rescue knots

[RescueMan]

what is the point of learning two dozen knots?

I'm aware of at least one very-active mountain rescue team which mandates that only the Figure 8 be used

Would you take your car to a mechanic who had only 2 tools in his toolbox? Or would you go to a doctor who prescribed only 2 drugs?

That "mountain rescue team" could not be MRA certified, as they do not possess the minimum competency for rope rescue.

Figure Eight on a Bight
Figure Eight Follow Through
Double Figure Eight Knot
Bowline with safety (Yosemite acceptable)
Münter Hitch
Butterfly Knot
Clove Hitch with safety
Trucker’s Hitch
Ring Bend (Water Knot) in webbing
Double Overhand Bend (Double Fisherman’s Knot, Barrel) in 8mm cord
Prusik Hitch (3 wrap) in 8 mm cord on rescue rope

Tim's NCRC list is an absolute minimum list for a rescue rigger. Anyone who doesn't know at least these knots blindfolded should not be doing rope rescue.

It's all in the terminology - in my opinion, "double figure 8" can be misleading.

A double-loop figure 8 is probably a better description.

Perhaps he was thinking of an in-line figure 8 follow-through - a knot commonly used to join ropes. This is shown as a "figure 8 join" in the Knots chart in Vertical.

A two-eyed figure-8 on a bight would be even more accurate. And the figure-8 join (ouch!) is NOT an inline knot but a figure-8 bend.

[quote="Tim White"]The “Truckers Hitchâ€

[knudeNoggin]

like any hitch - once the package is removed the hitch falls apart

!? What's a Timber Hitch, Anchor Bend (Hitch), Groundline Hitch, ... then?
--all will have some residue devoid of their object.

As for employing the Directional Fig.8 loopknot in a Trucker's H., that's
a bit much; in addition to simpler knots (the (slipped) Overhand, properly
dressed, e.g.), just tying a Bowline with a bight vice working end yields a
dual-eyed knot which will better bear the abrasion of the fall line through
its twin eyes (and there's a quite simple dua-eyed knot).

Btw, I don't much care for the Directional Fig.8 loopknot; this is essentially
a Reef/Square knot when loaded on the ends (not on eye); go for the
Directional Fig.10 (one more full wrap of the eye bight), which should be
easier to untie. (But don't use it in the trucker's hitch!)

*knudeNoggin*

[Tim White]

Robert! your bacK. Welcome.

Some great points and observations.

With the reef knot enclosed by two double overhand knots, much of the strain is taken up by the overhands and the strain on the strands in the reef knot are considerably reduced. This would definitely increase the strength of the bend.

Do you know of any testing supporting this? I’d like to see it if you do.

[RescueMan]

With the reef knot enclosed by two double overhand knots, much of the strain is taken up by the overhands and the strain on the strands in the reef knot are considerably reduced. This would definitely increase the strength of the bend.

Do you know of any testing supporting this? I’d like to see it if you do.

No I don't - I think it's too esoteric a knot to have been considered for testing.

But I believe that the "weakness" of the reef knot (which was designed only for closing a package - a sailer's duffel) is that it is an insecure knot - it can easily untie itself under load.

It's basically just two interlaced larksfeet with only one strand of each loaded. You would never use a girth hitch if only one strand were to be loaded.

By enclosing the reef knot inside a double overhand bend (or backing up each tail with a double overhand knot), as the knot is loaded the overhands cinch up on the core reef knot and have the effect of sharing the strain between each of the two strands of the two central larksfeet.

Not only would this make the reef knot infinitely more secure, I believe it would considerably increase its load-bearing capacity because of the load-sharing.

Just a well-informed guess, but I would have no qualms about rappeling on such a knot. I would NOT use it for rescue, however.

- Robert

[Scott McCrea]

It's basically just two interlaced larksfeet with only one strand of each loaded. You would never use a girth hitch if only one strand were to be loaded.

By enclosing the reef knot inside a double overhand bend (or backing up each tail with a double overhand knot), as the knot is loaded the overhands cinch up on the core reef knot and have the effect of sharing the strain between each of the two strands of the two central larksfeet.

So, hypothetically, would this have the same effect as joining two 'endless' loops with a bend that looks like a square (reef) knot (I'm not sure what to call it)? It would look like this:



It would apply equal pulls on each strand of the bend, right? I've always heard that you shouldn't join sewn webbing loops this way. Is that true or an old wives tale?

[RescueMan]

So, hypothically [sic], would this have the same effect as joining two 'endless' loops with a bend that looks like a square (reef) knot (I'm not sure what to call it)? It would look like this:



It would apply equal pulls on each strand of the bend, right?

Nice sketch. Your girthed loops depict, I believe, one of the effects of enclosing a reef knot inside a double overhand bend (fisherman's), which is to allow load-sharing between the strands. But I suspect that the reef/fisherman's bend also distributes some of the strain inside the double overhands as well once the entire knot tightens up on itself, and hence reduces the strain within the reef knot.

I've always heard that you shouldn't join sewn webbing loops this way. Is that true or an old wives tale?

Old wives' tale.

The "problem" with the girth hitch is that, placed around a large-diameter tree for instance and turned so that the loaded leg comes off the side of the tree, the forces on the capturing eye of the loop can be up to double the actual load. (This is the same "problem" in using a high directional or a change of direction (CD) pulley in an MA sytem - it is merely something to consider when rigging).

If you're using 4,000 lb webbing to girth a tree, then you might reduce its functional strength to 2,000 lbs. Are you going to worry about hanging on a 2,000 lb anchor?

Girthing two slings as you've depicted does not cause the kind of reversal which significantly multiplies the applied load. But it does create a very secure and stable connection which will minimize relative movement at the join.

- Robert

[knudeNoggin]

With the reef knot enclosed by two double overhand knots, much of the strain is taken up by the overhands and the strain on the strands in the reef knot are considerably reduced. This would definitely increase the strength of the bend.

Do you know of any testing supporting this? I’d like to see it if you do.

It should be apparently false, and you can check this yourself by tying the
knot and loading it--little force makes it through the Reef bend to the Strangle
back-ups, and they hardly tighten. (Indeed, this is part of the reason for
using the Thief vice Reed/Square: to have slippage sufficient to tighten the
back-up knots.) But if the data cited (as that "10%" figure in Vertical) results from
slippage and not breaking, then, yes, back-ups make a big difference.

For a more stable, Reef-like bend, make a 2nd tuck of the ends--a Reverse
Surgeon's bend; set the knot by pulling on the ends. This is more secure and
easier to untie.

you shouldn't join sewn webbing loops this way. Is that true or an old wives tale?

So-called "girth hitching" of sewn tape slings reduces their strength by about
30-50%*; the break occurs (for roughly equal-strength slings, i.e.) in the sling making
the "girth hitch". I quote that term for the resultant knot is quite different from
that formed around a solid object, although with material cross section like tape,
it's also quite distinct from what obtains in rope (which is symmetric).

Simply making one further tuck/hitch of the material adds strength and eases
untying.

[* The report on this at http://www.climberware.com is a bit confusing. Another
tester (a sling vendor) reported 50% for HMPE slings. YMMV]

*knudeNoggin*

[Scott McCrea]

[* The report on this at http://www.climberware.com is a bit confusing. Another tester (a sling vendor) reported 50% for HMPE slings. YMMV]

I think you mean this link, right? http://www.climerware.com/cknot1.htm

[fuzzy-hair-man]

[* The report on this at http://www.climberware.com is a bit confusing. Another tester (a sling vendor) reported 50% for HMPE slings. YMMV]

I think you mean this link, right? http://www.climerware.com/cknot1.htm

Thanks, I needed a picture...

I guess I'm being difficult but when we are talking about strengths of knots are we talking static pull strength or dynamic loads? I ask because it seems to me that in a static pull test the slipping / shock absorbing behaviour does not add to the strength (a reliable knot cannot slip forever) so at some point / force it has to grip and no longer slide anymore.

This said (correct me if I am wrong) I understood that a knot's strength as a % of rope strength was primarily determined by how small the radius of the bends the rope was forced to make in the knot. The reason being that if you bend a rope the strands on the outside are forced to pull tighter than those on the inside of the bend, the tighter the bend the more this is exaserbated. Given that the outside strands are taking more strain a tighter bend will tend to break these strands first then loading the strands further in until the rope is broken, ie a bend means all strands are not loaded equally, the most loaded strands become the knot's weak point the smaller the radius of the bend the more severe the weak point.

This would seem to suggest to me that in a static pull test once any slipping action has been pulled out a rope's strength is primarily decided by the knot's form and how tight the bends in the rope are this isn't changed by back ups or slipping behaviour as far as I can see.

Dynamic tests? well that's a different kettle of fish

[RescueMan]

when we are talking about strengths of knots are we talking static pull strength or dynamic loads? I ask because it seems to me that in a static pull test the slipping / shock absorbing behaviour does not add to the strength

Knot testing is done in both ways - with slow-pull machines or with drop tests.

While the looseness of a knot (typically from failure to dress & set properly) contributes to the extension of the system, it does not tend to add strength as you are suggesting.

In fact, quite the opposite. Poorly dressed and set knots tend to be weaker.

I understood that a knot's strength as a % of rope strength was primarily determined by how small the radius of the bends the rope was forced to make in the knot. The reason being that if you bend a rope the strands on the outside are forced to pull tighter than those on the inside of the bend, ...the outside strands are taking more strain

This was more true with natural fiber ropes, which allow almost no stretch, so that any unbalanced tension (such as in a tight radius) stresses individual fibers to the breaking point - sometimes causing a cascading effect as each successive fiber is stressed.

With the stretchy fibers of nylon rope (both dynamic and static are the same nylon), this effect is apparently NOT the principle cause of rope failure at a knot.

Imparting stress, particularly sudden dynamic stress, to a nylon rope causes the fibers to move against each other as outside bend filaments stretch. This relative movement - nylon on nylon - creates considerable heat. Nylon melts at about 460 deg F, but it starts to weaken at 350. Nylon's high coefficient of friction and low rate of thermal conductivity contribute to the sudden internal heat buildup under load.

This is why wetting a nylon rope weakens it - saturation lowers its "glass transition temperature" - the same effect as raising its temperature - and increases its plasticity.

For this reason, the weakness of a knot in nylon rope is not simply a function of the radius of the bends, but also of the configuration of the knot which would allow dissipation of internal stresses over a larger area.

This is why a knot with more turns (figure-8 vs. overhand, figure-9 vs. figure-8) tends to be a stronger knot.

- Robert

[NZcaver]

It's all in the terminology - in my opinion, "double figure 8" can be misleading.

A double-loop figure 8 is probably a better description.

Perhaps he was thinking of an in-line figure 8 follow-through - a knot commonly used to join ropes. This is shown as a "figure 8 join" in the Knots chart in Vertical.

A two-eyed figure-8 on a bight would be even more accurate. And the figure-8 join (ouch!) is NOT an inline knot but a figure-8 bend.

Of course. It is a bend, not an inline knot. Mea Culpa.

Welcome back, Robert.

[fuzzy-hair-man]

While the looseness of a knot (typically from failure to dress & set properly) contributes to the extension of the system, it does not tend to add strength as you are suggesting.

In fact, quite the opposite. Poorly dressed and set knots tend to be weaker.

I didn't mean loose knots add to strength I meant there are knots such as the barrell knot which are good at absorbing force (caviate on shock absorbing knots being unreliable AFAIK). There was a report testing cowstails with dynamic falls using various materials, products, and knots. The upshot was the barrell knot performed above figure 8, figure 9 and was the best method to attach your carabiners to your cowstail when using rope (dynamic rope is best).

In a dynamic test this slipping shock absorbing behaviour should reduce peak load right? (a good thing) but with a static pull test and I didn't see how the slipping / shock absorbing behaviour would help the knot gain a higher breaking strai?n all of this should have been pulled out by the time the rope gets close to breaking.

When we refer to strength as a % of rope strength are we refering to static pull strength or a measure of dynamic performance / strength? I guess I'd have to go back and read the original article

With the stretchy fibers of nylon rope (both dynamic and static are the same nylon), this effect is apparently NOT the principle cause of rope failure at a knot.

Imparting stress, particularly sudden dynamic stress, to a nylon rope causes the fibers to move against each other as outside bend filaments stretch. This relative movement - nylon on nylon - creates considerable heat. Nylon melts at about 460 deg F, but it starts to weaken at 350. Nylon's high coefficient of friction and low rate of thermal conductivity contribute to the sudden internal heat buildup under load.

This is why wetting a nylon rope weakens it - saturation lowers its "glass transition temperature" - the same effect as raising its temperature - and increases its plasticity.

A rubber band will only stretch so far before it breaks, but heat becomes the issue before nylon's stretch limit is reached right?

For this reason, the weakness of a knot in nylon rope is not simply a function of the radius of the bends, but also of the configuration of the knot which would allow dissipation of internal stresses over a larger area.

This is why a knot with more turns (figure-8 vs. overhand, figure-9 vs. figure-8) tends to be a stronger knot.

Good to know, Thanks! So larger knots which use more rope and therefore have more turns tend to be stronger? (nice conflict there).

The last bit, (figure 8s vrs figure 9s) makes me wonder why figure 9s aren't used more in the US (the list of knots mentioned earlier doesn't include them) and they are common here(Australia) and in the UK (Life on a Line recommends them anyway) part I imagine is down to rigging (9s are good for rebelays, particularly if a rebelay were to fail) but they are also useful anywhere you need a loop knot, unless you want to do a rethreaded loop round a column, but then it's not impossible. People say they can't tell verify a nine as easily as an eight but I find it reasonably easy and they aren't very hard to tie. Is there something I'm missing?

Any ideas? Just something I have wondered about

[RescueMan]

There was a report testing cowstails with dynamic falls using various materials, products, and knots. The upshot was the barrell knot performed above figure 8 (dynamic rope is best).

Since that test was a dynamic drop test (FF 2), rope with more stretch and knots with more turns came out ahead. But the dyamic rope made more of a difference than the particular knot, and in low-stretch rope the knot differences were not very significant.

http://home.comcast.net/~onkaluna/COWSTAILS_crr01364.pdf

In a dynamic test this slipping shock absorbing behaviour should reduce peak load right? (a good thing) but with a static pull test and I didn't see how the slipping / shock absorbing behaviour would help the knot gain a higher breaking strai?n all of this should have been pulled out by the time the rope gets close to breaking.

In a dynamic (drop) test, all strain-reducing elements help reduce the peak load (shock load) and the purpose of the testing is to acertain peak loads.

In a static slow-pull test, the goal is to determine minimum breaking strength (MBS). Any slack in the system (in knots, for example) will actually reduce MBS by causing additional heating.

I recommend pretensioning all fixed knots (meant to stay in place) such as those on a cowstail or on prusik loops, with one person pulling on a 9:1 MA tensioner while holding the knot tails with pliers to keep them from pulling into the knot. While this might take away some of the shock absorbing quality of the knots, it also prevents that tail from disappearing when you fall on it - and I like to keep my tails very short.

When we refer to strength as a % of rope strength are we refering to static pull strength or a measure of dynamic performance

This is a slow-pull static MBS measurement. Again, dynamic testing is usually to ascertain peak force - not breaking strength (though sometimes elements do fail).

The last bit, (figure 8s vrs figure 9s) makes me wonder why figure 9s aren't used more in the US - Any ideas?

Yeah. Since the strength of knots is not an issue (unless you're using very small diameter rope), and since the figure-8 family is among the stongest of common knots, there's simply no reason to use a bulkier knot except in extreme applications.

- Robert

[chh]

A little bit off topic, but as per joining slings together with here's some interesting recent test results.

http://www.bdel.com/scene/beta/qc_kp.php#current

[RescueMan]

joining slings together with here's some interesting recent test results.

Thanks for the link.

What these tests indicate to me is not so much to avoid girth hitches (though the symmetrical hitches, as I suggested earlier, are superior), but to avoid anything but nylon (or polyesther) in ropes, cordage, and slings.

Tom Moyer's tests of "high strength cord" also demonstrate that these tech cords are weaker and more problematic in actual use than nylon, which comes out on top of the list in most tests and certainly in the overall ranking.

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

- Robert