Friction hitch without access to ends?

[xarax]

If the rope is dynamic, then it will further elongate and reduce in diameter when the added load is applied, so the hitch has to handle this, being able to reduce to below one diameter where it is trying to nip the main line.

   I have never thought of that !  - but now you mentioned it seems to me reasonable, but probably insignificant : if elongation of a dynamic line is, say, 4%, then the reduction of the diameter of the line would be about the square root of this ( so the total volume of the material remains the same ) -  that is, about 2% : a very small, and probably insignificant factor in what we discuss.

[SS369]

If the rope is dynamic, then it will further elongate and reduce in diameter when the added load is applied, so the hitch has to handle this, being able to reduce to below one diameter where it is trying to nip the main line.

   I have never thought of that !  - but now you mentioned it seems to me reasonable, but probably insignificant : if elongation of a dynamic line is, say, 4%, then the reduction of the diameter of the line would be about the square root of this ( so the total volume of the material remains the same ) -  that is, about 2% : a very small, and probably insignificant factor in what we discuss.

Small things can lead to big things. I would not discount this as insignificant. Depending on the particular hitch, one small move for knotkind, one giant step for slippage.   

This small stretch and reduced diameter has been brought up by rope manufacturers as a cause of sheath slippage, etc., and that can effect many things with the rope's use. Knots included. They are bonding sheath to core fibers with some unique methodology now in regards to this occurrence. With the new techniques, load transference within a knot is now being changed to some degree.

SS

[xarax]

one small move for knotkind, one giant step for slippage.   

 

[shadowjack]

Hopefully I will be able to test some of the ideas in two days under my body weight in static and dynamic ropes (and may be combinations). I will be backed up, so do not worry!
I'll start with Blake's tied with bight and then rat-tail. If any of them holds, I'll stop. If not will try other ideas. I hope to videotape experiment.

[Dan_Lehman]

Hopefully I will be able to test some of the ideas in two days
under my body weight in static and dynamic ropes (and may be combinations).
I will be backed up, so do not worry!

Or you might be only a foot or so above ground.

I'll start with Blake's [aka "Prohgrip"] tied with bight
and then rat-tail. If any of them holds, I'll stop.
If not will try other ideas. I hope to videotape experiment.

I can't imagine a rat-tail tied w/o ends
(perhaps that's why we've no picture from X.).

I'd like to understand what sort of application
you have in mind that puts these constraints
on the system --the lack of access to ends,
and the need to shift load (and how this is
done!) !?

--dl*
====

[DerekSmith]

Most good gripping hitches that could cover this application rely on a difference of rope diameters (prusiks, 60% of main line size approx.) and type of material used (soft second rope for hard first rope and vice versa).

SS

You make a most important point Scott, I had forgotten it.

I will often Prusik the end of my climbing line back onto the down leg from the belay point, that is, self to self.  Although it is dynamic rope and obviously the same diameter, the Prusik holds well.  But of course I forgot that the principle load is taken between my belt and the top belay point, far less force remains in the returning line and that is all the Prusik has to contend with.  Also my VT hitches are tied with a 60% diameter line, so again do not suffer from the mismatched diameter problem.

@shadowjack  - the VT hitch should hold better than the rat-tail because the rat-tail has already lost its leverage by being made in the open configuration.  Hitches like the VT and the KC by contrast utilise the opening of their wrappings to create leverage and gripping tension.  But having said that, both are known to behave poorly when they are tied using rope of similar diameter.

[shadowjack]

About possible scenario. The most simple is following:
You are rappelling down single line. Something got sucked in rappel device (loose closing, for example) and it jammed. To unjam it is simple matter, but you must first unweight it. You are free-hanging some 20 m above the ground. It's an emergency, and you do not have additional gear or partner to pass you something. You try to pull up the end, but it's stopper knot snags badly on rock and you can't free it from your position. You are stuck. All you have is several meters of bight below your rappel device. You must attach it above rappel device, tie foot loop, shift weight to it, maybe temporarily tie in with bowline-on-bight to your harness, unjam device, undo friction hitch and loops, rappel down and free stuck end. Of course you must tie "catastrophe knot" below you rappel device to attach yourself to line in case friction hitch fails (bowline-on-bight again).
Shifting weight to different rope is common operation in rescue and self-rescue. Escaping belay, untying knot, you name it. It's usually achieved with Prusik loop, but you might not have it on hand. Ends of rope could be used for something else - lowering something, etc., so you do not have access to them during operation.

[xarax]

I can't imagine a rat-tail tied w/o ends
(perhaps that's why we've no picture from X.).

I can t imagine why GLASSES are so rare nowadays !

However, there is always the possibility to enlarge anything that a computer screen shows :

  If the second line is of a much smaller size than the first, you can perhaps try to tie a double-line rat-tail stopper
  the two-line Standing and Tail ends of this double-line rat-tail stopper

a double-line climbing gripping hitch is an even bulkier, and an even more ugly, thing, than a double-line rat-tail stopper !

( A double-line VT hitch is not less bulky, ugly and difficult to dress than a double-line rat-tail-stopper. I do NOT recommend those knots, they are NOT practical ! )

( So, I was talking about a double-line rat-tail-stopper, right from the start... Also, I stressed that, for many reasons, this is NOT a practical knot, IMHO )

[xarax]

the VT hitch should hold better than the rat-tail because the rat-tail has already lost its leverage by being made in the open configuration.

Noope ! If one joins the ends of the rat-tail-stopper properly ( as I had done (*), in the pictures shown at the thread about it - but there are also many other ways one can do the same thing ), there is no "open configuration" any more : the whole hitch works as a crossed-coils spring, that can be elongated in length = shrink in diameter, around the main line.

Derek, please, TIE one rat-tail-stopper !  It does not BITE anybody beyond the main line ! 

  Hitches like the VT and the KC
  are known to behave poorly when they are tied using rope of similar diameter.

Welcome ! 
In contrast, the rat-tail-stopper is NOT behaving poorly - it behaves wonderfully  ! However, this is only "known" to people who had actually tied and tested it. around tensioned lines of the same material and diameter as the hitching line - as I did, for example...

(*) About the "closing" of the rat-tail-stopper :

  Now, on the matter of the closing knot. This is of not much importance, really. The only thing we have to achieve is a connection/bend of the two ends, so that that the pulling of the each one of them would be transferred to the other, too. We do not want the one rope to creep on the surface of the Main line more than the other, and the whole hitch to loose its initial symmetry. So, we should preserve a symmetry of loading as much as we can, somehow. The most simple and easy  way is to tie, with each leg/end, an overhand knot around the Main line, and to interlink those two overhand knots in some way. Two interlinked half hitches/nipping loops would also do the job. An other way is to "weave" the legs/ends around the main line and between them at the lower section of the hitch, so they would also be fastened together.

[xarax]

I will often Prusik the end of my climbing line

  I do not like the use of the name "Prusik", to denote a double-wrap Cow hitch - I think it was just a duplication of names, which made the knotting jargon even more repulsive to the general public than it already was -  but to use it as a VERB, is even more odd, to my eyes... I believe we should use common language, and common terms, wherever we can.

[Dan_Lehman]

About possible scenario. The most simple is following:
...
Shifting weight to different rope is common operation in rescue and self-rescue.

Hmmmm, this is quite different than the vague
notion I had : here, you load your 2nd rope
from below, and I was trying to think of some
case where this rope was running beside and
needed to support ... from above!
(To assume that you have just enough slack
below, yet a stuck rope to keep the end away,
is getting pretty picky!     In my vague scenario
it would just be so lonnnng 'til the end (thinking
of doing this exchange in caving, near upper
anchors) that one wouldn't care to use the end.)

In your case, you need also to have sufficient
material in the lower end (aka *2nd* rope) in
order to make some attachment structure
--an eyeknot, likely--; and THEN hitch that
rope into position to be effective.

Frankly, I would simply try putting in a rolling hitch
letting the one, away side of the bight used in
tying fall away, having an effective single-strand
(usual) knot until the closing half-hitch which
would be a bight.  The tying becomes cumbersome
with each additional wrap of to-be-tight & fall-away
bight strands.

--dl*
====