A 'gotcha' in the standard Y-hang method

D

Dep

Guest
Here is something that happened to me the other night on an SRT trip which proved instructive...

The site in question is a circular brick-lined shaft 50ft deep and about 8 feet in diameter. Flush with the ground it has a heavy metal grill over it for safety and a sliding hatch in the top.
There are two heavy girders parallel about four feet apart that lay across the top and form the main structure. The rest is filled in with metal bars about 4 inches apart. The opening is a square hole in the centre about 4 ft square.
A very simple SRT site - a Y-hang is rigged between the two girders and a re-belay to get down onto the main pitch.

I rigged a large Y-hang from the two main girders tied with the standard bowline-on-the-bight. From here I ran a re-belay loop up and over the lip of the shaft and off to a tree. To avoid dirtying the rope I left it coiled in the bag and tied it off about 4 metres below the Y-hang to be released by the first person down – me. A fairly tried and tested rig at this site. I did all of this sitting at the top of the shaft, I guesstimated the lengths of the Y-hang arms; attached them to the girders, rigged the re-belay and then started to lower the whole assemblage into place.

I lowered in the heavy rope bag first, let the line slide through my hand until I got to the knot, let this go and continued to pay out on the re-belay line until the Y-hang eventually came tight onto its intended belays. All simple no-brainer stuff. Just as I was letting go of the re-belay loop to drop it into place, there was a twang and the whole thing went out of shape.

The knot appeared to have capsized allowing one loop to run straight through the knot into the other; the Y-hang then slid across until it was all at one end – one giant loop. I have seen this happen before (from below and had to prussic up it!); the rig is still safe in so far as it will not come undone, but it puts all the load onto a single belay which is definitely not ok.

I was rather concerned as I was certain I had tied the knot correctly (I can tie this one with my eyes closed – literally) so I hauled it up and checked it. It all looked ok except that the knot had capsized. After a while I decided I must have made a mistake so played safe and took it all apart and started again; this time making very sure I tied the knot correctly; checking and double checking. I lowered the whole assembly into place and twang – it happened again!

This time I knew the knot was correct so the fault had to lie with the way I was loading it whilst lowering it into place. The weight of the bag (with 45m of rope in it) is pulling down on the main rope. As soon as I start to lower on the re-belay loop the strain is across the two knot-tails at 180 degrees, which inverts the knot.

So, confident that I had found the flaw I rigged it all again, and this time I lowered the whole assembly in by the Y-hang loops of the knot. With the load only on the main rope and no load at all on the re-belay loop it al dropped into place as intended.

So an unusual and not entirely obvious potential flaw in the standard Y-hang rigging method to watch out for. Using a figure-eight-on-the-bight is better, the knot is less prone to capsizing but this is still possible. The only sure solution is to be careful how you load the knot when it is not in its usual working position, and where possible lower the Y-hang into place by its ‘bunny-ear’ loops not by the re-belay loop.
Click to expand...
 
cap n chris

cap n chris

Well-known member
Nicely explained, Dep; s'happened to me lots of times - I now prefer to use ABs for general rigging since they're easy (and CAN be untied, despite what Joel wrote elsewhere, as long as you're using 9.5+mm rope) and use less rope.
 
J

Joel Corrigan

New member
Er, I mustn't have explained myself properly in th previous post as I almost exclusively use Alpines when i'm rigging as part of a Y hang, as I feel that bunny knots of any type use too much rope and are less versatile.  My only grievance with alpines, as I believe I state clearly in another thread, is that I feel they're unsuitable as stand-alone knots for single rebelays.  God, hate it when I get involved in geeky topics.  If I keep this up I'll have to join the knot guild....   
 
Stu

Stu

Active member
Dep said:
The only sure solution is to be careful how you load the knot when it is not in its usual working position, and where possible lower the Y-hang into place by its ‘bunny-ear’ loops not by the re-belay loop.
Click to expand...

Or use the rope from the tree to access the girders from below i.e. rig above your head, whilst on preferred abseil device. Though I'm assuming you did it your way to avoid having to use slings around the girders. It took me three re-reads to get your drift but once I fathomed it, it explained it all... good effort!
 
SamT

SamT

Moderator
To back joels comments up  -

if you are on a particularly large y hang (I consider 4 foot apart to be fairly large), By using an A.B. - you leave yourself with a single strand that can be jumared up to one of the belays. something that you cant do on bunny ears (b.o.b. or fig8). This can be pretty handy on some awkward wide belays.

However - it has to be said - on most normal - 18inch apart pitch head Y hangs - I just use a fig8 bunny ears knot. I never use a b.o.b. for no other reason than I just have got used to using fig8s'.
 
D

Dep

Guest
SamT said:
To back joels comments up  -

if you are on a particularly large y hang (I consider 4 foot apart to be fairly large), By using an A.B. - you leave yourself with a single strand that can be jumared up to one of the belays. something that you cant do on bunny ears (b.o.b. or fig8). This can be pretty handy on some awkward wide belays.

However - it has to be said - on most normal - 18inch apart pitch head Y hangs - I just use a fig8 bunny ears knot. I never use a b.o.b. for no other reason than I just have got used to using fig8s'.
Click to expand...

Hi Sam, yes I see what you and Joel mean, that has some uses provided the highest point you can get to up the single belay line is high enough to be at a safe place to mount/dismount.

Unfortunately this is not the case at the site I described, here the two main girders are 6" deep, and on the slack in a sling, plus a krab and the highest point in the Y-hang loop is a good 15" below the top.
The first time we ever rigged the site we did something similar, the highest point was too low - getting out was an epic!

How do you descend the ABK single loop Y-hang you describe?
Or do you lift up the rig, get your stop onto the main line - lock off and lower yourself by hand until you are on the rope?


A big Y-hang here has the rope nicely in the centre of the shaft use of a double loop knot allows us to use the long knot-tail as a rebelay loop and taken up and over the edge to a third  more distant belay point.

This allows you to sit on the edge and rig a stop just immediately below the lip when in place, or to jumar right up to the very top lip before a final stand on the Y-hang.
Although similar the two different rigs have about 15" vertical difference in how high you can go ascending and that tips the balance between anyone (mixed ability bunch) being able to get out as opposed to the strong agile climbers only.

Fig8 on the bight is indeed a tougher knot than the simper bowline-OTB (and I like the way it snugs down) but it is still possible to capsize it so the Y-hang loops run through - it is however less prone to this than the bowlineOTB as far as I can ascertain by making them up and then shaking them about until they loosen enough for this to happen.

I stress that the original point was not that there is any problem with any of the three knots so far mentioned but simply that when rigging from a higher position than the belays one should be very careful how the rig is loaded whilst lowering it into its working position.

Because I had a weight on the main rope and was holding the whole thing by the rebelay loop as I lowered it I was effectively pulling the knot tails apart.

But food for thought though as the ABK is not affected by opposed pulls on the knot-tails and given me an idea.
I might try this idea out the next time I am there - the hardest part will be convincing some of the others that it is a good idea! :)
 
SamT

SamT

Moderator
Yes - I realise that in this situation, jumaring up to one of the girders is of no use. It was just a food for thought.

I reckon what you are doing seems fine. The whole setup seems pretty bomb proof. and Im not so sure you should be too worried about the knot slipping so that the load is all one side of the Y hang , not ideal - but it doesnt sound like these girders are going anywhere.
 
D

Dep

Guest
Indeed - the girders are pretty serious.
It's just disconcerting when a familiar and ultimately simple rig suddenly does something odd like that.

The first time it ever happened was in the same shaft. But the knot held on long enough for me and another person to descend and only when he went to ascend first did it suddenly go out of shape. (I didn't trust his knot-tying ability so opted to ascend on it as well as I knew it was still safe rather than allow him to re-rig the rope where I could not watch and check!)
 
D

damian

Active member
SamT said:
if you are on a particularly large y hang (I consider 4 foot apart to be fairly large), By using an A.B. - you leave yourself with a single strand that can be jumared up to one of the belays.
Click to expand...

Dep said:
How do you descend the ABK single loop Y-hang you describe?
Or do you lift up the rig, get your stop onto the main line - lock off and lower yourself by hand until you are on the rope?
Click to expand...

I imagine that's the answer, Dep. This is why, on wide y-hangs using butterflies, I prefer to tie an additional butterfly in between the two y-hang butterflies. The sole purpose of this is then to provide an additional attachment point for cowstails, thus protecting both ascent and descent.
 
D

Dep

Guest
Dep said:
Three ABKs?
Click to expand...

damn we need the modify buton back!

I meant to add...

Surely multiple knot 'y-hangs' do not have the same geometry as a true single knot Y-hang - and as you transfer the load from one point to another it will shift - whereas a single knot system does not?

 
D

damian

Active member
Dep said:
Dep said:
Three ABKs?
Click to expand...

damn we need the modify buton back!

I meant to add...

Surely multiple knot 'y-hangs' do not have the same geometry as a true single knot Y-hang - and as you transfer the load from one point to another it will shift - whereas a single knot system does not?
Click to expand...

True, but I don't see that that's a problem.

As for 3 ABKs, the 3rd I mentioned is not actually connected to anything, but rather provides a spare loop to clip into for pretection. It is midway between the two y-hang ABKs. (Actually, I use cavers' butterflies, not ABKs, but that's another thread entirely!)
 
P

potholer

New member
Bowlines-on-the-bight are prone to distortion at the best of times. Tightened oddly (or distorted by loading without complete tightening), it's possible for them to turn into a kind of slipknot.
On some thicknesses/stiffnesses of rope, it can take a deal of tightening, working round the knot several times, pulling each of the 6 cords tight in turn, before the knot is solid enough to be loaded without shifting.
 
You must log in or register to reply here.
Top