To knot or not to knot

[Storker]

I just need the line to hold a regulator to my neck or a dive computer to my wrist. I'm not particularly concerned with the bungee breaking due to load. Should I be?

I can't see any reason for that.

 

[kelemvor]

Warning to All: All knots reduce the rated tensile strength of line, often between 50 and 75%!

Now you've got me thinking. Is the reduction in strength due to the presence of the knot or stressing of the material? What I'm getting at is this: if I use a piece of bungee to practice knots, am I quickly going to "wear it out" by tying and untying the knot several times?

 

[Jim Lapenta]

Not likely. Unless you really stretch the crap out of it over and over. What is more likely to happen, if you don't singe and glue the ends, is that the outer sheath will begin to unravel. If you keep tying a knot in the same spot over and over for practice, it may begin to unravel there as the sheath fibers stretch and begin to break down.

This may look silly bit I have something similar that I picked up in a book store. https://www.amazon.com/Pro-Knot-Kno...3495&sr=8-1&keywords=knot+tying+kit+with+cord

I will sometimes, while watching tv, grab it and pick one or two knots to work on. It freaking helps big time! You get to where you look at something and make a decision that it needs this knot or that one and you just tie it.

 

[Jim Lapenta]

One thing to be aware of with bungee is the grade and age of it. There is some cheap stuff out there that I've seen the center start to break down. Crappy rubber or whatever. The older it is the more likely it is to lose it's elasticity and even break as well. Kinda like old socks or that 10yr old pair of comfie underwear you can't seem to toss.

 

[Resqdivemedic]

Any knot can weaken a cord (rope, bungee) some more than others. I don't think repeated tying or untying will cause wear to the point of failure. In rescue, every time we need a knot we tie it. We never store our lines with knots left in them because that can weaken the rope. My theory about knots is only learn the ones you need to know. There are so many knots for so many tasks it will become a full time job remembering and tying them. I am a big fan of the figure 8 family of knots because they tend to cause less stress on the rope than traditional knots like the bowline or square knot. When I teach knots to firefighters, I keep it simple and joke with them with the phrase "if you don't know the knot, tie a lot". I think a simple overhand or surgeons knot will act as a good stopper.

 

[Storker]

I am a big fan of the figure 8 family of knots because they tend to cause less stress on the rope than traditional knots like the bowline or square knot.

The figure eight knots are great, since they hardly ever lock up and are easy to untie. Just like the bowline, which is one of my favorites, and which I often prefer over the round turn/two half hitches when I moor my boat.


And about the weakening of the rope: There's a simple solution to that. Just use a rope rated for at least 200-400% of the strain you want it to handle. For us hobbyists, that's the simplest solution, and as a bonus the thicker rope is easier to handle, easier to tie and less prone to tangle. Unless you choose to splice the rope, you will have to tie a knot or three. And splices are only suitable for certain uses, like anchor lines or permanent moorings.

 

[Akimbo]

Now you've got me thinking. Is the reduction in strength due to the presence of the knot or stressing of the material?...

This warning was more for knots that lives or property depends on as opposed to line failures that create an inconvenience. For example:

How much strength does my rope loose when I tie it in a knot?

Line that is too fragile (low in tensile strength) to hold dive accessories, gauges, or second stages are also so small they are difficult to use or can cut into skin and suits. On the other hand, you want to be more careful in choosing line and knots for DSMBs, decompression lines, or lifting/pulling heavy loads. Sorry if I got off your topic but it is a safety issue that many people aren't aware of.

Google "knot strength reduction" if anyone wants to know more.

 

[Akimbo]

One thing to be aware of with bungee is the grade and age of it...

True, but it is wise to think of all line, bungee, and straps as a consumables. I know you are diligent on packing predive checks but not everyone reading this thinks of it. When in doubt, swap it out.

 

[dberry]

If you haven't seen it, this is an incredible (free) site for all kinds of knots. There's an app for it, too.
Animated Knots by Grog | How to Tie Knots | Fishing, Boating, Climbing, Scouting, Search and Rescue, Household, Decorative, Rope Care,

 

[Storker]

Is the reduction in strength due to the presence of the knot or stressing of the material?

Most conventional knots pinch the rope. The overhand is a prime example and a very good reason that knots like the fisherman's hitch should never be used with lines that lose a lot of strength when pinched, like monofilament fishing line. For monofilament, you'd use a friction knot (a knot that locks due to friction rather than pinching the line, like e.g. the blood knot) rather than a pinching knot. But even standard braided or laid/twisted rope loses some of its strength when pinched, so you always have to factor in the strength loss from knots when you choose your rope.