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Prussik strength

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 nickb 10 Jan 2021

Hi all

I'm looking for any info or investigations into the holding and breaking strengths of various prussik knots. 

Is a classic stronger than a French, what about a v.t. that kind of thing. 

Any info would be great thanks 

 David Coley 11 Jan 2021
 Cobra_Head 11 Jan 2021
In reply to nickb:

Why? Is there a reason you need a strong prussik?

 McHeath 11 Jan 2021
In reply to Cobra_Head:

Dunno about you, but I like using the strongest knots available for everything. Following situation: you've come off seconding an overhang and rather than being lowered off into the sea you prussik up the rope. The rope goes up to a single wire over the lip, then diagonally to your partner on the belay. The wire rips. I'd rather have a strong knot. 

In reply to McHeath:

You should be backing up your progress and tie in to the rope you're ascending at reasonable intervals.  The strength of the prussik knot is academic, so long as it holds the rope sufficiently enough to make progress.  Better to choose a knot/rope combo that is easy to manage in a stressful situation. 

 nickb 12 Jan 2021

The reason I'm asking is for water rescue actually. Mainly for raft rescue were the stress on the system can be high, e.g. River flowing into wrapped raft. 

The main prussik taught is a classic but i would prefer something releaseable if strength loss isn't too great.

Thanks for the help

 PaulJepson 12 Jan 2021
In reply to nickb:

Would the holding strength not be more important (as in the 'grippy-ness') in that instance than the actual breaking strength of the prussik? In that case I think the Klemheist is the grippiest. 

In reply to nickb:

Yes, as Paul suggests above, my experience is the Klemheist works well on wet kit.  Particularly effective if tied with a nylon (not Dyneema) sling and very easy to release, even under load.  This is from experience in crag environments, not water.  Think it would be difficult to exceed the breaking strain of a sewn sling used in such circumstances.

(Did the Swiftwater Rescue Tech course many moons ago but unfortunately cannot remember the recommendation for "rope clamps", sorry.  Just don't put any knots in the ropes....)

 David Coley 12 Jan 2021
In reply to nickb:

I know nothing about water, or boats, but releasing a prussik under load is not always easy - and can't be guaranteed in an emergency especially if you don't know the load before you set the system up. I would suggest considering keeping the prussik and release  elements separate. The document I pointed to shows a classic prussik can be very, very strong. So, how about combining this with a munter-mule-overhand on the locker. This also allows for a more controlled release.

The other classic, non-controlled, prussik release mechanism is a knife. Be it a real one or a 1mm cutting cord.

As you probably know, any line under large forces suddenly released is a fast moving dangerous object. But again, I know nothing of water and boats.

 PaulJepson 12 Jan 2021
In reply to nickb:

There are also lots of fancy prusik cords available now (like the Beal Jammy), which is fully rated at 22kn. I've just got one as I thought it might be handy in case of running out of runners. I've had to resort to using a length of 5mm prusik before so it may as well be full-strength! 

 Rick Graham 12 Jan 2021
In reply to PaulJepson:

> There are also lots of fancy prusik cords available now (like the Beal Jammy), which is fully rated at 22kn. I've just got one as I thought it might be handy in case of running out of runners. I've had to resort to using a length of 5mm prusik before so it may as well be full-strength! 

Be careful what you wish for or are trying to achieve.

Some test results would be useful but three likely failure modes are prussik slipping/ melting , sling failure or rope failure.

That may be why trying to tie a prussik using a wire sling is non sensical .

A lot of mechanical devices damage the rope before metalwork failure.

 innes 12 Jan 2021
In reply to nickb:

There's more data in here for fellow dweebs....

https://www.kajakksenteret.no/files/Belay%20and%20brake%20loading%20test.pdf 

Number of turns on the prussik being an obviously important consideration.  n.b. some of the materials might be different to what I'd carry (6mm cord and chunky nylon slings in this study).  

 Cobra_Head 12 Jan 2021
In reply to McHeath:

> Dunno about you, but I like using the strongest knots available for everything. Following situation: you've come off seconding an overhang and rather than being lowered off into the sea you prussik up the rope. The rope goes up to a single wire over the lip, then diagonally to your partner on the belay. The wire rips. I'd rather have a strong knot. 


Would you not backup your ascent up the rope? Why would you trust you life to a piece of string?

You might need a strongest knot and bits of kit, if your not very good using the ones you have to their best.

The OP has explained now why they are interested in having a strong prussik, which is a far better reason than yours.

 Cobra_Head 12 Jan 2021
In reply to nickb:

Bachman knots are very useful if you are considering moving a lot of rope through it.

 PaulJepson 12 Jan 2021
In reply to Rick Graham:

I meant more in having something that could be dual purpose. If you're using a prusik as a prusik and need 22kn then you're doing something wrong. However there's no harm in having something that is rated to be used as a thread/runner in an emergency. 

 Rick Graham 12 Jan 2021
In reply to PaulJepson:

Fair comment.

Misunderstood the direction of your post.

 HardenClimber 12 Jan 2021
In reply to David Coley:

This is a slow onset of load, so a bit different from a shock load, where there might be more initial slippage etc.

One of the major issues with falure of clamping system is damage to the main rope - which is why metal mechanical clamps oten are not that great.

for one off use a tandem prussik may be good

(Life on a line, Dave Merchant, p134-5, also on line)

 EdS 12 Jan 2021
In reply to nickb:

What specifically are you using the knot for?

Tensioning a tension diagonal? 3:1 (or more ) MA for hauling the raft from the bank?


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