Debunking the Knot Efficiency Chart
Knot strength charts are misleading and - for newer high strength cordage - actively harmful.
We've all seen it: a chart of knots and their respective ratings. Perhaps in your rescue handbook, or even in a rope manufacturer's manual, this figure is illustrated and with a fixed percentage of relative strength.
Additionally, maybe a climber was proudly tying a double bowline loop, and a bookish individual reminded them "that knot is only 70% strong", perhaps recommending a figure 8 loop. Yet where does the number come from?
(As an aside, here I'll clarify this strength as knot efficiency, to distinguish it from the overall security of the knot dressing. The term 'strength' is often misapplied to both concepts: for example, some books say the basic bowline is 'weak' because it is easily jostled into undressing.)
First, knot efficiency is certainly underappreciated. I've heard someone proudly report their rescue line as rated at exactly 11,520lb/f, despite marking its length every 5m with overhand knots. In the circus world, the rescue 8 descender is popular for tissu, despite its awful efficiency and consequent tearing effect. Snow climbers girth-hitch 6mm slings across sharp aluminum snow pickets or metal edged skis. Meanwhile, ultra high efficiency knots (eg Bimini Twist) that might be valuable in an emergency context are essentially unknown outside the world of fishing.
So to the extent casual riggers consider efficiency at all, we should be grateful their noggin is in the right place. Yet how accurate are these charts and where does the data come from?
First let's recap knot efficiency, to the extent scientists understand it. Generally cordage derives strength from its internal components: at the most basic level, fibers. Bending the cordage puts these fibers on different "tracks". You can think of these like runners on a track. The outside runner has to do more work, while the inside runner sits idle.
For natural fiber cordage, you can even observe this by bending the rope around a sheet of iron. As the rope pulls to its breaking strength, fraying begin in the overstressed outside track.
Clearly this phenomenon is not specific to knots but occurs even when bending around a pulley sheaf or pole. When discussing this, riggers mention the D:d ratio, that is how the two surfaces compare in diameter, with 4:1 ratio being the ideal minimum.
This definition does not fully explain observed behavior, but in general, as great knot expert Clifford Ashley wrote: "the strength of a knot is the ease of its curve".
From this definition, we can assume certain hitches and loops are more efficient. Softer angles will more evenly stress the rope. Can they really be exactly rated though?
The most frequently cited data for climbing knots seems to derive from an 1987 study by CMC Rescue at ropemaker Wellington Puritan. This study was done on Goldline, a cheap hawser-laid (non-kernmantle) nylon rope using an old mechanical dynamometer. (Today we use electronic load cells like the Rock Exotica Enforcer and have rigorous procedures specified by the Cordage Institute and UIAA. And of course, asymmetric laid rope is now wholly relegated to the industrial world and unknown to recreationalists).
This 1987 study was subsequently copied into hundreds of manuals and guidebooks, and invariably without mentioning the original fiber or cord. A newer study by John McKently was presented to a rescue conference in 2014 and debunks some of the lore around it.
For example, loop knots (fig8, bowline) seem to have little difference in efficiency when loading the rope "end-to-end" outside the loop. The differences between nylon 6 and polyester were quite substantial and dwarfed any differences between the knots themselves.
Of course, it would be better to repeat these efficiency tests with a true UIAA 892 fall tower, but this would require hundreds of iterations and falls to approximate the breaking strength. For now, we'll need to be content with pull tests.
McKently essentially seconds the conclusion of Tom Moyer's paper at the same conference 14 years earlier, which doomed Black Diamond's new Gemini II cordage. Our vaunted figure 8 loop is only 42% efficient in certain cordage and certain fibers. Overall, knot efficiency is a property of temperature, humidity and particularly fiber. Efficiency is mostly unrelated to the knot design itself.
The message is clear, choose the best ropework for the task. If operating on (say) a 5:1 safety factor, any efficiency differences will be subsumed in the safety factor. Real focus should be on D:d ratio, edge management and especially fiber selection. If you need to choose between knots, opt for simple and clear, over any historic efficiency difference.
As a footnote, here's a comparison of loop knots when pulled loop-in-loop.
We've all seen it: a chart of knots and their respective ratings. Perhaps in your rescue handbook, or even in a rope manufacturer's manual, this figure is illustrated and with a fixed percentage of relative strength.
Additionally, maybe a climber was proudly tying a double bowline loop, and a bookish individual reminded them "that knot is only 70% strong", perhaps recommending a figure 8 loop. Yet where does the number come from?
First, knot efficiency is certainly underappreciated. I've heard someone proudly report their rescue line as rated at exactly 11,520lb/f, despite marking its length every 5m with overhand knots. In the circus world, the rescue 8 descender is popular for tissu, despite its awful efficiency and consequent tearing effect. Snow climbers girth-hitch 6mm slings across sharp aluminum snow pickets or metal edged skis. Meanwhile, ultra high efficiency knots (eg Bimini Twist) that might be valuable in an emergency context are essentially unknown outside the world of fishing.
So to the extent casual riggers consider efficiency at all, we should be grateful their noggin is in the right place. Yet how accurate are these charts and where does the data come from?
First let's recap knot efficiency, to the extent scientists understand it. Generally cordage derives strength from its internal components: at the most basic level, fibers. Bending the cordage puts these fibers on different "tracks". You can think of these like runners on a track. The outside runner has to do more work, while the inside runner sits idle.
For natural fiber cordage, you can even observe this by bending the rope around a sheet of iron. As the rope pulls to its breaking strength, fraying begin in the overstressed outside track.
Clearly this phenomenon is not specific to knots but occurs even when bending around a pulley sheaf or pole. When discussing this, riggers mention the D:d ratio, that is how the two surfaces compare in diameter, with 4:1 ratio being the ideal minimum.
This definition does not fully explain observed behavior, but in general, as great knot expert Clifford Ashley wrote: "the strength of a knot is the ease of its curve".
From this definition, we can assume certain hitches and loops are more efficient. Softer angles will more evenly stress the rope. Can they really be exactly rated though?
The most frequently cited data for climbing knots seems to derive from an 1987 study by CMC Rescue at ropemaker Wellington Puritan. This study was done on Goldline, a cheap hawser-laid (non-kernmantle) nylon rope using an old mechanical dynamometer. (Today we use electronic load cells like the Rock Exotica Enforcer and have rigorous procedures specified by the Cordage Institute and UIAA. And of course, asymmetric laid rope is now wholly relegated to the industrial world and unknown to recreationalists).
This 1987 study was subsequently copied into hundreds of manuals and guidebooks, and invariably without mentioning the original fiber or cord. A newer study by John McKently was presented to a rescue conference in 2014 and debunks some of the lore around it.
For example, loop knots (fig8, bowline) seem to have little difference in efficiency when loading the rope "end-to-end" outside the loop. The differences between nylon 6 and polyester were quite substantial and dwarfed any differences between the knots themselves.
Of course, it would be better to repeat these efficiency tests with a true UIAA 892 fall tower, but this would require hundreds of iterations and falls to approximate the breaking strength. For now, we'll need to be content with pull tests.
McKently essentially seconds the conclusion of Tom Moyer's paper at the same conference 14 years earlier, which doomed Black Diamond's new Gemini II cordage. Our vaunted figure 8 loop is only 42% efficient in certain cordage and certain fibers. Overall, knot efficiency is a property of temperature, humidity and particularly fiber. Efficiency is mostly unrelated to the knot design itself.
The message is clear, choose the best ropework for the task. If operating on (say) a 5:1 safety factor, any efficiency differences will be subsumed in the safety factor. Real focus should be on D:d ratio, edge management and especially fiber selection. If you need to choose between knots, opt for simple and clear, over any historic efficiency difference.
As a footnote, here's a comparison of loop knots when pulled loop-in-loop.
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