Ever wondered how much a person can hold or how much force they can apply to a rope when pulling with one hand? Or both?
The average person, based on a gloved hand gripping 11mm kernmantel rope, can hold 200N or 20kg with one hand or 400N (40 kg) with two hands according to Dr Dave Merchant in his book titled 'Life on a Line - The Underground Rope Rescue Manual'.
You can download the eBook here - Dr Merchant is more than a little opinionated, but the book is an interesting read never-the-less.
The figures are drawn from a large sample of the 'cave rescue' population where one-hand strength ranged from 100N to 400N - the 200N number was settled on for the purposes of the book.
One person could conceivably apply more force if they gripped a knot or used a hand ascender on the line.
Interesting concepts for aerial arb
If the 20kg and 40kg loads are correct, Dr Merchant's data presents some interesting scenarios for aerial arb.
By applying these figures you can see why its impossible for your groundie to pre-tension the rigging line without some form of mechanical advantage.
Likewise, if an injured climber on a single line needs to be lifted by his rescuer while aloft, a pulley or counterweight system would need to be employed before that could happen. A single rescuer simply cannot lift an injured person by pulling upward on the climbing line alone.
Also, the 20kg and 40kg figures are assuming 'steady, hold'.
That means, at some point in time, you would have to let go the rope to move your hand, essentially dropping the load back to 20kg while you're moving hand over hand.
In the pre-tension rigging scenario if you had a second person tailing in the slack as the first applied downward force, say around the Port-A-Wrap bollard, then the lead groundie would be able to maintain a constant 40kg load.
Progress capture devices and prusik minding pulleys could be utilized in the same way to ensure the 40kg of force was consistently maintained.
But 40kg is still a relatively low figure.
Think about this before asking your groundie(s) to lift that next load!
Even if he is superman it is highly unlikely he will be able to lift much with the rigging line set in a ratio of 1-to-1.
Same goes for lifting a climber in a single line rescue.
Check out the Rope Deviation through a Pulley Simulator on Dr Merchants site as well - you'll need Flash - it won't work on an iPad