Paracord rope strength

Question

I have recently come into possession of many thousands of feet of paracord, more than any practical use would dictate. So this started me down the addictive path that is weaving and forming it into various more practical or at least decorative uses. One of the extremely easy knitting styles using a spool loom allows for some pretty elaborate rope design, and also allows for compact bundling of cordage into ropes that unspool into much longer cord lengths. Since this is ultimately still one strand just doubled back x times per pin in the loom, I would assume it gains no strength in this process, and by nature of all the additional bend radii I would further assume that it actually looses some strength in this process...

So what I would like to know is there any definable equation / references by which one can calculate the relative strength of a rope constructed of single cords in relation to the knit?

I am not looking to make climbing rope or push its limits, I never really see this being used as anything more than novelty items like leashes, gear holding, bracelets, and gun slings. I am just curious what the deduction is from the 550# relative strength of the cord to start with and how much it decreases with a 2 pin knit vs a 3 pin, 5, etc... Just so I would have some frame of reference if any of it ever had to be load bearing.

Errata: An example of the rope in question, this is a three plat knit using single cord over three pins in a spool loom. It produces a rope diameter of about 1" and compacts the cord about 4.5"(ish) per link, so 50' of rope is actually a manageable 200+ length of cord.

Original milspec 550 cord, I looped some over a limb, supported my body weight (190#) and only a few inches off the ground, bounced as hard as I could in effort to test its breaking strength, it held with no obvious signs of damage.

So though I would never climb on a homemade rope as a first choice, it seems it would not be an issue if I did need to, it is somewhat stretchy, I got about a 2' stretch out of about 20 feet of rope before it seemed to hold me in a solid position.

Using that homemade loom and a Philips head screwdriver as a tool, I made about 25' in an evening, so practical way to store a decent length of cord...

Answer 1

You'll need to test to be sure, but it will lose quite a bit of strength.

When I sailed, I was taught that you lose as much as 80% strength by tying a knot. So if you have 500 lb paracord, by this estimate it could support about 100 pounds reliably when knotted. The weakening is at the point of the knot, so multiple knots doesn't further decrease the strength, it just gives you more probable failure points, all at 20% of the original strength.

The problem here is that the 80% loss is a conservative rule of thumb. That's good if you need to plan for something where your life depends on the line NOT breaking, but your knotted rope can probably hold more than 20% of it's rated strength.

I found a rope maker who tested some of their own ropes, and described both the testing protocol and the results. The result was that you lost between 35% and 68% of a rope's strength, depending on the type of line and the knot. Splices lost significantly less strength, but since you aren't splicing that's not really relevant. There are probably other lines and knots that lose even greater strength.

Edit:

This answer is about an unbroken cord with a knot in it. Rope is designed to be strongest when all fibers in it are straight, so the bent fibers, while bent, are significantly weaker than the surrounding straight fibers. The more severe the bend, the greater the weakening. If you untie the knot, the rope goes back to being straight and regains its strength.

The picture looks like any weight on the braided cord would be supported by the equivalent of multiple strands, and the braid might not compromise the rope's strength as severely as a true knot.

Braiding a single cord as you did functions kind of like wrapping a line between two anchor points several times. Any force placed on the ends will pull on all the bends, so the full force won't be on any single bend. In your picture, the force would probably be distributed between several bent strands, so it could support several times what a single knotted cord would support. But all this means is that if you really want to know, you need to test it.

Answer 2

A single strand of 550 paracord will hold body weight, so when you say, "load bearing" are you implying more than body weight? If not, then I think it's a non-issue, especially if you're weaving or knitting verses knotting; knots significantly decrease the breaking strength of ropes and cord, bending: not as much.

Paracord will be more than suitable for any application that you would use any other type of cord for. I'd argue that it'd be more durable than anything else you knit together.

You could make some pretty heavy duty hammocks out of paracord, and they would hold the weight of more people than you could ever fit in them.

Answer 3

If I slice across your knit rope, in a flat cut, how many strands would I cut.

Looks like 8.

Now use the best lies you can find about the strength reduction in a bend in kernmantle rope. My recollection is that it is about 80% of the original strength. (60% in a stranded rope) (The core fibers in a kernmantle rope have room to slip, so all the strain isn't concentrated on a few of the fibers.)

So your new root = 8 * 0.8 * original rating. or 6.4 * 550

Think like block and tackle: How many strands support the load?