Page 29 - Mark Gommers "Bowlines Analysis

A theoretical analysis of force acting on a Bowline structure

Let’s examine what happens as load is applied to a Bowline structure…

Force enters the core of the Bowline via the Standing part (‘SPart’). At the opposite end, each leg of the eye
sustains 50% (half) of the force. The nipping loop is loaded at both ends – 1) by the SPart and 2) the ongoing
eye-leg. During initial uptake of strain, the greatest degree of rope movement within the core occurs with the
SPart. Tension force causes the nipping loop to clamp and compress the bight. As load increases, compressive
forces acting on the bight also increase. The radius of the nipping loop also plays a role – it is theorised that a
smaller radius (sharp bends) induces higher stress and strain. Compression of the bight eventually reaches a
critical stage – when the force required to further compress the bight exceeds the breaking load of the rope. At
this point, rupture occurs. It is thought that rupture occurs at a region where the SPart exits from the nipping
loop.
Note: This paper has not pinpointed the precise location of the point of rupture – measurements are only indicative of the
approximate position (ie region). High speed camera equipment and carefully placed cotton thread ‘markers’ would be
required to pinpoint this position with greater accuracy.
T0 at the collar location
Standing Part
(‘SPart’) Note: Tension is initially 0 at the collar. As force
increases, some of that force propagates to the collar –
although the collar will never jam.
Capstan effect

Experiments are inconclusive. It is T1 In every pull test this author performed, 2 things were

possible that the SPart acts as a observed:

‘capstan’ at the position marked ‘C’ 1. The collar draws down and folds over the nipping
loop; and
during initial stages of loading. 2. The tail pivots and is displaced upward – caused by

However, as load increases, the the upward motion of the SPart leg of the nipping loop.

compressive force of the nipping loop

dominates and the capstan effect is no The nipping loop plays a key role in these observations.
longer observable or measurable. c Note that the nipping loop is not a perfect circle – it is a

closed helix (each leg of the nipping loop is offset by 1
rope diameter). The SPart leg of the nipping loop draws

The nipping loop grips and up while the ongoing eye-leg side draws down causing

compresses both legs of the ‘asymmetric’ compression of the bight (like a scissor
bight. Compression effect). This has the net effect of inducing a kink in the

bight structure.
continues until it reaches a

critical stage. The SPart also acts as a rigid post that the collar is
braced against. As the core structure collapses the collar
Compression is levered down the SPart. It should be pointed out that
zone even after extreme loading, a Bowline can easily be
untied by simply manipulating the collar.

Returning eye T0
leg Tail Ongoing eye
leg
T½ T½

Fixed anchor
This nipping loop is in fact a closed
helix structure
(image showing right hand chirality)

Page 28 of 59 Bowline Analysis Version 2.7a 16 July 2016 © Copyright Mark Gommers