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Force test on a Purcell Prusik
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camtraks


Dec 6, 2010, 7:02 PM
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Force test on a Purcell Prusik
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I am considering the use of a Purcell Prusik as an adjustable personal anchoring device in certain applications. I was wondering if anyone has done a force test on this to see where the Prusik starts to slip? If so, did you find a point where the slip rate caused the rope to start melting?


ptlong2


Dec 6, 2010, 7:43 PM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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Obviously it will depend on the diameter of the cord, the type of prusik (number of wraps), and the nature of the loading.

Some good information here:

http://www.caves.org/...DaisyChains-2005.pdf

and here:

http://www.caves.org/...yardsPartII-2006.pdf




(This post was edited by ptlong2 on Dec 7, 2010, 2:46 PM)


catbird_seat


Dec 6, 2010, 9:22 PM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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The Purcell has the lowest force:

For example, Fall Factor 1

Yates Spectra Daisy + Shorty Screamer
11.1 kN
Climb High 25mm Nylon Daisy
12.8 kN
Purcell Prusik (7mm nylon cord and 3 wraps)
9.1 lM


summerprophet


Dec 7, 2010, 8:37 AM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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The purcells outperform most anything else out there. The 'slippage' acts as a load limiter and I believe small slippage begins to be observed around 7 to 9 kN. (don't quote me on that, my material is elsewhere)

As well, rope melting, while technically correct, implies a far greater impact than what really happens, realistically it is a surface glazing of the cord, rather than flames and sputtering plastic.

I use purcells for rescues, but despite their usefullness, never made it into my freeclimbing setup. To bulky compared to just clove hitching the lead line.

If you Google, Kirk Mauthner +Purcell test, you should come up with the original testing.


healyje


Dec 7, 2010, 3:36 PM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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The topic has been flogged to death several times at this point. Bottom line? You don't want to take a short fall onto an anchor on any material. NEVER put yourself in a position to take such a fall. Any time such fall potential exists you should be on a rope.


camtraks


Dec 7, 2010, 4:37 PM
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Re: [bill413] Force test on a Purcell Prusik [In reply to]
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bill413, while many in this thread have devolved into a discussion about Nazi's which has nothing to do with the question I posed, I have been doing some reading on the original subject. I have to say that you have misinterpreted the data you are looking at. The low forces you have sited are a positive. It reflects the fact that the Purcell has excellent energy absorbtion. In the studies I have read, high forces seen by various lanyards is a sign of weakness in the device. As a static measurement it may be impressive, but it shows weakness in the the dynamic world of climbing. I would be fine to see this thread come to an end if contributors aren't interested in staying on subject.
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Partner philbox
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Dec 7, 2010, 7:12 PM
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I've hidden a whole bunch of posts in this thread. Some were flaming the OP with no reason, some were completely off topic. Play nice guys, this is the Lab forum after all. Who cares if this subject has been thrashed around before. Always good to explore whether any new understandings of a subject are out there.

So if you post does not show up, it was me. A couple of those posts did have some parts that could have been spared so if you want to repost then do so with the parts that are on topic.

Thank you all and I hope you understand my actions.


bill413


Dec 8, 2010, 6:34 AM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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camtraks wrote:
bill413, while many in this thread have devolved into a discussion about Nazi's which has nothing to do with the question I posed, I have been doing some reading on the original subject. I have to say that you have misinterpreted the data you are looking at. The low forces you have sited are a positive. It reflects the fact that the Purcell has excellent energy absorbtion. In the studies I have read, high forces seen by various lanyards is a sign of weakness in the device. As a static measurement it may be impressive, but it shows weakness in the the dynamic world of climbing. I would be fine to see this thread come to an end if contributors aren't interested in staying on subject.
In reply to:

I have not misinterpreted any data. Lower forces on a climber is generally a good thing.
I have posted to this thread about Godwin's law and about Sets. I have not posted about forces.
You clearly have me confused with someone else. Hopefully not Majid.


(This post was edited by bill413 on Dec 8, 2010, 6:38 AM)


camtraks


Dec 8, 2010, 7:17 AM
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Re: [bill413] Force test on a Purcell Prusik [In reply to]
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Sorry, plugged into the wrong place in the thread I guess.Point is someone interpreted the low force data as a negative when it is not....obviously you get it.I think PHILBOX must have removed the reply that I was trying to target. Just out of curiousity, if this subject has been beaten to death, would anyone like to point me to other (informative) forums on this site which discuss the subject...how about it philbox?


Partner philbox
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Dec 14, 2010, 12:59 AM
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Re: [camtraks] Force test on a Purcell Prusik [In reply to]
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camtraks wrote:
Sorry, plugged into the wrong place in the thread I guess.Point is someone interpreted the low force data as a negative when it is not....obviously you get it.I think PHILBOX must have removed the reply that I was trying to target. Just out of curiousity, if this subject has been beaten to death, would anyone like to point me to other (informative) forums on this site which discuss the subject...how about it philbox?

I really don't mind if a subject is warmed over and discussed again. Feel free to keep this thread going. It'd be different if it was a shoe thread though. Laugh

I love Purcell Prussiks. Does anyone else use the three on two method of tying. Very easy to adjust but still retains the ability to hold the load that is imposed. Once loaded and then released it is also easy to adjust.


binrat


Dec 28, 2010, 5:33 PM
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Re: [philbox] Force test on a Purcell Prusik [In reply to]
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philbox wrote:
I love Purcell Prussiks. Does anyone else use the three on two method of tying. Very easy to adjust but still retains the ability to hold the load that is imposed. Once loaded and then released it is also easy to adjust.
Thats the only proper way to tie it. I use the Purcell for many things including cleaning anchors on a route.


billl7


Dec 28, 2010, 5:41 PM
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binrat wrote:
philbox wrote:
I love Purcell Prussiks. Does anyone else use the three on two method of tying. Very easy to adjust but still retains the ability to hold the load that is imposed. Once loaded and then released it is also easy to adjust.
Thats the only proper way to tie it. I use the Purcell for many things including cleaning anchors on a route.
What is the "three on two" method? Do you mean the three-wrap prusik around two cords?


Partner philbox
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Dec 28, 2010, 7:26 PM
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billl7 wrote:
binrat wrote:
philbox wrote:
I love Purcell Prussiks. Does anyone else use the three on two method of tying. Very easy to adjust but still retains the ability to hold the load that is imposed. Once loaded and then released it is also easy to adjust.
Thats the only proper way to tie it. I use the Purcell for many things including cleaning anchors on a route.
What is the "three on two" method? Do you mean the three-wrap prusik around two cords?

No. The three on two method is that instead of tying a normal three wrap prussik you have one side of the prussik wrapped three times and the other side of the Prussik wrapped twice. The load side of the Prussik which is towards the load is wrapped three times to hold the load and the side away from the load is wrapped twice. This ensures very easy adjustability and ensures that the Prussik will definitely hold the load.

It's a bit of a trick to tie though. You have to tie the wraps first and then send the cord that you are wrapping around through the wraps. I generally wrap the wraps around two of my fingers. Three around one finger and two around the other finger.

You can't tie this method on a rope with a standard Prussik loop unless you do it at the end of the rope and pass the rope through the wraps.


moose_droppings


Dec 28, 2010, 7:57 PM
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If you enlarge the pic you can see the three and two raps on the purple PP.



Edited to add Majid arrows.


(This post was edited by moose_droppings on Dec 28, 2010, 8:07 PM)


billl7


Dec 28, 2010, 9:57 PM
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Re: [moose_droppings] Force test on a Purcell Prusik [In reply to]
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Thanks, Phil, Moose. So the three-strand end of the prusik goes towards the adjustable loop?

Interesting to see the sections of webbing. I imagine they are to reduce chafing. I assume that part gets girthed to the harness tie-ins? I only ask because I've seen the purcell prusik used in more than one kind of application.

Regarding chafing, another option I had been considering was to tie that loop off with a figure 8 such that there are two cords to girth hitch - adds a little more bulk but removes the need to join the original cord with a separate knot (e.g., double fishermans).

Bill L


(This post was edited by billl7 on Dec 28, 2010, 10:00 PM)


dmushrush


Apr 6, 2011, 12:29 PM
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Any opinions on tying the main loop with a frost knot as opposed to double or triple fisherman? I like how it handles/unties.


majid_sabet


Apr 6, 2011, 2:16 PM
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dmushrush wrote:
Any opinions on tying the main loop with a frost knot as opposed to double or triple fisherman? I like how it handles/unties.

The whole propose of the prusik loop on parcell prusik is have an adjustable loop where you could shorten the distance between harness and the anchor.


dmushrush


Apr 6, 2011, 2:55 PM
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I mean use the frost knot as the knot to make the two loops one of which then has the prussik tied to it. So as opposed to tying the loop with the fishermans, then tying this big loop into two loops with an over hand or figure eight the frost knot makes the two loops. Does that make sense?
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cintune


Apr 6, 2011, 5:40 PM
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dmushrush wrote:
I mean use the frost knot as the knot to make the two loops one of which then has the prussik tied to it. So as opposed to tying the loop with the fishermans, then tying this big loop into two loops with an over hand or figure eight the frost knot makes the two loops. Does that make sense?

No it doesn't. Whatever you're describing isn't a Purcell, and Frost knots are for webbing.


(This post was edited by cintune on Apr 6, 2011, 5:41 PM)


majid_sabet


Apr 7, 2011, 12:06 AM
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Re: [dmushrush] Force test on a Purcell Prusik [In reply to]
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here is a purcell prusik. you make two of them from a 3 meter (6 or 7) mm cord and a 2 meter cord.

three wrap on the prusik side and triple fig 8 at the tie-off part. The cord end where the knot is should be facing opposite of each other when knot is dressed and then clove hitched to your harness.








(This post was edited by majid_sabet on Apr 7, 2011, 12:12 AM)


dmushrush


Apr 7, 2011, 4:42 AM
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I know this horse is pretty much dead but I am finding this discussion interesting.

I've been using the wrong term. The knot I was referring to (and which this seems to be the only reference for ) is called the Frost 8. Interesting at least.

Check this video. Also, although the demonstrator is talking tree work his videos are interesting.

http://www.youtube.com/watch?v=wg2HB7hCfz8


cintune


Apr 7, 2011, 9:30 AM
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Seems kind of bulky; if you feel a need to isolate the harness loop a dogvine fisherman's would be slimmer:
http://www.climerware.com/dogvine.shtml


(This post was edited by cintune on Apr 7, 2011, 9:31 AM)


dmushrush


Apr 7, 2011, 9:48 AM
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I like that knot! Thanks. I guess I hadn't thought about it but there is no need for the big loop to be made into two loops to be used as a Purcell.


binrat


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dmushrush wrote:
I know this horse is pretty much dead but I am finding this discussion interesting.

I've been using the wrong term. The knot I was referring to (and which this seems to be the only reference for ) is called the Frost 8. Interesting at least.

Check this video. Also, although the demonstrator is talking tree work his videos are interesting.

http://www.youtube.com/watch?v=wg2HB7hCfz8
The frost eight is the original way to tie it.


knudenoggin


Apr 19, 2011, 11:45 PM
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philbox wrote:
billl7 wrote:
binrat wrote:
philbox wrote:
I love Purcell Prussiks. Does anyone else use the three on two method of tying. Very easy to adjust but still retains the ability to hold the load that is imposed. Once loaded and then released it is also easy to adjust.
Thats the only proper way to tie it. I use the Purcell for many things including cleaning anchors on a route.
What is the "three on two" method? Do you mean the three-wrap prusik around two cords?

No. The three on two method is that instead of tying a normal three wrap prussik you have one side of the prussik wrapped three times and the other side of the Prussik wrapped twice. The load side of the Prussik which is towards the load is wrapped three times to hold the load and the side away from the load is wrapped twice. This ensures very easy adjustability and ensures that the Prussik will definitely hold the load.
...

Whoa!! Sadly --and let's hope not tragically--, this is bassackwards!

Friction hitches can hold either "coiling away" (ProhGrip/Blake's, Bachmann, Klemheist) or "coiling towards" (Hedden, Tautline/Rolling) the loaded ends of the knot. The former tend to take some bit of *extending* to develop their grip (but as they extend further, they grip more --Chinese finger-trap mechanics).

Friction hitches don't hold (so well), however, if something pulls into the knot pressing it towards the load. THIS is part of the behavior of the Prusik hitch, which can be seen to contain both forms (away/towards) of coil : the away coil will extend and its loaded end, twin to the other, will press into the towards coil, and cause it to slip --YMMV on the degree of this, but as was discussed briefly on RC.com with the late Dirtineye in re the --by above terminology-- "3 on 1", which got mistakenly introduced/recommended (by upside-down images) in On Rope (2nd ed.) that knot might not hold at all; how much improved will putting on a 2nd away coil be? --well, it should be a little better than the 2-on-2, common Prusik.

Thanks for the URLinks to the two test reports. It's not clear to me from a quick read that they examined the cord inside the Prusik (for damage from frictional heat, abrasion) --that is where the worst of it should be! I note, however, that on the reported "failures" (breaks), rupture came at the end-2-end joint (Fig.8 of some sort). It should be noted that that knot could be positioned next to the Prusik and thus bear half the force it sees on the harness side of things. (It could also be incorporated into the Prusik --e.g., a grapevine knot or an "EDK" as the cross-bar part.) This would leave the Purcell structure completely *clean* on its 2-strand span to the harness.

*kN*


irrational


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How well a friction hitch will hold is completely dependent on the cordage type and how the two ropes interact. Static kernmantle ropes do not perform as well as 16 strand static lines. I have a tenex purcell prusik that I've used at work for the last few years. Also a schwabisch http://proclimber.org/...gShed/Schwabisch.jpg works nice too!


delrio


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Acceptance of the Purcell Prusik in Europe:

http://www.gipfeltreffen.at/showthread.php?t=51482


rescueman


Jul 5, 2011, 10:39 AM
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binrat wrote:
The frost eight is the original way to tie it.

I don't know if it's "original", but that is the method used in the Mountain Rescue Association Technical Rescue Rigger's Guide by Rick Lipke. And it appears to be what's shown in Moose Dropping's picture.

And, contrary to Majid,
majid_sabet wrote:
The whole propose of the prusik loop on parcell prusik is have an adjustable loop where you could shorten the distance between harness and the anchor.
purcells are used in a set of three for the most compact and light personal ascending system. But that requires that the top loop be large enough (~8" long) to form a prusik over the mainline. The doubled adjustable loops are for cinching footloops.

And, contrary to Philbox,
philbox wrote:
You can't tie this method on a rope with a standard Prussik loop unless you do it at the end of the rope and pass the rope through the wraps.
you can tie this over a rope by slipping the mainline through the single top loop and then wrapping just like a regular 3-wrap prusik.


(This post was edited by rescueman on Jul 5, 2011, 10:43 AM)
Attachments: 3 over 2 prusik tie.jpg (15.5 KB)


healyje


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In reply to:
Purcell Prusik (7mm nylon cord and 3 wraps)
9.1 lM

9kn is not a fall you want to take - ever. That it's less than a slightly more horrendous fall is besides the point. There is no leash of any design or material you want to take a short fall onto.


binrat


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rescueman wrote:
binrat wrote:
The frost eight is the original way to tie it.

I don't know if it's "original", but that is the method used in the Mountain Rescue Association Technical Rescue Rigger's Guide by Rick Lipke. And it appears to be what's shown in Moose Dropping's picture.

And, contrary to Majid,
majid_sabet wrote:
The whole propose of the prusik loop on parcell prusik is have an adjustable loop where you could shorten the distance between harness and the anchor.
purcells are used in a set of three for the most compact and light personal ascending system. But that requires that the top loop be large enough (~8" long) to form a prusik over the mainline. The doubled adjustable loops are for cinching footloops.

And, contrary to Philbox,
philbox wrote:
You can't tie this method on a rope with a standard Prussik loop unless you do it at the end of the rope and pass the rope through the wraps.
you can tie this over a rope by slipping the mainline through the single top loop and then wrapping just like a regular 3-wrap prusik.
1. Yes it is, and its good that you have read Mr Lipke's pocket guide.
2. It is designed as a means of ascending, but is commonly used as a personal anchor system for climbers and others.


rescueman


Jul 10, 2011, 7:42 PM
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binrat wrote:
1. Yes it is, and its good that you have read Mr Lipke's pocket guide.
You sound a bit sarcastic. Not only have I read Rick's manual (which is the best on the market) and every other rope rescue reference book, but I've contributed to the current best practices for rope rescue, have published critiques of common practices, have been recognized worldwide as an expert in technical rope rescue, and was the last moderator of the original SARBC rescue forum.

In reply to:
2. It is designed as a means of ascending, but is commonly used as a personal anchor system for climbers and others.
Yes, it's useful as a force-limiter for edge work, but why would a climber carry an unnecessary extra piece of gear when you can do the same with your rope.


trenchdigger


Jul 10, 2011, 11:05 PM
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I keep a 3/2 purcell tied with 9' of 6mm cord attached to my harness at all times. Rather than girth hitching to my belay loop, I tie it directly into the tie-in points with the loop at that end sized to match the belay loop. I store it in the shortened form with a locking carabiner attached to the end. I use it regularly as a "personal anchor" when setting up top-ropes, accessing rappel anchors from above, as part of my ascending system, and as my attachment to a prusik on an edge line for rescue work when not using an aztek kit.

I prefer the adjustability and shock absorption (of course striving to avoid situations where that could be necessary) over daisy chains, PASs, and other commercial options. I also prefer the simplicity and convenience of always having this system available over the minimalistic approach of simply utilizing other coincidentally available gear like slings or the rope. 9' of 6mm cord and a locking carabiner is not a lot of extra "unnecessary" equipment.

I tie the 3/2 prusik with the 3 wraps toward the loaded end of the prusik and 2 toward the inside of the loop of the purcell and it grabs well and adjusts easily. I've anecdotally experimented with it tied the other way and it works as well as a two-wrap prusik, maybe slightly better. Regardless, in the form of a purcell, tying the prusik wraps incorrectly is not a death sentence. The worst case scenario is a purcell that doesn't grap and simply slips to full extension only to stop as the loop closes down on the attached carabiner.

To each his own. I like the purcell and use the one tied to my harness every time I climb.


binrat


Jul 11, 2011, 4:54 AM
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Re: [rescueman] Force test on a Purcell Prusik [In reply to]
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"rescueman wrote:
Yes, it's useful as a force-limiter for edge work, but why would a climber carry an unnecessary extra piece of gear when you can do the same with your rope.

It makes a quick adjustable leash for climbing where you want to come to an anchor and clip in.
Edit to add: Cheap, easy to make, light weight, easy to use, compared to the manufactured ones.

I thought that the SARBC rope rescue forum went bye bye a few years ago.


(This post was edited by binrat on Jul 11, 2011, 4:59 AM)


billl7


Jul 11, 2011, 5:27 AM
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Re: [rescueman] Force test on a Purcell Prusik [In reply to]
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rescueman wrote:
binrat wrote:
2. It is designed as a means of ascending, but is commonly used as a personal anchor system for climbers and others.
Yes, it's useful as a force-limiter for edge work, but why would a climber carry an unnecessary extra piece of gear when you can do the same with your rope.

Most of my outside climbing is multi-pitch, and I exclusively using the rope to attach at belay anchors. In other aspects of climbing / training, I can imagine using the purcell prusik but those activities don't seem to be a driving force for me to switch.

Bill L

Edit: If I did use a purcell, I'd go with 7mm nylon cord. If I recall correctly, 6 mm failed or was damaged in falls nearing worst case.


(This post was edited by billl7 on Jul 11, 2011, 5:29 AM)


rescueman


Jul 11, 2011, 9:22 AM
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Re: [binrat] Force test on a Purcell Prusik [In reply to]
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binrat wrote:
"rescueman wrote:
I thought that the SARBC rope rescue forum went bye bye a few years ago.

It died a peaceful death, perhaps because it didn't have all the bells and whistles of the new forums like this one.

Shame, though, since it was the most professional discussion forum, in which we could actually disagree respectfully.


binrat


Jul 11, 2011, 11:05 AM
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Re: [rescueman] Force test on a Purcell Prusik [In reply to]
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rescueman wrote:
binrat wrote:
"rescueman wrote:
I thought that the SARBC rope rescue forum went bye bye a few years ago.

It died a peaceful death, perhaps because it didn't have all the bells and whistles of the new forums like this one.

Shame, though, since it was the most professional discussion forum, in which we could actually disagree respectfully.
Yes, I have to agree I was on there weekly, checking things out.


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