This is the second post in a mini series about shedding weight from your rigging. The goals are to minimize rope waste, reduce the number of pieces of gear carried, maximize efficiency and never sacrifice safety. Ropes from 8mm-11mm in diameter, and manufactured by Sterling Rope, Petzl and Highline Rope will be tested.
Sterling Rope generously donated 4 short lengths of static rope for some of these tests. They asked nothing in return, other than to send them the data.
Product links are at the end of the post. Please consider purchasing your rope and other caving gear from the CES, as all proceeds go to support caving expeditions and research.
In the book Alpine Caving Techniques it states that when one leg of a Y anchor is much shorter than the other, you can save rope if a figure 8 knot is used on the top anchor and a butterfly knot is used on the lower anchor, compared to a figure 8 with bunny ears. This post will look at a Y Belay or Y Hang with equal legs, tied with a double figure 8, double bowline on a bight, or a butterfly-figure 8 combo.
To start, let’s look at how much 10mm static rope is needed to tie each knot. All knots were dressed and pulled tight before measuring. The butterfly and figure 8 were tied tight around a 1 inch (2.54cm) diameter pipe to create a small, uniform bight. Each bight took on average 4.25 inches (10.80cm) of rope. Rope lengths are rounded to the nearest 0.25 inches (6mm). All referenced knot lengths are their respective averages.
All figures are in inches.
Now let’s define some variables before we write the equations.
L=Length of long loop/leg
S=Length of short loop
V=Double figure 8 a.k.a. figure 8 with ears
W=Double bowline on a bight
X=Figure 8 on a bight (including the length of the knot)
Y=Butterfly
Z=8 inch tail
Y BELAY (Y HANG) WITH EQUAL EARS (this is the easy one)
Butterfly and Figure 8 on a bight
Length of rope =(L+X)+(2S+Y)+Z —> (L+21.5)+(2S+17.75)+8 —> L+2S+47.25
Double figure 8
Length of rope =2L+2S+V+Z —> 2L+2S+(37.75+8) —> 2L+2S+45.75
Double bowline on a bight
Length of rope =2L+2S+W+Z —> 2L+2S+(25+8) —> 2L+2S+33
To figure out when Butterfly and Figure 8 on a bight is better than a Double bowline on a bight, let’s set the equations equal to each other, and solve. L+2S+47.25=2L+2S+33 Thankfully the S variables cancel out and we get L=14.25 This means that when the long loop/leg is less than14.25 inches, a double bowline on a bight uses less rope. For loops greater than 14.25 inches long, a Butterfly and Figure 8 on a bight uses less rope. Here’s the crossover chart showing how much rope is needed per knot.
Double figure 8 vs Butterfly and Figure 8, that’s easy. 2L+2S+45.75=L+2S+47.25 L=1.5 Tie a Butterfly and Figure 8 on a bight anytime the loops/leg are over 1.5 inches to minimize rope waste.
We also know that a double figure 8 uses, on average, an extra 12.75 inches of rope compared to a double bowline on a bight.
Up next is Y Belays with uneven loops.