polyester rope.

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Agrophobic

Guest
Hi guys!
Can anyone tell me the advantages and disadvantages of polyester versus nylon for caving rope?
I have "aquired" 100m of marlow 11mm polyester abseil rope as used by forces personell to drop off cliffs and out of choppers but is it any good for caving use?
Apart from being twice the price of lsk.
cheers.
 

nickwilliams

Well-known member
It's a _lot_ less stretchy than nylon. Fantastic for prusiking, not good if you fall on it. It's probably also thicker, heavier and will hold more water than nylon.
 
M

MSD

Guest
It also creaks and grunts when you prussik on it, which takes a bit of getting used to!

Mark
 
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Agrophobic

Guest
Cheers guys (y)
I'll give it a try, less stretch sounds good for prussiking long pitches.
is it weaker than nylon or just doesn't absorb the shock?
thanks for the warning about creaks and grunts, I'd have been a bit alarmed if i hadn't been warned.
As for the spelling that's easy- polyethyleneterephthalate. :tease:
pronunciation is harder.
 

Stu

Active member
It's of similar or greater strength (flavours vary).

Not that it's a worry but I'm pretty sure it's not C.E.  rated to the same standard which doesn't mean it's not safe; it might have implications for use with a group for example.
 
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Agrophobic

Guest
cheers Stu.
Personal use only  (y)
it's rated to a M.O.D standard but i can't find the spec for that even though i work for the mod.
If it's good enough for special forces it's good enough for me.
 
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phil.gregson

Guest
A 10.5mm Polyester kernmantle rope will have a very similar strength to a Nylon one of equal diameter.

The issue is with the stretch.

'Low stretch rope' as used for SRT conforms to EN1891:1998 - Personal protective equipment for the prevention of falls from a height. Low stretch kernmantel ropes.  This places many requirements but the most relavent one here is that of peak force following a fall - Off the top of my head I think it must have a max peak force of 6kn at fall factor 0.3.  Polyester kernmantle ropes are extremely low stretch and do not (indeed, cannot) meet this requirement.

What does this mean?

Well, they are extremely static and therefore great for prussiking (I've done a about 900' on one and it didn't feel too bouncy at all - it felt like bloody hard work going up as well as down but not bouncy).

However if you fall significantly it will transmit a large amount of force to you - which you won't like and may damage you severely, if not kill you.

If anchors or re-belays should fail it will not only transmit significant force to you, but will also transmit it to other anchors in the system.

In an industrial context it has the advantage of being acid resistant and its use can be risk assessed in for this reason as long as careful attention is given to potential fall factors when in use and rigging.

On balance, though, I wouldn't use it for caving for several reasons:

Not as safe
Not everyone using it may be aware of its differing properties
More expensive.

Hope that helps

Phil
 
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Agrophobic

Guest
Point taken Phil. :-\
I see you would need an extremely low stretch rope for 900' or you would bounce.
With regard to polyester not meeting en1891- what would the peak force of 80kg at ff 0.3 be?
If this is nowhere near 6Kn after allowing for a safety factor (of say two, maybe en1891 specifies more)
i'd be happy to use it.
Giving careful attention to rigging to reduce fall factors as allways.
 
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Agrophobic

Guest
OK... i just crunched those numbers and a FF of 0.3 would put a peak force of 2.7Kn on the rope, assuming an 80Kg caver.
So if en1891 specifies 6Kn that's a safety factor of just over two.
Judging by this, poyester sounds a bit dodgy.

Also from www.aditnow.co.uk/documents/personal-album-4/Caving-skills.pdf
"static ropes which have no stretch, such as black marlow abseil rope should not be used for srt as their lack of stretch will result in high loads being placed on the anchors and the caver if there is a fall."
 
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phil.gregson

Guest
I think you may have misunderstood.

The 6kn figure I quoted is not the max force the rope will take at 0.3FF but the max force the rope will transmit to the person on the end (or the anchors on the other end). i.e. how much of the fall energy it will absorb.

So when you are calculating the energy and peak force of a falling body the FF is imaterial.

An 80 kg person falling 2m would have an impact force of (Im guessing here) around 10kn.  A normal 'low stretch' rope should be able to reduce that to  6kn at FF3 - the static rope would not reduce it anywhere near that far.

Phil
 
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Agrophobic

Guest
Cheers Phil.
I relised this after my last post.
Basicaly. IF polyester is used, rigging should be bomb proof as the rope won't absorb the force if a belay fails. and care should be taken to ensure you minimise the distace you fall (preferably to zero, though this is not always practical) as the cowstails will be taking all the shock load and the FF will be greater as the rope won't absorb much?
 

Bob Mehew

Well-known member
Force is defined as mass times acceleration.  Thus peak impact forces relate to the rate of deceleration, or more simply how fast you stop.  So if your were say held by a metal chain the peak impact force would be much higher than if your were held by a rubber band.  Peak impact force is mostly down to the energy absorption properties of the material holding you.  And if you have an understanding of maths, down to not just deceleration but also rate of change of declaration known as jolt or jerk.  These properties are nicely shown by silly putty, see http://www.doitpoms.ac.uk/tlplib/glass-transition/demos.php for the videos or http://www.campoly.com/documents/appnotes/sillyputty.pdf for simple photos.  One cannot simply increase or reduce peak force by the ratio of different masses, that does not seem to work.  I have not yet found a technique to predict it and alas I have no access to a machine to measure it. 

Phil's guess at 10 kN is just that.  But I would say that the military are clearly aware that they don't want their soldiers arriving on the ground with back injures in the middle of a battle, so I am confident they will have put a specification on this property.  The only problem is that the military work with fit and trained young people, so they are likely to take a similar line to peak impact forces as Air Forces did and tolerate much higher peak impact forces, perhaps as much as 20 kN.  For more info, see http://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf for more info.  Access to MoD specifications seems to be controlled, see http://www.uktdl.com/default.htm .  I can understand why they may well not wish to let even Agrophobic have access as it could compromise preplanned events involving abseiling.

So I reckon the bottom line is how much does Agrophobic value his back and how fit and young he is compared to say a Marine. 

Bob

PS - I trust Agrophobic has read the French report on Cows Tails at http://british-caving.org.uk/rope/lanyard_tests_v6.pdf
 
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Agrophobic

Guest
Cheers Bob, i'll read and digest those links.
i'm certainly not as fit as your avarage marine. and while no where near drawing my pention, i'd love to be 21 again :LOL:
to take this topic even further from original- if you are clipped into a rebelay piont by your cowstails is this not the same as being attached to the main belay by a polyester rope? ie: theres nothing to absorb the force etc of a fall axcept your cows.
 

seddon

New member
Agoraphobic - remember, absorbing forces is part of what cowstails are for. Otherwise the ideal cowstail would be steel wire - which it ain't - rather than dynamic rope or tape with other force reduction features built in.

Even a very low stretch rope will possess some shock absorption value. This will be related to size of fall / length of rope available to absorb shock, so again fall factors do come into the picture. So you are quite correct to say that rigging may be an effective way of managing the risk of using this type of rope. Certainly, you'd be worse off if a main belay failed while you were just underneath it, or a rebelay failed if it was a short distance below a main belay, than if you were on a less static rope. This is why, when using spits, we double up the bolts and prussik sensibly in these circumstances.

Having said all this in it's favour, I've not used the black Marlowe for over fifteen years - just too heavy and bulky. Many of the long drops can be avoided by careful rigging and some effort and it's only rarely now that you get the 100m+ single hangs where it really comes into its own.

On a more cheery note - your average marine is certainly a wiry individual, but they'll probably be carrying more when they abseil than you do. Combat load used to be 35lb as I recall, plus a rifle; all of which makes both the dead weight, and the weight slung around the body, more likely to cause injury in the event of a fall; if the MOD accepts that the gear is good for those circumstances there is perhaps some safety margin built in. (Mind you, they have a higher acceptable casualty rate than most caving clubs - and operational abseiling has more alarming factors involved than high peak impact forces).

Anyway - you've got the rope. Don't sling it, just be aware of its limitations and use it accordingly.
 
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Agrophobic

Guest
cheers Seddon.
Sounds like good advice (y)
Hopefully lugging it around the hillside will make me as fit as a marine :LOL:

Bob- the french cowstails link certainly makes good reading, i'm glad i tie my own.
 
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phil.gregson

Guest
Please DO NOT say that if the military do it it must be safe!

I've seen the military abseil training - they hurt people regularly and accept a certain casualty level!

Also, chucking a rope down a cliff and abseiling off is a situation far removed from SRT - shock loading of anchors, for instance, is not an issue as the concept of 'back-up' is unheard of.

In the days of spits, prior to eco hangers, there were a few incidences of individuals suffering, in some cases quite severe, back injuries due to shock loadings on low stretch rope when anchors failed - this would be much worse on a true static rope, as polyester is.

The figure of 10Kn is a guess, incidentally, but its an educated guess and not far off.

The use of any equipment by an individual should be down to risk assessment of the suitability of the equipment, the competence of the individual and the environment and conditions underwhich it is to be used.

However, my advice remains the same:

This is not really suitable rope for general SRT use - there are standards for rope for this use for a reason.

Phil
 
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Agrophobic

Guest
"Please DO NOT say that if the military do it it must be safe!"
I'd certainly agree with that! having seen the SBS run a RIB up the beach by accident and rip the bottom out of it.
These posts are part of my risk assesment. (y)
 

Bob Mehew

Well-known member
Agrophobic said:
to take this topic even further from original- if you are clipped into a rebelay piont by your cowstails is this not the same as being attached to the main belay by a polyester rope? ie: theres nothing to absorb the force etc of a fall axcept your cows.

Not strictly true as there other components which do their little bit.  We have not talked about how much "peak force lopping" the resin component of an Eco anchor provides for example.  [Answer not much but it is important in considering how the rock reacts.]  But the major components for "peak force lopping" in a set up of cows tails attached to an anchor are the flesh in your body (death due to high peak forces is often due to the internal organs ripping away from their soft surroundings - look for the helicopter example in Crawford's paper) and the knots and rope in the cows tails.  Our problem is we don't really know how much each contribute. 
 

Bob Mehew

Well-known member
phil.gregson said:
In the days of spits, prior to eco hangers, there were a few incidences of individuals suffering, in some cases quite severe, back injuries due to shock loadings on low stretch rope when anchors failed - this would be much worse on a true static rope, as polyester is.

Phil - Can you quote any references?  I have trawled BCRC data back to 1995 without finding any, though I guess you are referring to earlier events.  I have got Nigel Roberston's event in December 1989.
 
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