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Earth: Crevasse Rescue part 2

Posted by Joseph Anderson

Crevasse Rescue part 2:  Increasing Mechanical Advantage

During July of 2010 we posted this article breaking the stages of crevasse rescue down into the most basic steps, from arresting a fall all the way up to building a 3:1 mechanical advantage.  Since it is likely enough that there will only be YOU present as the rescuer, that is how we have approached this entire operation.  With that said, as a single rescuer you will almost certainly need to increase the mechanical advantage beyond the basic 3:1 described in the previous lesson.  After teaching this skill for years it is clear that this next step of increasing the mechanical advantage becomes confusing for the student.  Mastery of this step usually equals mastery of the entire skill.

Here is the break down, we’ll start with the 3:1.  You are pulling on the rope with the strength of one person.  As the rope you are pulling on slides through the pulley, the same rope on the other side of the pulley is being pulled with the strength of one (you) as well.  As that rope slides through the pulley the weight that the pulley is attached to is being pulled up with twice the strength of where the rope is originating from i.e. your hands.  That simple act of allowing the rope to slide through a pulley in order to pull the weight is doubling the strength output that is pulling on the weight.  But, where is the rope going after it slides through the pulley?

At the end of our last scenario the rope traveled back to the anchor and was freely passing through another pulley point at the anchor.  Remember, the force of the pull on one side of a pulley is the same as it is on the other.  What does that mean for this rope? Well now it finishes its Z pattern and goes down towards the weight ( i.e. rescue victim).  Of course before it gets to its rescue victim it has the tractor prussic attached to it which is being pulled with the force of two of your pulls (doubling your pull).  Now you get to add the force of the strand moving freely through the anchor to the force of two coming from the prussic thus turning your mechanical advantage into a 3:1.

Now it’s time to increase the mechanical advantage.  If you understand what I discussed in the previous paragraph and post than great, this part should be easy.  If you don’t than take a pencil and paper and draw it out.  That’s the best way to break it down for the sake of comprehension.

So the simplest way to explain this next step is to consider the rope you are pulling for your 3:1 and think, “Now I just want to double the strength for the 6:1”.  The easiest way to double the strength is to add a 2:1 directly to the rope that you were just pulling on.  In order to do this:

  • Take an additional rope.
  • Tie the end into your anchor.
  • Attach another prussic to the first rope where you were previously pulling.
  • Clip a carabineer into that prussic
  • Take the rope you just added to the anchor and pass it through the carabineer you just added to the new prussic.
  • Now pull the rope, you have a 6:1 and your victim is getting cold so stop burning day light and get to work.

Still don’t understand?  The Crevasse Rescue class is the best way to properly learn these skills.

Once you have mastered this step you are on your way to learning crevasse rescue.  Stay tuned for part 3.  Meanwhile, if you think you understand a 6:1 pulley system than now try rigging up a 5:1.