Does Your Knot Reduce the Strength of Your Rope?
Throughout our day as an arborist, depending on the exact work order, we could be tying a knot hundreds of times. When it comes to a large removal or even speed lining the limbs of a large conifer, the amount of knots tied throughout the day can be daunting! Obviously every knot that is tied should be TDS, Tied, Dressed, the Set. This is one of the most basic rules when tying knots used for life support or rigging! We want to make sure that the knot is first tied correctly, next we want to make sure each strand of rope is laying correctly next to the other strands and that we don't have any unnecessary twists in the strands, and lastly we want to set the knot by loading it in our hands or within our climbing system to make sure that it is being tight and compact as a knot should be. With all of these little things covered, we know that our knot should be performing for us at the top of it's game, right?
Well, some knots are created differently than others and can be stronger or weaker than their counterparts. Essentially every knot that you tie in a rope will make the rope weaker than it is by itself. This is because knots cause tight bends in the rope whose outer circumference is greater than the inner part. This causes the rope to load unevenly, technically loading the rope across it's width when under tension, instead of it's length which completely undermines it's strength. This uneven loading is what makes all the knots that we use in tree care operations decrease the overall strength of the rope at the knot.
Let's take a look at some common knots and how their strength is affected by tying and then loading them.
The King of Knots! The bowline is use to attach the rope to the tree for base anchors in SRS climbing systems or for attaching the rigging line to limbs or logs being lowered from the tree. It's overall statistics are:
Percentage of retained strength of the rope with Bowline: 70-75%
Percentage of strength loss of rope with Bowline: 25-30%
Also known as the clove hitch, this is a great hitch for attaching the rigging line to a limb or log being removed from the canopy or for quickly attaching a rope to a redirect while climbing SRS. The Clove Hitch is also used in closed climbing systems to attach the rope to the climber's harness before tying their friction hitch. Another great use for the clove hitch is for sending water bottles up into the tree for the climber!
Percentage of retained strength of the rope with Clove Hitch: 60-65%
Percentage of strength loss of rope with Clove Hitch: 35-40%
The Double Fisherman is a great cinching knot for attaching the working end of your climbing line to a carabiner in place of a splice. This knot is relatively compact and is easily identifiable. It does cinch down quite hard after use and is hard to undo.
Percentage of retained strength of the rope with Double Fisherman: 65-70%
Percentage of strength loss of rope with Double Fisherman: 30-35%
The Anchor Bend is another great way to attach a carabiner to the working end of a climbing line. The Anchor Bend is more bulky than the Double Fisherman but is still very identifiable and easy to tie. The Anchor Bend does not cinch down as hard as the Double Fisherman so it is easier to untie after use.
Percentage of retained strength of the rope with Anchor Bend: 80-85%
Percentage of strength loss of rope with Anchor Bend:15-20%
This blog is meant to be a primer for what we need to be looking at when building rigging systems and the forces as well as the restrictions to those systems. We can go further in depth in a later blog post about Safe Working Load and the likes. All knots and hitches pictured were done so to illustrate the said hitch or knot, not to illustrate their exact uses. Some hitches like the Clove Hitch should have a backup tied with the tail around the standing part of rope. Any Bowline used for life support should be backed up with a Yosemite Tie Off or a stopper knot. Stay safe!