A simple shape 8 overhand knot (a double collar structure), interweaved within two nipping loops (a double nipping structure), generate the double bowline knot shown in the attached pictures.
What I have found interesting - because I think that it may be beneficial to the overall security of this knot - is the "complementary" character of the two structures, in the following sense : When we pull only the two ends of the nipping structure, the knot, as a whole, tends to "fold" in one direction, while, when we pull only the two ends of the collar structure, the knot tends to "fold" in the opposite direction. So the collar structure, as a whole, prevents the nipping structure from being deformed in a way that would enable the two nipping loops to "open up", and release their grip on the segments of the two collars they encircle. I would like to watch how this knot would behave under extreme loading - if the nipping loops would be able to "open up", overcoming the resistance of the interweaved collars, before the final rupture of the rope... Where are you, J.P ?
P.S. I would like to mention that if we use, as a double collar structure, a fig.8 knot ( instead of a shape 8 overhand knot that we have used here ) the result is not so satisfactory. Why ? Because a fig.8 knot collar structure is perfectly symmetric, and the segment of the rope between the two nipping loops, having to pass from the one or the other side of this structure, destroys this symmetry. In the case of the shape 8 overhand knot, which is not so symmetric, the two interweaved structures complement each other in a more "consonant" way.
