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Step 6Rappel Down
This is where prussiks are (kind of) awesome!
All you have to do is squeeze the prussik knot, and it loosens and you can slide down the rope!
The downside is that you have to successfully do this to two knots at once;
fortunately, if you screw up, you just come to a halt on the rope.
You can also control your descent rate by how much you grip the knot,
which controls the friction between the rope and the knot.
I'm squeezing pretty hard, and making the accompanying face to show what it's like to loosen two four-turn prussiks enough to get it to slide. It worked well enough to prove the theory, but I abandoned the prussiks in favor of using an 8 for the rappelling.
All you have to do is squeeze the prussik knot, and it loosens and you can slide down the rope!
The downside is that you have to successfully do this to two knots at once;
fortunately, if you screw up, you just come to a halt on the rope.
You can also control your descent rate by how much you grip the knot,
which controls the friction between the rope and the knot.
I'm squeezing pretty hard, and making the accompanying face to show what it's like to loosen two four-turn prussiks enough to get it to slide. It worked well enough to prove the theory, but I abandoned the prussiks in favor of using an 8 for the rappelling.
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Of course this does rely on your being sensible, one of the golden rules is never to muck around with fast descents etc for the precisely the reasons you mentioned: you burn your ropes.
As an additional point, kind of backing up what you said, arborists use specialist ropes that are more resistant to heat and friction than the kind rock climbers use specifically to endure the kind of punishment they take over a lifetime of work.
But for anybody else reading this, #mahyongg's advice is good, solid advice and should be stuck to in most circumstances unless you're trained in other techniques.
Interesting, I never knew arborists (in the UK at least) use knots entirely.. I think in Germany it’s probably not even be allowed by the professional organisations and insurances.. not that I don’t believe a proper, pro use of knots would work absolutely fine, but I guess the belief in solutions with a little more technology is just over the top here. Plus the industry lobbies would do everything to sell a 80€ device over a 1€ of tech cord any day ;D
However, as you said - descending with prussiks should really be left to professionals and not be used by novices in any case, other than for test purposes when backed up with a second rope or such in my view.. although its good to know that it can be done, since it could be very valuable in an emergency. Another reason why any climber should carry one or two prussik loops in his/her gear loops ;D
Cheerio,
Jan
One other thing, when backing up a rappel on an ATC with a friction knot, it is important to note that the knot should provide almost no friction. I can put a prusik above my ATC and clip it into the belay loop. In that case, I keep the prusik extremely open such that it is still around the main rope but barely touching it, with my left hand. The right hand holds the brake side of the rope and provides the friction for the descent.
The alternative is to put a friction knot below the ATC. I say friction knot and not prusik because a prusik provides way too much friction and is a hassle. I use an autoblock knot attached to my harness leg loop, or I can girth hitch a sling through my 2 rope tie in points and extend the ATC to about the height where my neck is, and then clip the autoblock to my belay loop.
How to tie the autoblock