Author Topic: Double Overhand as a flat-knot for joining rappel ropes  (Read 10170 times)

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Double Overhand as a flat-knot for joining rappel ropes
« on: February 16, 2010, 09:46:03 PM »
I will not rehash the EDK discussions here, other than to say I am of the cautious sort who has never used one. I have gotten ropes stuck , and I'm also considering a 10.2mm dynamic joined with a 7mm static for a rappel/retrieve combination: these two in combination have me researching flat knots again.

While looking over and tying the many knots in use for joining rappel ropes, I realized I had never seen a double-overhand flat knot. That is, simply tying a double overhand as you would to form a stopper knot, but with two strands.

In my quick testing I was unable to produce a failure by rolling, as I can easily do with an EDK and sufficient coaxing.

The knot is fast and easy to tie and check, as all climbers are used to tying double-fisherman's. It is compact, and would appear to not require long tails if it is truly capsize-proof.

I can only assume the knot is unused because it jams too strongly. Yet, I haven't had any trouble getting them out.

Am I missing some weakness of this knot, or is it an excellent replacement for the EDK?

roo

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #1 on: February 16, 2010, 10:32:19 PM »
I suspect that people don't try it simply because it just seems like it would be hard to untie.  Just trying it myself, it wasn't the easiest thing to untie, but I was able to undo it.  I would probably reserve judgment until I can test it with some other rope, and in slightly sloppified configuration, as the bend in question could easily be drawn up in a number of different ways (which again may lead to some hesitation among potenital users, justified or not).

I'll have to wait to test it further, but I'll be watching this thread with interest.


P.S.  Just trying the bend in question in some highly elastic material, I was able to make it roll and consume the tails. 

« Last Edit: February 16, 2010, 10:59:20 PM by roo »
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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #2 on: February 17, 2010, 04:03:49 AM »
Much to the chagrin of my wife, I spent some time this evening watching women's boardercross and bouncing around in my harness on various configurations of this knot.

Ropes were a 10.2mm dynamic and an 8mm static. I tied 10.2 to 10.2 and 10.2 to 8. I loaded both through a ring (as in a single-rope rappel), and in-line (as if joining doubles or twins).

I will refrain from making any final judgments about strength or security - 160lb bouncing around is hardly a rigorous test. I am pretty sure it's better than an EDK, but I want to test more before I begin using it in the field.

As for ease of untying - I had no problems at all. It's certainly easier than a double-fisherman's.

One final observation: It seems more secure when tied with the thinner rope on "top", much as the EDK does. But, either configuration shows no sign of capsizing - it just flips the diagonal wrap portion over and cannot flip any further.

Dan_Lehman

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #3 on: February 17, 2010, 06:20:58 AM »
I will not rehash the EDK discussions here, other than to say I am of the cautious sort who has never used one.
I have gotten ropes stuck , and I'm also considering a 10.2mm dynamic joined with a 7mm static
for a rappel/retrieve combination: these two in combination have me researching flat knots again.

The (infamous) EDK discussions are a regular feature of various
rockclimbing forums, to be sure.  Their continuation is assured by the
lack of research done by those coming new to such forums, and re-raising
questions & issues addressed previously & repeatedly.  But not all treatments
are good.  Sometimes there are test data cited although invariably the test
that was performed has less to do with how the knot will be used and what
the fear of it is -- it is just a slow-pull strength test, and, well, any knot is
strong enough for rappelling.

But some good information is out there, as well as decades of usage
testified to by users:  so, I wonder why it is that at this late time you
are still cautious about employing the knot?

It will help to forget the ill-fitting adjective "flat" and refer to the EDK
as the "Offset Ring Bend" -- the knot is offset from the axis
of tension; it is not "flat".   (Sure, "flat" has been uttered & echoed; big whoop.)

Quote
While looking over and tying the many knots in use for joining rappel ropes,
 I realized I had never seen a double-overhand flat knot. That is, simply tying a double
 overhand as you would to form a stopper knot, but with two strands.

Good for you -- explore a bit, look around.  Although, again, with some recent
(re)Search some other offset rap-rope unions should've popped up; I'll cite 'em.

Quote
In my quick testing I was unable to produce a failure by rolling, as I can easily do with an EDK and sufficient coaxing.

??!  You must have tied the ORB fairly loose and with a bad orientation
of different-sized ropes to get an "easily" flyping ORB -- for the knot can take
some rather stiff "shock" loading w/o doing that.  And it has been tested to
some extent in different-diameter ropes (though exact orientation wasn't clear)
to hold for well more than any easily generated force (several hundred pounds).
Maybe this "coaxing" involves the laying on of hands?   8)
(I've tried even with the Offset Fig.8 bend and not had it flype easily,
and that's one that has shown greater vulnerability to doing so.)

Quote
The knot is fast and easy to tie and check, as all climbers are used to tying double-fisherman's.
 It is compact, and would appear to not require long tails if it is truly capsize-proof.
I can only assume the knot is unused because it jams too strongly.

But the knot isn't a Dbl.Fish/Grapevine (which, btw, has been tried & tested,
via Jost Gudelius, from Franz Bachmann/Heinz Prohaska's idea); and I hesitate
to assert what "all climbers" can do re knots -- what they sometimes canNOT do
is pretty amazing (e.g., discern the difference between a Square & Granny knot).

As for "require long tails":  let's get past this nonsense!  To try to recommend
to someone, in a life-critical use, a knot that is asserted to be "fine" BUT "just
leave long tails" is really a lousy road to travel.  Who in their right mind is
going to swallow that sales pitch?  Why must the tails be long if the knot
is fine?
is the obvious thought.  :P
There are some really simple ways to prevent the flyping, by doing something
with those tails or using a slightly different knot.  I hope to see the long-tails
warning fade away via some alternative knot.  Eh, we'll see.  (There is rumor
of some climbers having mistakenly rapped on the long tails ... .)

Quote
Am I missing some weakness of this knot, or is it an excellent replacement for the EDK?

Consider what the design goal for a rap-ropes union knot is:
1) secure (slack & through moderate load, preferably any load);
2) offset, so to flow over rough surfaces easily;
3) compact (to lessen getting hung up somewhere (though I then
wonder if compactness would ever let it fall into a crack ...));
4) easily tied (sometimes in conditions of hastened retreat, stress, darkness);
5) non-jamming (although, really, in rappel, even a lardbutt shouldn't be
able to jam a knot much in commonly used cordage)

The effective geometries for desideratum #2 [that's a 10pt. word! :o]
call for some ring of rope to "choke" two ropes together at their
entry into the offset knot; ideal would be snug-fitting perfect ring,
but of course that's impossible.  The closest one can come to that
is a loop of rope.  But if you loop two adjacent ropes, you
effectively open the minimum spiral (of looping one) a little,
and you involve more rope in the knot than is necessary and
even helpful for this important securing.
.:.  It is better to have one of the ropes make choke and make
it well.  And it's better to do this with the thinner rope (if they
are different sizes), because not only is it likely that that rope
is more flexible and able to bind tightly here, but then it will
be the thicker rope that this choking rope would have to be
forced up and around for the knot to flype ("roll").  And there
is a further benefit, as will be seen -- in that this positioned
rope provides the end better to tie off.

Extensions and alternatives to the ORB have been presented
in this forum, here
http://igkt.net/sm/index.php?topic=1238.msg10536#msg10536
and on rockclimbing.com, here
http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2091962;page=unread#unread

-- with images.

(Please note the aspect of knot orientation demonstrated by the
Overhand-stoppered ORB; quoting RC.com post:
Please note that the same "knot" is shown in two orientations--the
extremes of how the knot body can be rotated in the surface plane


Btw, the testing that you are doing by bouncing your 160# is the sort
of testing anyone can do, and which you have aggravated by using
just a single line (you will not be rapping on a single rope with a knot
in it, will you, ever?!).  You can go further by rigging a simple 2:1
(theoretical -- not so good, actual) pulley and bouncing on that,
to put in, what, 400# or so?  Beyond this testing would be some
sort of check of knot stability when loaded and rubbing rock,
but in practice the conditions of knot placement are pretty well
under user observation & control, and shouldn't pose a problem.
And, in many cases, you can test the particular ropes that you'll
be using (I guess not always -- although, even on-site, one could
do some bouncing and examine the result before rapping).

--dl*
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Dan_Lehman

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #4 on: February 17, 2010, 06:27:15 AM »
One final observation: It seems more secure when tied with the thinner rope on "top",
much as the EDK does.

"on top" leaves me still unsure what you mean.  I've cited some posts
that have images.  I think that you would tied them the other way
'round (golden rope making the choke, orange rope away)?!  And that,
as was argued previously, seems well the worse orientation, for various
reasons (flexibility of thicker rope, and resistance to its movement by
thinner, being less).
!?

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #5 on: February 18, 2010, 03:21:19 AM »
One final observation: It seems more secure when tied with the thinner rope on "top",
much as the EDK does.

"on top" leaves me still unsure what you mean.  I've cited some posts
that have images.  I think that you would tied them the other way
'round (golden rope making the choke, orange rope away)?!  And that,
as was argued previously, seems well the worse orientation, for various
reasons (flexibility of thicker rope, and resistance to its movement by
thinner, being less).
!?

--dl*
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No, I meant that the thinner of the cords provides the critical choke around the standing ends.

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #6 on: February 18, 2010, 03:46:58 AM »

But some good information is out there, as well as decades of usage
testified to by users:  so, I wonder why it is that at this late time you
are still cautious about employing the knot?

You said it:

Quote
As for "require long tails":  let's get past this nonsense!  To try to recommend
to someone, in a life-critical use, a knot that is asserted to be "fine" BUT "just
leave long tails" is really a lousy road to travel.  Who in their right mind is
going to swallow that sales pitch?  Why must the tails be long if the knot
is fine?
is the obvious thought.  :P

It probably won't invert, but if a little effort gets me something more secure, I'm all for it.

But, not too much effort. I need to be able to tie my rappel knot in -20? F temperatures, wearing thick gloves, being buffeted by 50mph winds, while suffering from oxygen deprivation due to altitude. Easy is the operative word.

Quote
It will help to forget the ill-fitting adjective "flat" and refer to the EDK
as the "Offset Ring Bend" -- the knot is offset from the axis
of tension; it is not "flat".   (Sure, "flat" has been uttered & echoed; big whoop.)

Good for you -- explore a bit, look around.  Although, again, with some recent
(re)Search some other offset rap-rope unions should've popped up; I'll cite 'em.

Thank you for the correction - I agree that technical discussions are much easier when we all speak precisely. "Girth Hitch", anyone?

I have read the threads you pointed out - the backup overhand made by the thinner cord has the most appeal to me due to it's simplicity. However, I don't like the lack of security of the backup. Will it come undone if the wind flaps the ropes before I load the rappel? Possibly, especially with frozen dynamic ropes.

Quote
??!  You must have tied the ORB fairly loose and with a bad orientation
of different-sized ropes to get an "easily" flyping ORB --

Indeed I did. I believe in very worst-case testing.

Quote
I hope to see the long-tails
warning fade away via some alternative knot.  Eh, we'll see. 

I hope so as well, but if my partners and I can settle on a nice replacement I'll be happy.

Quote

Consider what the design goal for a rap-ropes union knot is:
1) secure (slack & through moderate load, preferably any load);
2) offset, so to flow over rough surfaces easily;
3) compact (to lessen getting hung up somewhere (though I then
wonder if compactness would ever let it fall into a crack ...));
4) easily tied (sometimes in conditions of hastened retreat, stress, darkness);
5) non-jamming (although, really, in rappel, even a lardbutt shouldn't be
able to jam a knot much in commonly used cordage)

I would revise that list of priorites:

1) Secure
2) Easy to tie (with no almost-correct configurations which are potentially dangerous)
3) Offset
4) Compact
5) Non-jamming - not that this isn't important, because with a tag line I have to re-tie at every rappel anchor. But, I have yet to find a knot that's difficult to undo when tied with at least one large-diameter dynamic rope.

I can't find a knot that meets that list better than the one I suggested. Some certainly constrict the standing ends better, but I don't see that degrees of offset-ness are of any importance. Compactness certainly lends itself to some of the other knots, but by a small amount which I'm not sure is of any importance, given how much simpler the double overhand is.


Dan, thank you very much for hashing this over with me! I'll point this out to my climbing partners and, if I can find a length of dynamic rope, do a breaking load test. No load cell, just to ensure it doesn't flype.

Dan_Lehman

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #7 on: February 18, 2010, 07:58:33 AM »
Thank you for the correction - I agree that technical discussions are much easier when we all speak precisely. "Girth Hitch", anyone?

Ahhhhh, yeah, THAT one.  It amazes me that soooo many folks can weigh
in on the "girth hitching" of slings issue without noticing the various geometries
that arise.  Heck, Black Diamond's Kolin Powick used to have a test report on
their site (I think it disappeared, alas) in which he tested the like-tied-to-'biner
form and the hard-to-exactly-orient "Strop hitch" form but NOT the one likely
to arise naturally from the former, which is like a Square-knot form but just
a little asymmetric in actual shapes of the respective slings (with apparently some
significance re strength or break point)!  Geesh ... .

Quote
the backup overhand made by the thinner cord has the most appeal to me due to it's simplicity.
However, I don't like the lack of security of the backup. Will it come undone if the wind flaps the ropes before I load the rappel?
Possibly, especially with frozen dynamic ropes.

Yes, it has simplicity, which as you remark can be a big plus
under stressful conditions (when NOT knotting in the arm chair).
Can't you set the knot a bit prior to full loading?  For this knot,
the force in prying the knot open should pull on the knotted
(extra, stopper knot) end and so help to set it beyond what
you've done in tying it.  And, with that knot being in the thinner
of the ropes (where different), it should better grip & hold onto
the thicker end.

Quote
Quote
??!  You must have tied the ORB fairly loose and with a bad orientation
of different-sized ropes to get an "easily" flyping ORB --

Indeed I did. I believe in very worst-case testing.

That's fairly wise, but do be fair about the reality of some of those
"worst cases" :  I mean, you ARE experienced (highly so, I hope,
if you're facing the conditions you mention), and with like companions,
so who's going to tie a knot THAT badly, really -- maybe that's UNfair.
Part of the benefit to the ORB is simple, easy-to-get right tying
and dressing and checking.  Perhaps though you're looking at the
flapping-in-the-breeze loosening?  (I mentioned Jost Gudelius and
the Grapevine:  Prohaska/Bachmann suggested tying that offset,
and there you gain in slack (flapping) security, paying with bulk.)

FYI -- and this is on-line -- one fellow recommended a full ORB as
the back-up stopper; to me, this is just overdoing it.  BUT, consider
that it is in a sense "same technology" (same idea & knotting movement,
you just involve both ropes tied around nothing instead of the thinner
tied around the thicker); it could be an easy "tool" to carry in case
you judge conditions of the ropes to warrant that.  (Not sure how
you go about testing those rope conditions  ("Honey, why is there
rope in our freezer?!"   ;D  ))

Quote
2) Easy to tie (with no almost-correct configurations which are potentially dangerous)

I hope you understood the "rotation in the plane" variations shown by
the ORB+stopper (two identical knots but in different rotations relative
to the axis of tension); that is an aspect of the offset knots (probably
in general) that folks have not generally noted.  The one end comes
in and makes a sort of forward arc, or --rotating the body-- makes a
backward loop (or in-between these extremes, arcing laterally).

Quote
I can't find a knot that meets that list better than the one I suggested.
 Some certainly constrict the standing ends better, but I don't see that degrees of offset-ness are of any importance.

Note that in the Offset 9-Oh --where the emphasis is on the full turn
of (thinner, if ...) rope at the choke point-- this choke also enhances
security:  for, with an iterative setting method, one hauls hard on
the thin end and tightens this choke, then hauls on the SParts to clamp
down on the two ends, and then on the ends again --and that choke
prevents the SParts from loosening back into the knot, and so preserves
the good "clamp" upon the ends.  (If anything, the good choke will
make the knot weaker -- but that is only a concern in some knot-testing
contest, and known to the test device, not pertinent to abseiling, where
the significant aspect is coming untied, or being stuck.)

Quote
Compactness certainly lends itself to some of the other knots, but by a small amount which I'm not sure is of any importance,
 given how much simpler the double overhand is.

Hmmm, I don't see the Offset Double Overhand as being all so simple,
as it can be prone to mis-dressing (crossed strands), which can occur
in who-knows ways (read:  hard to test against), and that would
worry me about its resistance to inversion.  And constricting the SParts
is the first defence vs. flyping.  The ORB note puts each rope to a
specific task, if you will:  the thinner chokes, the thicker gets stopped
trying to pull through; whereas you lop them together in a Double
Overhand, sort of.

I'm unclear about where this knot will be, such that it can be
flapping around in a breeze (or gale)?  In the typical rappel situation,
the knot is at the rappel anchor, start point, without much leeway
for flapping (sometimes set against the anchor to arrest movement).

??

As for testing, again, a break test really isn't what you want -- those
are way out-of-bounds loads (and you get one crack, one orientation,
or burn through a lot of rope).  Rather, do your force-enhanced
bouncing, where you can iterate through all sorts of loadings (not
ruining the rope given the forces), and be much better assured.
(And if done w/partners, they can share the assurance, and the
practice in tying.)

I'll point out that one can tie the stopper knot first,
and then set the ORB tightly against it -- not sure that's
a better way, but it's a way.  One advantage to the Overahand
stopper is that it can pretty well be set snug to something,
unlike any other stopper (Fig.8 or Dbl.Oh or Ashley's) --and
here is a case for that.

--dl*
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« Last Edit: February 23, 2010, 11:34:50 PM by Dan_Lehman »

sharky

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #8 on: February 18, 2010, 01:22:11 PM »
I would suggest tying that knot between two large trees, and set up a comealong on it to see if you have any chance of collapse or failure. If you have consistent results over 10 tries, then it is probably safe.
Sharky

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #9 on: February 18, 2010, 02:14:37 PM »
Dan-

Again, many good points. The backed-up ORB certainly has a lot to recommend it - I will try it out and see if there reasonable cirumstances that can comprimise it's security, or if a simple tightening of the main ring bend against the backup will hold snug.

My main concern with that lot has been the possibility of trying with the strands in the wrong positions, with the thick strand poviding the choke. But, although that is theoretically worse, it still seems to be flype-proof when backed up, so it's probably a moot point.

Dan_Lehman

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #10 on: February 18, 2010, 05:57:26 PM »
My main concern with that lot has been the possibility of trying with the strands in the wrong positions,
with the thick strand poviding the choke. But, although that is theoretically worse, it still seems to be
flype-proof when backed up, so it's probably a moot point.

Good point.  So, a test case should be this, AND both with the
thick end (proper one given misoriented beginning) and then
with the thin end (belatedly recognized as that that was to be
tied...) making the final, stopper knot.

My natural tying is to bend the paired ropes around my left hand,
and in that case, the thinner should be on the left, up into the
V between thumb & index finger.  Then the pair is turned around
my index & middle fingers, and tucked out the just-formed loop;
tensioning should pull the thinner (and now it should be on the
right) in towards oneself, whereas the other end is pulled more
up & away -- the choking/thinner strand is thus set well in
anticipation of the draw of the loading to rotate it around
anti-clockwise.  So if you & partners get into this tying habit,
you're halfway (or more) there.

The Offset 9-Oh puts care at the formation and then the
setting should be pretty straightforward -- pull on everything
and repeat.

--dl*
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Dan_Lehman

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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #11 on: February 18, 2010, 06:18:50 PM »
I would suggest tying that knot between two large trees, and set up a comealong on it to see if you have any chance of collapse or failure. If you have consistent results over 10 tries, then it is probably safe.

This is a good plan to increase the force upon the knot well
beyond that that should be ANYwhere seen in practice.
(Btw, I'm still unclear on where the knot is occurring and
if it's single-strand loaded or is in just one half of the abseil
support --the usual case.)  Given the factors at play here,
ten tries is not much scratching the surface:  NB, one doesn't
test "the knot" but some particular knotted material
-- no amount of testing in two equal low-elongation ropes here
would give assurance of the union of dynamic ropes, and then
there is the potential variety of dressing & rotation-orientation
of the knotted ropes, the positions of thick&thin, as has been
noted.  And in just dry rope, or ... (the Tom Moyer testing
of the ORB & Offset Fig.8 found some differences w/wet rope).
(cf. www.xmission.com/~tmoyer/testing )

It should be possible to run the test of a particular orientation
of the knot, slack off a little --but still maintain some tension--
and then rotate the knot as I've described above, which
is like dialing a different position, pointing the ends to a
different angle.  I'm coming to think that the surest orientation
might be that where the thinner rope (in proper, choking position)
makes the forward arc, and the thicker/non-choking makes the
loop-arc.  In the extreme rotations, the ends should be roughly
parallel to the axis of tension; in the mid-way rotation, the ends
are roughly perpendicular.

Come-along isn't necessary:  a couple 'biners, a Butterfly to clip
one into form a Trucker's Hitch structure, and you're getting
a 2:1 or so actual advantage which should get your line pretty
tight (maybe not ready for slacklining, but ...), and able to be
retightened after some initial bounces.
Hmmm, but at 120deg angle you're putting only the load
of your bouncing; in a vertically oriented line with a pulley
structure of some sort it seems that you'd exceed this more
easily.  The come-along will generate a steady force; the
bouncing will give "shock" forces, a bit more like rappelling.

--dl*
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Re: Double Overhand as a flat-knot for joining rappel ropes
« Reply #12 on: March 07, 2010, 09:22:59 PM »
I have been using this knot to join a 10.2mm dynamic climbing rope to a 7mm tagline, and I'm very happy with it. Getting it untied is no problem, it is very easy to tie, and the bulk is appreciated for blocking against the rappel ring or quick link. It jams very slightly into the ring, but a moderate amount of pressure on the 7mm static line pulls it loose.