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The Autoblock - Your Third Hand on Dicey Raps

Submitted by admin on 2001-11-22

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Not too long ago, people thought that backing up rappels was strictly for sissies, or something you'd only resort to in special situations - bringing down an injured or incompetent climber, for example. If you needed to free both hands, or wanted to hang out a while shooting pictures or whatever, you'd wrap the brake strands of the rope around your leg a few times(ouch!) or tie it off somehow (tricky and awkward).

If you think about it, though, there are few climbing situations where you have as little standing between you and the big splat as when you rap. Everything depends on your brake hand. Yeah, you depend on your belayer's brake hand while you climb, but she's comfortably anchored in one spot, with nothing much else to do. When you rap, you're swinging around, trying to free stuck ropes with one hand or trying to find the next rap station, all the while gripping the rope - literally - for dear life.

And what if your grip fails? You hit your funny bone and your arm goes numb (it has happened). Rockfall knocks you unconscious or breaks you arm. You have some kind of wierd seizure or fainting spell. You drop below an overhang into a wasp nest the size of a sofa pillow. The list goes on.

Autoblock AnimationThe traditional backup method was to put a prussic knot on the rope above the rappel device, attach it to your harness, and hope it would catch you if you lost it. The problem was, once the prussic cinched up and took your body weight, it was the dickens to get it loose again. You had to keep a hand on it the whole time you were descending, to keep it from catching. And you were really SOL if your attachment to the prussic was too long and it clamped down on the rope beyond your reach. Generally an unsatisfactory setup.

Enter the autoblock, aka French prussic, basically a less grippy relative of the prussic and Klemheist knots. Why would you want a weaker knot? Cuz it goes below the rappel device, on the brake strands of the rope. The prussic in the setup described above would have to hold your body weight or more; the autoblock just has to be as strong as your brake hand to hold. It does the job well, but it's easy to release - just slide it down the rope.

Setup is totally simple. Use your prussic loop, a loop of 6mm perlon cord tied with a double fisherman's knot, about a foot long. You always carry at least one of these anyway, right? Girth hitch it to your leg loop on the brake side, or clip it in with a locker. (Some people like to set it up that way and leave it, so it's always ready and they're less likely to get lazy and not use it.)

After you set up your rap device on the rope, just wrap the loop around the brake strands of the rope, in an upward direction, two or three times. Some experimenting will determine how many wraps work for you. Then clip the end of the loop back into your leg loop with the locker. If your prussic loop is too long, you can shorten it with an overhand knot.

As you descend, keep your brake hand over the autoblock, sliding it down as you go. Keep an eye on it to make sure it doesn't get discombobulated. When you want to stop, just let it ride up until it cinches. It takes a little practice to get the feel of it, but you'll learn to love it, especially when you're rapping with a heavy pack or you have the heeby-jeebies. It takes a while to trust the autoblock enough to let go completely with the brake hand. Even then you should do so only with extreme caution, keep a close eye on it, and consider tying a backup knot (an overhand on a bight with both strands of the rope below the autoblock, with a biner clipped into it, is easy and foolproof).

Webbing can be substituted for the perlon cord in a pinch, but spectra webbing is not recommended due to the possibility of heat damage. If you find that the autoblock is riding too close to the rappel device, which could prevent it from cinching properly, try extending your connection to the rappel device with a doubled runner looped through your harness or rappel loop. Happy landings!

Note: This article is republished by permission of All rights reserved.


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I really like the peace of mind this setup gives me. I actually enjoy my rappels.

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