The Bowline Knot

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The Bowline Knot

Submitted by An Outdoor Idiots Team Member.

Page 3 of 4 Previous Next [Index]

Failures of the Bowline Knot

So what exactly is all this about slippage, inversion, three-way loading and the need to tie a "stopper knot" in order to make the Bowline safe?

Well, firstly, it is probably a good idea to forget some of the phrases that are commonly used, particularly phrases like "3-way loading" (that could mean anything) and the idea that failure is only likely with heavy loads (untrue). Phrases and ideas like these seem to appear so commonly, with no further substance, that they are perhaps cut and pasted from one source of information to another, with little understanding about what they actually mean.

Let's start afresh, and start by breaking down the Bowline knot into understandable components. The Bowline knot can be considered to consist of a loop and a u-bend (these terms will be used for the remainder of this article). I've tied a Bowline with two different coloured ropes, to make this clear. In Fig. 1, the red rope demonstrates the u-bend, and the white rope demonstrates the loop:

NB: Please don't think of the following diagrams as showing the sheet bend knot, or be put off by the fact that one rope happens to be thinner than the other. The best way to follow this page is to have your own Bowline knot tied, in the normal manner, and manipulate it while following these diagrams and the text. These diagrams show two different coloured ropes purely to make it easy to visualise the ways in which the knot can be distorted.

Fig. 1: The Bowline consists of a loop and a u-bend
With that in mind, it becomes easier to describe the ways in which the Bowline knot can fail. First, though, I'll be clear on what I mean by "fail": I am specifically referring to the idea of pulling certain parts of the rope in certain directions, in such a way that the knot becomes useless. That being the focus, there are two ways in which the Bowline can quickly fail.

Failure 1: Inversion

The first is the phenomenon that is usually called "inversion". To achieve this, simply grab the two parts of the rope that emerge from the u-bend, and pull in opposite directions, as in Fig. 2:

Bowline Failure Modes (2)

Fig. 2: Inverting...
With very little effort, the knot will start to look as in Fig. 3:

Bowline Failure Modes (3)

Fig. 3: Inverting more...
You can see the problem already. The part of the rope that was forming the u-bend is now simply passing straight through a mess of rope that was forming the loop. All knotting properties have been lost. To push the point a little further, I've gone on to pull the red rope away from the white rope a bit, for Fig. 4:

Bowline Failure Modes (4)

Fig. 4: Really inverted.
Note that the above photograph shows an "inverted" and loose Bowline knot. This is nothing more than a rope passing through a slipknot.

Failure 2: Slippage

Now for the second way in which the Bowline can easily fail. I suspect this type of failure is the phenomenon that has lead to phrases like "can fail if subjected to a three-way load," "not suitable for heavy loads," "can slip" and various others. So what do you need to do in order to demonstrate this failure? Tie a Bowline between a tree and a Land Rover and apply phenomenal force in peculiar directions? Get a climber to have a nasty fall at a jaunty angle? Well, those would probably work. I seem to remember the Land Rover idea has been used somewhere to test this.

For simplicity, though (Fig. 5), you could simply hold the parts of the rope that emerge from the loop together (1), then pull gently on either part of the rope that emerges from the u-bend (2):

Bowline Failure Modes (5)

Fig. 5: Slippage.
The phenomenon shown in the above photograph can, to put it mildly, be called "slippage." It seems this type of failure is much less publicised than it should be. The lack of clear descriptions of this type of failure is perhaps what has lead to the propagation of ambiguous phrases and the false idea that the knot needs to be loaded heavily in order for it to slip. It occurs easily when the parts of the rope that emerge from the loop are held parallel to each other, as shown. The less parallel they are held, the harder it is for the rope that forms the u-bend to slip. I suspect that terms like "three-way loading" come from an arrangement where they are not held completely parallel, and so more force is required to make the u-bend slip - but it still slips.

In any case, if the Bowline is used to attach a harness to a rope for example, then it doesn't take much for the rope to become arranged as in the above photograph. The climber simply turning 90 degrees will do it. And if, for example, the climber is squeezing through a crack or chimney, it becomes a matter of luck as to whether they will still be attached to a Bowline when they emerge from it - either of the two types of failure might have occurred.

The Stopper Knot

The "stopper knot" is sometimes thought of as the cure-all which makes the Bowline safe. The "stopper knot" can be a simple overhand knot, tied in the loose end that emerges from the u-bend, and around the other part of the rope that emerges from the u-bend, as shown in Fig. 6:

Bowline Failure Modes (6)

Fig. 6: A stopper knot.
You can see that both of the failures of the Bowline knot are countered very well by the stopper knot above. Note that the arrangement should be tighter than shown in the photograph - it is shown loose in order to make it clear how it is tied. The Bowline should be tighter, and the stopper knot should be right up against the Bowline. The stopper knot should prevent both inversion and slippage.

It is not quite a cure-all, however. To simply say that a Bowline, with a stopper knot, is safe, is to miss one further complication. There are many ways to tie a Bowline, and it is quite possible for the knot to come out as in Fig. 7:

Bowline Failure Modes (7)

Fig. 7: A dangerous Bowline configuration
The above arrangement differs to the "normal" arrangement in one important way: The loose end of the rope no longer emerges from the u-bend, as it has in all the other photographs. Rather, it emerges from the loop. It is still possible to tie a sort of stopper knot with the loose end, as in Fig. 8:

Bowline Failure Modes (8)

Fig. 8: A dangerous Bowline configuration with a sort-of stopper knot
However, now that the two primary failings of the Bowline knot have been covered and explained in terms of the u-bend and the loop, it should be possible to look at the above photograph and come to two frightening conclusions: First, there is nothing to prevent inversion - the two parts of the rope that emerge from the u-bend can be pulled in opposite directions (Fig. 9). Second, the arrangement is absolutely perfect for encouraging slippage - the two parts of the rope that emerge from the loop are being held parallel to each other by the stopper knot.

Bowline Failure Modes (9)

Fig. 9: Oops.

A Safely Tied Bowline

So to be complete, if you are to regard the Bowline as a knot that even approaches something to be trusting your life to, you should check that the following two conditions have been met after you tie it:

The free end of the rope emerges from the u-bend.
You have tied a stopper knot in the free end, around the other part of the rope that emerges from the u-bend.

Fig. 10 is a repeat of Fig. 6, showing just about the only configuration of the Bowline that can be regarded as safe. It should all be tighter than shown in the photo:

Complete Bowline with Stopper Knot

Fig. 10: An acceptable arrangement, complete with stopper knot.

Page 3 of 4 Previous Next [Top of Page]

Page 1: Introduction

Page 2: Tying the Bowline Knot

Page 3: Failures of the Bowline Knot

Page 4: Appendix: Animal Testing

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