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superbum
Mar 6, 2003, 7:51 AM
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OK, If i were to back up a fixed, single rope rappel, american style, I would use a prussik, attatched to my harness via a locker, and slide it down the rope with my guide hand on descent. Fine.
How about on two ropes or a doubled over rope? Would you need two prussiks? Would two knots be harder than one to guide down the ropes? If I only used one on a doubled over rope and my brake hand fell off, the half without the prussik would goo screaming up and through the anchor.
How do ya back up a rappel on dual ropes?
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neph
Mar 6, 2003, 11:59 AM
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I just make the prussik around both ropes...
Although, I put the prussik below my rappel device and clipped to my leg loop, to back up my brake hand, not above the rappel device. I have tested this, and it works for me.
I don't know if a prussik around two ropes will hold full body weight, as it would have to if you put it above the rappel device.
-Nate
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apollodorus
Mar 6, 2003, 12:20 PM
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Yeah, that's right. Put the prusik around both ropes. And make sure that it's short enough to you that you can grab it if it locks up. And that it's soft enough perlon mini-rope that it grabs. Some of that "great" stuff for sling chocks is too stiff. You want 7mm, or so. Do the prusik three times around.
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adamwvt
Mar 6, 2003, 1:28 PM
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Yeah, It a great technique. I have my rappel piece extended on a runner girth hitched to my harness, and the prusik on my leg loop with a small locker, around two ropes. The friction knot acts as a fireman's belay, so im still supported by my rappel piece when I stop. I can set it up pretty quickly now, but my friends still think Im overdoing it. I say up yours to that, It makes cleaning gear on rappel a snap, and it's good form!
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burz
Mar 6, 2003, 1:31 PM
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I recommend the Auto - Block for backing up a rappel on a doubled rope. Here is an address for some info, not sure how to make a link:
http://www.gunks.com/index.php?pageid=108&pagenum=4&smGroup=2&smID=6
The auto-block is smooth and efficient, just clip the beaner to your leg loop and attach it below your rappel device. It does not jam-up, because it has less bite. But when backing up a rappel you don't need much bite, that's what makes this knot so great.
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texasclimber
Mar 6, 2003, 2:11 PM
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The "Auto Block" looks to me to be the same thing as a prusik!
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burz
Mar 6, 2003, 2:32 PM
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Stephen,
When tying a prusik the rope is wrapped back through itself twice, or more if you want to increase friction. This inherintly will cause it to bind up (it is a more secure knot). The Auto-block is just a loop of cord wrapped around the rope three times and then both ends are clipped to a beaner which is attached to your leg loop. It is not wrapped back through itself. Is this clear? See the link, they discuss prusiks earlier in the article.
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c_kryll
Mar 6, 2003, 3:59 PM
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Registered: Nov 18, 2002
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The prusik, bachmans, auto-block, each has it's own advantages and disadvantages. The most obvious choice is try them all and choose for yourself which you prefer. Myself, I prefer using a bachmans, attached below my belay device and clipped to my leg loop.
Chris
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climb4life
Mar 6, 2003, 4:10 PM
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hey there...
i am a search and rescue member for the gunnison county. if want back up on double rope, put the prussik around both rope. wrap it around 3 times, not 2. although wrapping around 2 times does work, but i highly recommend 3 wraps. i also prefer to add a biner, preferably oval so that you can also clip things to it, like another belay device for lowering another person down, or need to get down and lower something like a haul bag or anything like that.
7mm is the standard size. more importantly, make sure that the knot (double fisherman) is not right at the bottom where you hang and hang on the knot. many climbers have failed to do so. make sure the knots are out of the way like on the sides.
climb on
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fitz
Mar 6, 2003, 6:52 PM
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Registered: Jul 15, 2002
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I agree with most of the previous posters, there is nothing wrong with wrapping a prussik around both lines. However, I personally prefer some form of the autoblock on the brake side for several reasons:
1. It leaves a hand free. Having a prussik above the rap device keeps both hands occupied, since the prussik has to be slid to keep from biting.
2. Less clutter at the tie in point.
3. Seems to lock more reliably. I haven't done anything scientific, but I've never had an autoblock not lock when I stop. On the other hand, I have often had to fiddle with a prussik to get it to bite.
The above said, I still occassionally use the prussik-above-the-device method when I am rapping with a really heavy load.
-jjf
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beyond_gravity
Mar 9, 2003, 5:02 PM
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Make sure you tie your cord short...if you stop for whatever reason and lift your leg up you don't want the prussik to raise to the level of the rappel device.
I personally don't bother with the biner, I just wrap the prussik and tie it around my leg loop with a overhand knot (Euro Death knot)
Two wraps will work fine below a belay device...you can do 3 if you don't feel safe but your not going to die if you only do two.
For prussiking, rescue, or any prussik that is not backing up a friction device, allways use 3 or more wraps.
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dirtineye
Mar 29, 2003, 7:49 PM
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Always set an auto block before using it, or it may not block anything.
IF you want to know how well this or any other rapell backup works, set a rappell, get on from the bottom, jug up a little and test your backup a few feet off the ground.
Personally I'd use a 4 wrap prussic and an auto block with about as many wraps as I could get on it.
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