RescueMan
Rope Rescue Specialist
I just emailed ISA-ARBOR with some comments for improving their excellent book, The Art and Science of Practical Rigging. These included a couple of mistakes that need correcting.
The gist of my comments are below:
On page 49, there is an inconsistency between the text and the picture and an inconsistency between what is described and conventional language.
In the discussion of the "slip knot", the text instructs to take a counterclockwise turn while the picture shows a clockwise turn (though the knot will come out the same either way). More importantly, what is described as a "slip knot" is not what is commonly known as a slip knot, which is an overhand noose knot which will cinch around an object. What is described and pictured is a slipped overhand (which is stated later in the text). It would be helpful to differentiate this knot from what every kid learns as a slip-knot by using the term "slipped knot" rather than "slip knot" and by describing a possible function, such as an easily spilled stopper knot. The picture makes it appear that the knot is a loop knot, but the loop will spill as soon as it is loaded, causing a potential catastrophe.
On page 88, there is what I believe to be an error of fact and also a case of misapplied terminology.
In the middle of the page there is a statement that the input force, with one person pulling, is approximately equal to their body weight. I suspect that few arborists can climb a fixed rope hand over hand, thus lifting their body weight while gripping a rope.
Studies done by Rigging for Rescue of Canada on the grip strength of rope rescue personnel, demonstrated a wide range with an average one-gloved-hand grip strength on a ½" rope of 50 lbs. When I figure mechanical advantage systems, I calculate the number of available haulers times 50 lbs each (assuming they are using a hand-over-hand method with only one hand on the rope at a time). Few people are aware of how little force can be exerted on a rope by pulling and it would be helpful to clarify this common misunderstanding.
Also, in the second to last paragraph, the text states that "it would be possible to rig a system to disadvantage, where the anchor force is greater than the output force." The text earlier defined mechanical advantage correctly as "the output divided by the input," but then uses the term "disadvantage" inappropriately. While the system described would place more load on the anchor, there would still by a 4:1 increase in output. A mechanical disadvantage system would create less output for a given input input (a 1:4 system, e.g.).
- Robert
The gist of my comments are below:
On page 49, there is an inconsistency between the text and the picture and an inconsistency between what is described and conventional language.
In the discussion of the "slip knot", the text instructs to take a counterclockwise turn while the picture shows a clockwise turn (though the knot will come out the same either way). More importantly, what is described as a "slip knot" is not what is commonly known as a slip knot, which is an overhand noose knot which will cinch around an object. What is described and pictured is a slipped overhand (which is stated later in the text). It would be helpful to differentiate this knot from what every kid learns as a slip-knot by using the term "slipped knot" rather than "slip knot" and by describing a possible function, such as an easily spilled stopper knot. The picture makes it appear that the knot is a loop knot, but the loop will spill as soon as it is loaded, causing a potential catastrophe.
On page 88, there is what I believe to be an error of fact and also a case of misapplied terminology.
In the middle of the page there is a statement that the input force, with one person pulling, is approximately equal to their body weight. I suspect that few arborists can climb a fixed rope hand over hand, thus lifting their body weight while gripping a rope.
Studies done by Rigging for Rescue of Canada on the grip strength of rope rescue personnel, demonstrated a wide range with an average one-gloved-hand grip strength on a ½" rope of 50 lbs. When I figure mechanical advantage systems, I calculate the number of available haulers times 50 lbs each (assuming they are using a hand-over-hand method with only one hand on the rope at a time). Few people are aware of how little force can be exerted on a rope by pulling and it would be helpful to clarify this common misunderstanding.
Also, in the second to last paragraph, the text states that "it would be possible to rig a system to disadvantage, where the anchor force is greater than the output force." The text earlier defined mechanical advantage correctly as "the output divided by the input," but then uses the term "disadvantage" inappropriately. While the system described would place more load on the anchor, there would still by a 4:1 increase in output. A mechanical disadvantage system would create less output for a given input input (a 1:4 system, e.g.).
- Robert