Waterknots in Dynema Slings

I've been wondering......

I have a sweet 20' dynema cordalette that I love. I also have a bunch of the dynema slings (8mm). Say I had to cut them to sling some rap anchors for a descent. I'd need to join them, I assume with a waterknot.

What's the skinny on joining dynema slings with a waterknot? Good to go?

Andy

I gained a lot more confidence with water knots (and other knots) in Spectra/Dynema tape after reading this a while back Andy. I've had my paddling waist loop tied with water knots in spectra tape for a couple of years now so far with no signs of slippage.
-ian

www.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf

xmission.com/~tmoyer/testin…

made clicky

Good find

Edit: as with any knot you loose strength, dynema/spetra is a very slippery material, I would make sure to tie backup knots

the sheath is nylon.

the failings of a water knot for a non-destructive is cyclical loading not friction.

Andy,

Slings can be joined together without cutting. You can make a square knot in them without having a loose end.

Jim

Andy-

For two reasons, please do not use a water knot for joining the ends of cut dyneema slings, particularly at rap anchors:

#1 as mentioned on the tom moyer's site referenced above, water knots have a tendancy to untie themselves. i've tested this myself and found this to be true, and it appears to happen much more quickly with slippery material like dyneema.

#2 it is very easy for for the ends of a slippery material like dyneema to pull through a water knot. it is for this precise reason that dyneema cord is tied with a triple fisherman's knot. (when it is tied with a double fisherman's knot, the ends of the knots actually pull through the knot before failure). i'd be willing to pull test this myself with a dynamomenter when i get a chance, but i'd be willing to bet that a dyneema sling that's been cut and rejoined with a water knot would fail at 1000-2000 pounds.

the combination of this slippage and the natural tendancy of the water knot to untie itself would make a cut and rejoined dyneema sling a very poor choice of material to leave at a rap anchor.

sincerely-
geir

Geir, I'm a little confused by your post. It seems to me that Tom Moyers data specifically concluded that Ultra Tape (and I do understand this is only one particular weave of nylon/spectra tape) knotted spectacularly well with all forms of loop knots (Water and Fishermens). All three knot types formed loops that failed in excess of 5000lbs in his testing on average. Why then do you think that such loops are likely to fail at 1000-2000lbs? And even if that were the case, how do you feel that in the rap situation that Andy poses that such loading would occur? I noticed that on your own website that you determined experimentally that it is very hard to generate loads in excess of 600lbs during rappel. I'd be delighted to donate some older Bluewater spectra tape identical to the type I use water knots in, for you to pull since you have a Dynamometer, I'd love to see further scientific results.

Obviously I'm not endorsing people go plan to equip routes like this, but I see no reason when things have got desperate and its necessary to cannibalize gear to build rap anchors that this particular example isn't a good way to go.
-ian

p.s. Found this spirited amateur effort whilst researching further tonight: electricant.net/beta/spectr…

Ian Buckley wrote:Geir, I'm a little confused by your post. It seems to me that Tom Moyers data specifically concluded that Ultra Tape (and I do understand this is only one particular weave of nylon/spectra tape) knotted spectacularly well with all forms of loop knots (Water and Fishermens). All three knot types formed loops that failed in excess of 5000lbs in his testing on average. Why then do you think that such loops are likely to fail at 1000-2000lbs? And even if that were the case, how do you feel that in the rap situation that Andy poses that such loading would occur? I noticed that on your own website that you determined experimentally that it is very hard to generate loads in excess of 600lbs during rappel. I'd be delighted to donate some older Bluewater spectra tape identical to the type I use water knots in, for you to pull since you have a Dynamometer, I'd love to see further scientific results. Obviously I'm not endorsing people go plan to equip routes like this, but I see no reason when things have got desperate and its necessary to cannibalize gear to build rap anchors that this particular example isn't a good way to go.

hey ian-

some good thoughts. here are my thoughts (bear in mind it's really late and i'm tired.) :)

-i want to draw a distinction between ultratape and the thin dyneema runners (which i took the original post as referring to). ultratape has a lot more nylon, and as such has much better friction for holding knots. moyer's paper ( xmission.com/~tmoyer/testin…) was written long before dyneema slings were out. it also does not appear that water knots were tested in moyer's paper. (again, it's late, and i might have missed something. i'm looking at the figure on page 5). bottom line on this one: i don't think these pull tests are applicable for dyneema slings.

-my principle concerns:

1) water knots untie themselves with cyclical loading ( xmission.com/~tmoyer/testin…). after reading this paper i did some breif tests of my own and observed that this untying seems to happen several times more quickly with spectra runners.

2) i've pull tested 5.5mm spectra cord tied in a loop with double fisherman's knots. the ends pulled through the knot at just under 1500 pounds ... and this was with cord that had a nylon sheath that presumably would increase the knot efficiency. dyneema slings - which lack the nylon sheath - would almost undoubtly slip under much lower load. (i'll be glad pull test this and find out for sure.)

you are absolutely right that rappel anchors take very low load. even 600 pounds is hard to generate. i suspect, however, there's a much better chance for the tails of the dyneema being pulled through a water knot with repeated use on rap anchors. the combination of the cyclical loading/untying and the slippery nature of dyneema really makes me worry. so i suggest keeping another material handy for leaving behind on rap anchors. 7mm nylon cord, 1/2" nylon webbing, your friend's harness, whatever.

just my two cent's worth. anyway, it's really late and i have to be up in 5 hours for work. like i said, i'll be glad to pull test a few dyneema slings joined with water knots to put a little more measureable concrete numbers behind what i'm thinking here.

hope all's well-
geir

Geir, the illustration at the top of page 5 was the one I was referring t0, its titled "Loop Knots (Bends) Comparison". You're right to bring up the point that Andy is taking about the latest skinny 8mm tape rather than the older fatter stuff I've been cutting up and using. I'm not sure its safe to extrapolate results from one to the other.

-ian