This information was excerpted from the newly revised Best Practices for Rigging in Arboriculture manual, which will be available this November. There are many good knot books available. Knots at Work: A Field Guide for the Modern Arborist by Jeff Jepson, is a detailed and affordable resource.
The most important criteria of any rigging knot is to make sure it is tied, dressed and set before use. This means tying the knot in the right way, then aligning all the parts to function properly. Many types of knots can work themselves loose under load if not properly tied. To better understand the knot in use, anticipate the load forces and rope angles the knot is expected to sustain.
Knots and strength loss
All knots reduce the strength of rope to some degree. Strength loss in rigging knots is a significant concern. When you tie a knot or hitch, the rope fibers are bent. When bent, the fibers cannot share the load equally. Some of the fibers must hold a greater percentage of the load than the others, creating an imbalance in strength. The fibers doing most of the work – those holding the load – are more prone to getting damaged. Knots with tight bends, such as bowlines, cause greater strength loss (up to 60%) than hitches and knots with more open bends, such as the clove hitch. To account for the variation in strength loss from different types of knots, allow for 50% strength loss due to knots and hitches when rigging.
Tie, dress and set
• Every knot that you tie should be properly tied, dressed and set.
• Tying the knot requires the appropriate combination of loops, wraps and bights.
• Dressing the knot is aligning the parts.
• Setting the knot is tightening it to make it ready for use.
Each time you untie or tie a knot, do a quick inspection on that section of rope.