5 mm perlon okay for all my prusiks?

[iain]

It is interesting to note when prusiks do fail, it is in the entrance to the barrel of the hitch and not the knot. Makes you wonder if you really do get near twice your breaking strength out of them.

 

[pindude]

"And, oh yeah, I’m talking regular plain old perlon accessory cord, not Kevlar, Spectra, Gemini, or Titan cord. " Excel is good at removing "w00t".

 

I wouldn't have any problem with the strength of 5mm for my personal prussiks, but for crevasse rescue, I'd want 6 mm because you can exert more force than body weight using a 3:1 Z-Pulley. To get the necessary grip on 8 mm, you might have to go with a kleimheist.

 

Interesting discussion since I was last on.

 

MS Word with "Find/replace" does the trick too, CBS. Not translated, in my first statement, was why not use 5.5-mm perlon? (not Gemini, Titan, Spectra-type cords as mentioned in the second cryptic sentence.) I've been using regular 5.5-mm cord for crevasse rescue since the mid-80s--it grips well down to 8.5 mm, but I haven't used it on 8.1-mm or less rope yet.

 

On another note, rescue folks I have spoken to, including the now-retired Arnor Larson of Rigging for Rescue, prefer prussiks over mechanical-type ascenders in a rescue situation. Especially with the weight of two or more people on a single rope, there is more concern about a mechanical ascender's teeth (this would include a Tibloc) tearing the sheath of the rope if somehow the rope were shock-loaded.

[Stephen_Ramsey]

It is interesting to note when prusiks do fail, it is in the entrance to the barrel of the hitch and not the knot

Iain,

 

Yes, the presence of the Prussik hitch lowers the maximum allowed tension (Tmax), above which the loop of cord will break, just like any knot or hitch. Evidently the Prussik hitch lowers Tmax more than the double fisherman's. However, that doesn't change the fact that if you are pulling the haul strand with tension T, the tension in the Prussik is going to be approximately T, not 2T. The knot does't increase the actual tension in the part of the cord away from the knot, it lowers the breaking point. Your point about the breaking strength just means that you might only have to pull with, say, 6kN instead of 10kN in order to break the Prussik. Even Mister T couldn't pull that hard.

 

Now, if you set up a ZxC, that changes things. Then if you are pulling with tension T on the haul rope, the tension in the prussik would be 2T.

 

Someone else made the point about a Prussik potentially slipping. This is IMHO a more reasonable thing to worry about than Catbird's implied concern about the Prussik breaking. That's why I use 5mm perlon with my 8.5mm ropes. It seems to grip the rope well.

Edited April 15, 2004 by Stephen_Ramsey

[iain]

I'm not disagreeing with you, but you seem to think I am. I was just pointing out something that was interesting. Also, you can generate a pretty whopping force if you jerk a 3:1 with two people hauling. Try it sometime if you have access to a dyno. I'm not saying 5mm is inadequate. In fact, I thought I just demonstrated it was not.

[Stephen_Ramsey]

Iain,

 

Yes, we're in agreement, but discussing physics is fun.

 

Also, you can generate a pretty whopping force if you jerk a 3:1 with two people hauling.

 

True, but let's suppose that you and your burly partner are pulling with combined force T on the haul strand. The "whopping force" will be on the strand of rope going to the victim, which will have tension 3T. The anchor will have a somewhat less whopping force of 2T on it. The "whopping force" will not be on the haul prussik; it will only have tension T. Personally, my worry would be with the anchor blowing, in any crevasse rescue scenario. IMHO.

 

Cheers,

Steve

Edited April 15, 2004 by Stephen_Ramsey

[catbirdseat]

I've heard it said that one should not use high tech cord for prussiks because Spectra has a lower melting temperature than nylon, but in reality, only the core is high tech material whereas the cover is ordinary nylon, which insulates the core from any heat that may be generated. And you shouldn't be generating any heat if you are careful when resetting the knot.

 

Gemini cord, which is made using a blended core of spectra and kevlar tends to be rather stiff. I would suspect that it would not do well, when tied as a prussik, at grabbing the rope.

 

If you are really worried about your prussik holding, you can have a backup prussik. Just make sure that the down hill knot is longer than the uphill knot so one doesn't push against the other. This is the very setup that some SAR people use for a belay when lowering two or more people (the only difference being that the prussiks are set on a Mariner Knot so they can be released if they grab).

[iain]

The reason for the two 3-wrap prusiks on a rescue belay is more for shock-load heat dissipation across more nylon rather than raw grabbing power. You can catch a rescue load (usually considered 200kg) with a single 8mm 3-wrap prusik on 11mm static. Single prusiks functioning as "ratchets" on rescue loads are considered quite adequate.

 

There is no reason to use anything but stock perlon for prusiks, rescue or recreational.

[Stephen_Ramsey]

Catbird,

 

(Sort of) apropos your comment, what's the melting point of Titan? I tried to flame the ends of my Titan cordalette, and it wouldn't melt at all... so I would guess that Titan's melting point is higher than ordinary nylon??

[catbirdseat]

Is Titan made with a Spectra core. It should melt easily. Spectra is an ultrahigh molecular weight polyethylene. It may be that it would rather burn than melt. It certainly is heat sensitive.

Edited April 15, 2004 by catbirdseat

[AaronB]

and the weight of the victim (ignoring friction) should be about 3T. Hence the 3:1 ratio of the Z-drag system.

 

So, floating prussik max load = 2T = 2*weight/3.

 

Ratchet prussik max load = 3T = weight.

 

Assuming all standard assumptions of course.

 

 

WTF is all this shit.. What private schools did you guys go to.. Damn.. I don't understand this shit. Yes or no.. Is usually what I understand. For future reference.. Don't rely on me for anything other than fasion orientation on choosing a daisy chain/Locking biner combo.. I go with the Pastels