As working arborists, ropes are a vital part of our daily work. From climbing to rigging, rope is a ubiquitous component of our systems. As soon as you give a rope a job, it becomes a line, and a line requires a connection to something to do its job. While there may be many ways to affix a line in any given system, knots are often the best and simplest solution. However, like all components, the knots we use have some specific requirements we must address for effectiveness, safety and efficiency.
In this article, we will look at the three requirements that turn a tangle of bends, bights and loops into a useful knot. We will then address some good-sense guidelines for knot use and selection. Last, we will look at the overall effect of knots in our systems and how we can tie all the attributes together (see what I did there?!) and form a comprehensive knotting guideline.
What makes a knot a knot?
There are countless ways to cross cordage over other cordage and/or itself to make a rope connection. Not all are created equal! If you have ever had to cut a line because the knot you tied was too tight to undo, you know exactly what I am speaking of. There are more bad ways to knot rope than good ones. Clifford Ashley, in his tome Ashley’s Book of Knots, famously quips, “A knot is either exactly right or hopelessly wrong.”
There are three main characteristics that make a knot useful for arborists. First, it must hold under load. A knot that does not hold when tension is applied is not a knot, it is a parlor trick. We tie knots to connect and hold. The reliability of this holding is paramount. This is not to say that a knot or hitch won’t slide under load, as is the case with climbing hitches, but it should not become untied or unstable.
The second characteristic is that a knot must be reasonably easy to tie. I use the word “reasonably” in the legal or philosophical sense: suitable; just; proper; ordinary; fair; usual. The word “reasonable” is generic and relative. As such, it applies to what is appropriate for a particular situation. In the case of knots, “reasonable to tie” differs for each situation and application. It does not imply that all useful knots are easy to tie, as many require a good deal of practice to master.
However, as we use knots repeatedly for many tasks, they must be able, for the most part, to be assembled fairly quickly and with a minimum of fuss and no extra tools. We could probably macrame a block to a tree securely, but how long would that take?
The third requirement is that, after a heavy load, the knot must be able to be untied. Again, situations and applications differ, but if we tie it, we generally must be able to untie it after use in a reasonable (same definition as above) time frame. The strongest knot that cannot be undone connected to the piece you just successfully rigged to the ground will hamper efficiency.
Strength and weakness
Strength is often the only attribute considered. While important, it is just one aspect of knot selection. How strong are our knots? I think a more precise question is, how much does a knot weaken our system? Just as a chain is only as strong as its weakest link, so too is a rope under tension.
To form knots, we bend and twist lines. Once a rope is bent, a portion of the rope fibers are no longer able to bear the tension. In many cases, they may even be compressed. In the simplest terms, the tighter the bend, the more strength loss. While I admit this is a simplification, the principle is what is important. Knots weaken rope to varying degrees because they alter the way the rope fibers are loaded. A rope is strongest in a straight pull. Bend it, twist it or add compression, and uneven tension and strength is lost.
A comprehensive and accurate listing of how much reduction there is for each knot is a bit of a holy grail. While it would not be impossible to construct such a list, its ultimate value would be suspect. As arborists in the field know, there are many variables – rope age, wear, cycles of loading, heat, abrasion, use and bend radius, to name just a few – and all greatly affect the strength of the rope and, thus, that of the knot.
In my experience, a bowline reduces rope tensile strength by about 40%. The bowline meets the three requirements for a knot listed previously and is a mainstay in climbing and rigging. In contrast, the simple overhand knot can reduce rope strength by about 60%. It, however, is very hard if not impossible to untie after heavy loading.
These two anecdotal examples, derived from my own experience and some reliable but not truly scientific testing, give us a small glimpse into not strength, but estimation.
As a young arborist, the guideline I was given was to assume a knot will reduce rope strength by about 50%. This is more rule of thumb than science, but remember, as discussed previously, a comprehensive “knot-strength-loss chart” would only apply to brand-new rope in the proper configuration. This is rarely the case in the field.
Suffice it to say that the strength of a knot, or, conversely, how much it reduces rope strength, is an interesting conversation, but ultimately, as a concern in the field, one that can be addressed only by overestimation and generalization.
It is of note that when a rope is loaded to failure with a knot present, the rope almost always, in my experience, fails outside the knot. Also, in my experience, in-field failures are usually because the rope was damaged or extremely old. Knots are a part of these failures, but often they are not the “weakest link.”
Strength is just one concern when it comes to knotting matters. We must also consider stability. Stability, as we define it for this discussion, is the ability of a knot to remain dressed (all parts in proper alignment) and set (holding firmly until altered) as tension is applied and/or lessened. That is to say, a knot will not come loose on its own. But not all knots are stable once the tension is released.
A clove hitch, for example, can loosen after loading if tied on a large enough or slippery enough item. The movement of the load a clove hitch is tied around can cause it to loosen. Clove hitches also can roll or shift if either end is pulled opposite of the intended direction. Hence the reason a clove hitch is often combined with a half hitch or two on the working end.
This is not to imply the clove hitch does not have its uses. You must consider the loading action and direction of a knot and how stable it is when deciding on which knot to use. Even the venerable bowline can become loose with varying tension, hence the reason it makes a poor knot to use to construct a terminal loop on the end of a climbing line for life support.
The direction of loading is another consideration when selecting knots. Again, if we look at the bowline, properly tied, dressed and set, the load can be applied in any direction and the knot will hold and be easy to untie. Compare that to the timber hitch. Pound for pound, it most likely introduces less strength loss than the bowline. However, if the direction of load is not opposite or 90 degrees away from the initial bight that forms the knot, it too will lose holding power and begin to “roll.”
How a knot is configured can have much to do with ultimate strength and ease of use. Take the cow or Stilson hitch, essentially a girth hitch with a half hitch. The half hitch keeps the knot stable and allows for multidirectional loading. A closer inspection of the hitch will show a “basket” configuration. There are two legs of the sling around the object and also through the bight. Therefore, the cow hitch doubles the rope much like a basket configuration does on a round sling. This inherently adds strength.
The configuration also allows for the connection of hardware, such as blocks and friction devices, close to the tree, with a minimum of slack between the hitch and the hardware. Compare this to, say, a running bowline. This is a fine knot for attaching loads to a rigging system, but it lacks the “basket” or doubling configuration, and would not allow for easy placement of hardware close to the stem of the tree unless you further modify it.
Some knots also can have a “cinching” configuration, meaning that the loop or bight they form constricts under load. The running bowline is a fine example of this in rigging. In life-support applications, the grapevine or scaffold knot cinches tightly and stays tight around a connecting link. It does this so well that it can be difficult to untie. However, if the connecting link is a carabiner, then the carabiner can be removed from the knot and untying becomes much easier. If the connection is a captive eye, then removing it to untie cannot be done and, in this application, might not be as desirable.
As we have seen, there is much that goes into the selection of knots. First and foremost, our knot must hold reliably and be reasonably easy to form and then untie when the task is done. As knots bend rope and these bends disrupt and/or change the sharing of load on rope fibers, knots decrease rope strength. By how much per knot is mostly an academic discussion and would be hard to apply reliably in the field. Therefore, using an estimated and conservative strength reduction of 50% allows for error and makes for easier math!
Couple this guideline with a 10-to-1 ratio for working-load limits, and you have a solid foundation for rigging that, in effect, is a 22-to-1 ratio. While this may seem excessive, the consequences of rigging failure are often extreme! Modern materials and rope construction allow us still to be able to lower heavy loads safely, and a conservative working-load limit increases the number of times a rope can cycle (one load is a cycle) before retirement.
How the knot is configured plays a major part in its efficiency and usefulness. Knots to attach hardware, as opposed to the knots that attach the load, undergo different forces from different directions, therefore they have different requirements. A clove hitch may be fine mid line or in a series of knots, such as rigging several branches at once, but as the final knot on a rigging load line, it can roll and become unstable if not properly secured with another hitch.
Using another hitch to secure a primary knot can work, but you would be wise to use a knot that works in the application without additional fuss.
In the life of the production arborist, ropes and the knots we tie with them are necessary tools we use daily. Choose knots suited for the task, based not only on strength but also stability, loading direction and configuration. The ultimate strength is indeed a concern, so be generous in your working-load limits and potential loads. Develop a good-sense set of guidelines for choosing and using knots that work with the cycles-to-failure of the lines they are tied in and complement the established working-load limits. The knots we choose and use really do matter.
Anthony Tresselt, CTSP, is a consultant serving as director of safety and training for Arborist Enterprises, Inc., an accredited, 31-year TCIA member company based in Manheim, Pennsylvania. He is also a writer, philosopher, student of gravity and independent trainer based in Manheim. His writing and thoughts can be found on his blog, gravitationalanarchy.com. His books can be found on Amazon. He is a co-founder of The Arborist Boot Camp (thearboristbootcamp.com), a transformational training experience for new tree workers. He is also a co-founder of Leadership Performance Mastery, an online, self-paced, transformative leadership course for anyone looking to improve his or her leadership, regardless of whether they lead one or a thousand (valuebasedleadershipjourney.com).