of 10:16 for the line diameter) the results would have been very different.