Teuns
I remember considerable discussion some 30 years ago in the caving literature about the actual usefullness of prussik knots for backup safety when descending or ascending. One caver had died after falling down a rope despite a prussik backup.
I recall 3 primary concerns about prussiks. These all concerned prussiks held in reserve above the abseil device. I do not recall any discussion about prussiks attached below the abseil device.
1) If the person falls more than ???? (a few feet?) sufficient speed will have been developed so that the prussik material will burn through and not hold. (This is the same phenomena as recently related by those who have lost control of their rappels. Once any speed is attained no practical amount of additional pressure from a hand or prussik can stop the descent.)
2) When a person feels themselves starting to fall, the natural response is to grab onto the rope tighter. However, a prussik has to be released in order to function. Being able to release pressure and set a prussik in a panic situation before any speed has developed may be very difficult or impossible for most people to accomplish.
3) There is a common perception that having additional “safety” gear can make one safer, while ignoring the very real increased danger of increasing equipment “clutter”. The more clutter, the more chance of not seeing errors in one’s basic and primary attachments.
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It has seemed to me, that a prussik is primarily useful before any slippage or panic occurs. One locks off the prussik at the first hint of awkwardness and takes care of any problems before they develop to a dangerous level.
It also seems to me that having the prussik on the brake side of the rappel device has many advantages and may reduce somewhat the danger of burning through in a falling situation. I suspect, though, that it will not eleminate the danger of burning through, especially with small diameter ropes.
Paul Martzen
Date: Wed, 14 Jul 2004 11:53:00 +0200 From: “Teuns Kok” Teuns.Kok@capetown.gov.za> Subject: TECH: Autoblock
I have some questions on the use of the autoblock.
1. The autoblock system is rigged from a harness legloop on the side of your braking/holding hand. Has anyone experimented with placing the Autoblock directly below or above the abseil device?
2. The Autoblock is basically a French prussik tied on a legloop. One of the advantages is that after being loaded, it can be released/pushed down again. This advantage should also impact on the “holding strength” of the Autoblock. Any info on the “holding strength” before slippage occur?
Teuns