Tying Up HMPE

Tying knots in rope or webbing made from Dyneema or other Ultra High Molecular Weight Polyethylene (which is abbreviated HMPE) is a topic of much confusion. There are many different materials, and many knots or hitches. The various ties can be tested via slow pull or drop tests. Search it up on youtube and I am sure you can find lots of information. 

When it comes to thinking about tying stuff in HMPE, there are three main considerations:

1. It is very slippery, so some knots and hitches that would be stable in other cordage will slip out when tied in dyneema. 

2. It is very low-stretch, which means that any tight bends (and all knots involve relatively tight bends) will weaken it quite a bit. 

3. Tests always have a lot of variation in results. All knots in cordage show a wide variety of results, but HMPE seems to show more than fibers like nylon and polyester. 

So, when it comes to tying stuff out of HMPE, I approach these issues in the following ways:

1. I use ties that either have been tested, and the tail does not pull through, or they are tied in the middle of a piece with all ends secure. 

2. I use spliced or sewn connections whenever possible, and give a large evidence-based strength reduction when tying it. 

3. I base my assumptions about strength on as much data as I can get, and I only consider the weakest result I have seen. It doesn’t matter how strong it was once, what matters is how weak it was once. 

For most ties made in HMPE, I figure you lose 66%. That is a lot. Many people will want to argue that they have seen a lot of tests done with Dyneema slings that broke at 50% when tied with a figure-8, or hitched together, or similar. This is true, but it actually is indicative of strength loss of over 60%. The slings are sewn into a loop, and the sewn connection weakens the webbing, probably by 20%. So the webbing used to make a 22kn sling has 14-15kn strength, or a loop strength of around 28kn. The different things that weaken a single piece of cordage are not mathematically related to each other. So tying a knot in that sling you are starting out with something of around 28kn, and then reducing strength from there. 11kn is is 39% of original strength, and that is the sort of strength you get when you test knotted slings. This supports my underlying idea that most knots will weaken an HMPE device by around 2/3. That said, many things tied in an HMPE sling will be 11kn, and that is good to remember. This is also why I give the original strength of the rope my anchor slings are made from. That is the figure you want to use when calculating how strong something is after you have tied it all up. 

On to the question of slip. A figure-8 loop will not pull out of dyneema rope. Most bowlines will. A flat overhand bend tends to pull out, a Flemish bend will eventually stop tightening down and hold. A triple fisherman’s will hold, I haven’t tested the double. Most tying of HMPE happens in the middle of the piece, with the ends secured, thus mitigating concerns about the slipping. 

There are some ties that are consistently stronger with this slippery material. Some special knots have been developed just for tying 100% HMPE 12-strand rope, such as the Estar Bend and Estar Loop. These are claimed to consistently exceed 50% strength. 

The other ties that preserve more strength from this slippery material are hitches that go around something, thus making the bends less severe. For the readers of this blog this mostly means tying around carabiners. This introduces more variability, because carabiners come in many different profiles. There is also significant variability in how the hitches are tied. For example, a clove hitch, if it has both ends secured and tight, may be tied with the cross-over either outside or inside the carabiner. The cross-over outside shows strength loss of about 66%, whereas the cross-over inside shows a loss of 40%, (when tested on a large, round carabiner.) I have tested my Ultralight anchor sling with one ended girth-htich around a large, round, carabiner, and the worst result I have is 70% knot efficiency. 

Likewise, in climbing circles there has been much discussion about making multi-point distributive anchors with a girth-hitch master point. If one is using dyneema slings it appears this is stronger than tying all of the strands in a knot to achieve the same thing. I am sure this is because the girth-hitch around the carabiner involves a larger bend-radius than the bend radius within the knot that would be otherwise use. Then there is controversy to have about whether or not such an anchor is redundant, but redundancy is a topic for another blog post. 

There are a great many composite ropes and cords that use HMPE mixed with other fibers. It makes sense that if you surrounded the HMPE with a sheath of some other stuff you would make a larger bend radius within knots, and thus you would retain more strength. The truth is that in slow-pull tests, the composite ropes are all over the map. I have had efficiencies of 40%-75% in ropes with HMPE cores, tied in a figure-8 knot. Simply put, if you haven’t tested it, you don’t know. 

Long ago I had an engineer tell me that the reason Polyethylene had such poor knotted strength was because of it’s low melting point. The idea was that as knots tighten under load they heat up (which is true) and that reaching the melting point is what causes the failure. This makes sense but the data does not support it. If it were true, than ropes made from heat-resistant fibers would have higher knotted strength. However, Technora and Dyneema rope of the same diameter show very similar knotted strength. An equivalent nylon rope will show much higher knot efficiency, and nylon has a melting point far lower than Technora. The efficiency of knots seems to be mostly about stretch. 

Using HMPE is a bit more complicated than using nylon and polyester cordage. Most of us find it worthwhile because of its light weight, abrasion resistance, and the way it doesn’t absorb water. It is hazardous to use knots with, and most people misunderstand how it actually works. I recommend seeking out test data specific to what you are doing, and ignoring all of the optimistic results. 

Here at RopeLite, I use Dyneema rope to make Ultralight Anchor Slings, Ultimate Anchor Slings, the Skimo Teather, Ultralight Extenders, and some Litter Bridles. If you think you want some other HMPE product, feel free to reach out, I love custom projects.