Rebelaying long pitches

[Imo]

Went down Titan the other day and when you reach the rebelay you have to continue to abseil the stretch out of the rope, which I would guess was at least 3m worth if not more. Got me to thinking....

Is there a general rule of thumb about how much stretch there is in the rope say per 10m (I know its going to vary a bit from rope to rope). It would be useful to know when rigging on expedition how much remaining rope you'd need to keep back to compensate for the rope stretch on longer pitchs. Its always a pain when you have to do knot bypasses immediately above rebelays when people haven't got it right.

 

[damian]

The answer, as you suggest, is that it will vary considerably from rope to rope. Thinner ropes will generally stretch about twice as much as thicker ropes and some makes will stretch more than others.

I haven't actually seen a figure for %age stretch of semi-static rope between 0 and 8Kn - the figure quoted by manufacturers is, IIRC, between 5 and 15Kn (to represent the amount of bounce when prusiking). My best guess is that it is something between about 3% and 6%, depending on rope diameter and type.

When rigging rebelays, personally I would suggest attaching the hanger and maillon first, then clipping short cowstail into the hanger (above the maillon) and abseiling until your weight has transfered to the hanger. Then the stretch will have gone from the rope above, allowing you to get the length exactly right.

As a bit of a diversion, as a general rule of thumb these days, people tend to (when possible) rebelay more often than in the past. Therefore, rope sections of about 30m are more normal than of, say 100m.

 

[Imo]

When rigging rebelays, personally I would suggest attaching the hanger and maillon first, then clipping short cowstail into the hanger (above the maillon) and abseiling until your weight has transfered to the hanger. Then the stretch will have gone from the rope above, allowing you to get the length exactly right.

....provided you know in advance how much slack you actually need to keep back in the bag before placing the ancor.

I think I'll factor 2m of stretch per 30m of rope to be on the safeside.

 

[Slov01]

Its always a pain when you have to do knot bypasses immediately above rebelays when people haven't got it right.

You could move the rebelay to the knot?  ie put a new bolt in.

 

[Cave_Troll]

is it not in the spec of the rope?
for a given load, a % elongation will occur. this will be different for "dynamic " climbing ropes or "static" caving ropes, and will probably be on the spec sheet

 

[Bob Mehew]

The European / British Standard for SRT ropes (BS EN 1891:1998) states that elongation should not be more than 5% when measured using a 50kg mass.  The standard for dynamic ropes (BS EN 892:2004) states the elongation should not be more than 10% for  single and twin ropes and not more than 12% for a single strand of a half rope when measured using a 80kg mass.  These maximum limits are for any rope diameter permitted within the standard.  Both standards require the mass to be loaded without shock, so a typical bounce would no doubt add a bit more extension.  Having said that, the rope will contract on being left with a hanging weight on it after a bounce, though the amount is much smaller.

I have not tried to measure SRT rope extensions at different loads; my attempt stopped when I found the rope length was temperature dependent!  (I would speculate it might also may be wet / dry dependent.)

 

[seddon]

So really the way that damian describes it is correct - no shock there. Also Marbach, Tourte, Elliot etc etc.

Personally I lack the computational capacity to consider pitch length, relevant rope BS EN, temperature, humidity, rope age, and mass of load in the context of percentage elongation. Possibly this means I've failed some sort of SRT entrance exam?

Ah well, back to ladders it is, then...

 

[paul]

So really the way that damian describes it is correct - no shock there. Also Marbach, Tourte, Elliot etc etc.

Personally I lack the computational capacity to consider pitch length, relevant rope BS EN, temperature, humidity, rope age, and mass of load in the context of percentage elongation. Possibly this means I've failed some sort of SRT entrance exam?

Ah well, back to ladders it is, then...

But have you passed the Ladder and Lifeline Introductory Course? 

 

[seddon]

Well, I am quite hairy, and have been known to frolic about under waterfalls in rubber diving suits (with due acknowledgement to Dave Elliot who coined the immortal line) 

 

[damian]

Well, I am quite hairy, and have been known to frolic about under waterfalls in rubber diving suits (with due acknowledgement to Dave Elliot who coined the immortal line) 

No, no, no .... unless you know how to "scamper about beneath waterfalls in underwater diving suits with double ropes and Fig-8 descenders" it's just no good at all! 

 

[seddon]

Failed even that one 

Back to non vertical systems it is, then.

 

[Brains]

When our club has rigged such places as the Berger, we have used a full 200m length of pre washed rope that had also been used (eg at Eldon Hole, to use as much of the length as possible, from both ends and middle etc.). In the cave a more than generous rebelay loop has been left where needed, and at the bottom of a big pitch the rope would be cut leaving a generous 10% spare. This has been adequate for our trips to the sump over the period of the booking.

Many years ago, before my time, a club group were down GG main shaft on a new rope. after the last man down there was plenty of spare on the floor, but on their return a human pyramid was needed to retrieve the rope end. Apparently not a completely ideal situation... :-\