More on my rope infatuation…

I don’t know what world this guy comes from, but I needed about thirteen feet of rope to tie a Swiss seat. I could probably cut a foot out wearing tighter pants, but nine feet? I didn’t even come close to being able to tie one with ten feet, let alone nine! Otherwise, great instructions.

I had to double the initial overhand knots to four twists in the rope because otherwise the front of the sling was too tight on my nuts. Also a great idea to get the sling really tight while crouching–otherwise it will slip and you’ll be in a lot of pain. Wearing tighter pants really helps because with loose pants on it’s difficult to get the crotch of the seat tight enough to avoid injury, and the fabric bunches up in a very uncomfortable fashion.

The military rope I have a 6ooft spool of courtesy of one of my prior roommates is very slippery. I think it’s polyester, 5/8″ three strand twisted rope. It doesn’t like to dress tightly without a lot of persuasion and I think if I use this for anything serious I’ll have to add a lot of backup in my knots. I couldn’t even get a regular bowline to hold tight enough by itself for my satisfaction.

I have no idea what the rating of this rope is. I’ve found similar rope online rated at 9000 pounds, 7825 pounds, etc. All over the place, but a lot. I don’t know what kind of safety rating they use for rope, so I’m not sure what loads this stuff could handle.

… research …

Apparently the minimum breaking strength is usually given and you’re supposed to determine your own safety factor, or as they call it the ‘design factor’ in selecting your rope. The risks of certain factors, like dynamic loads (quick loading like a climber falling and being stopped by his rope), knots, abrasion, dirt, sunlight, etc all come into play. Normal working load limits are determined on a design factor of between 5:1 and 12:1, apparently. Smart. It’s not hard for a dynamic load to get really high really fast.

So, figuring I’m not using the rope for anything like mountain climbing or similar situations where a heavy shock load would occur, I could probably load it with 750 pounds comfortably. That takes into account about a 10:1 safety factor, allowing for 50% strength loss due to knotting and adding another 5:1 safety factor on top of that in case other crappy stuff happens.

Wow, the excitement on this blog huh. I like stuff like this. That’s why I’m going into engineering.

Other fun facts:

• Polypropylene degrades quickly in the sun and is highly affected by heat due to a low melting point. It also floats indefinitely.
• Nylon can handle serious shock loads, stretching up to 40% before it breaks.
• Nylon and polyester can handle similar loads, but polyester breaks much easier during a shock load.
• Polyester rope is highly UV resistant.
• The same shock resistance nylon offers becomes a detriment under heavy static loads, because it will stretch much more under a similar load than polyester.

So, nylon is probably best where dynamic loads are a possibility, while polyester is preferred for static loads like a ship’s rigging that requires tautness.

There’s a dramatic difference between 3 strand and braided rope, too. I imagine that’s a function of both reduced friction and more actual material in a given rope diameter due to a tighter lay. For example, 16mm (about 5/8″) three strand polyester rope is rated at around 4100kg breaking strength, while 8 braid rope of the same material and thickness is rated to 5700kg. Amazing.

I haven’t even looked at all the different kernmantle ropes yet!

Alright, I’m going to do some more research into going north, so peace. Definitely have the song North to Alaska stuck in my head now.