Knots for connecting ropes when rappelling - a comparison

Which knots can be used to connect ropes and what are their advantages and disadvantages in different situations?

Knots for joining ropes when rappelling. There are several scenarios in which ropes need to be joined when rappelling: Firstly, to use the full length of double ropes. Secondly, to make a rope longer during rescue maneuvers. Or thirdly, when climbing using a single rope then rappelling by combining this with a thin take line. The discussion as to which knots are best for connecting ropes is long standing and the answers differ depending on the country, club/association or mountain sport discipline, as well as in the field of professional work at height. 
The article below looks at the strengths, advantages and disadvantages of the most commonly used knots for connecting ropes as well as what happens when ropes of different diameters are connected.

Loads when rappelling

To assess the suitability of a knot for connecting ropes, the first question is which forces arise while rappelling. The exact force depends on the weight of the person rappelling, the type of rope, and the behavior while rappelling.
Tests were performed to measure the forces exerted on the directional anchor when a person weighing 86 kg is rappelling. Figure 1 shows an example of a force-time curve. Brief force peaks occur when someone intentionally makes the descender abruptly arrest them or severely jerks (suddenly drops) while rappelling. These force peaks reach a maximum of 2.6 kN, approximately three times the body weight of the person rappelling. Higher forces are almost only possible if a descender were to fully arrest an actual fall. The tests were repeated with various belay devices and ropes, and the forces were not even exceeded when using a rope with low elongation pursuant to EN 1892. With smooth rappelling, the forces remain similar to the body weight at 0.9 kN. This differentiation makes sense as some knots that connect ropes have a tendency to roll when subjected to a constant force but, conversely, to tighten without rolling in the event of brief force peaks. Calculated for a person weighing 120 kg, a maximum force of 3.6 kN should be exerted on the directional anchor when rappelling freely and a maximum continuous load of 1.2 kN should apply. When using a single strand, the entire load is also exerted on the knot connecting the ropes. When double-strand rappelling, only half the load is exerted on the knot connecting the ropes.

Fig. 1: Load on the anchor point when a person weighing 86 kg rappels using a MEGA JUL set up as a double strand with a rope with an 8.9 mm diameter certified to EN 892  50% of the force is exerted on each rope strand

Practical requirements

As a basic prerequisite, the knot connecting the ropes must hold. In practice, it is also important for the knot to snag as little as possible on the directional anchor and when retrieving the rope. asymmetrical knots, such as an flat overhand, figure-of-eight, or double overhand bend, can roll on the side with fewer structures and are therefore less prone to snagging on rock edges. The smaller the knot, the narrower the cracks it can slip through.

Fig. 2: Flat overhand bend (left) versus double fisherman’s bend (right)—the flat overhand bend rolls over rock edges more easily. Asymmetrical knots are less prone to snagging on rock edges

Fig. 3: The smaller the knot, the more easily it can slip through narrow cracks.

Flat overhand bend (left) versus figure-of-eight bend (right)—the smaller flat overhand bend slips through narrow cracks more easily
 

Strengths of knots for connecting ropes

The following knots were tested when used to connect ropes: flat overhand bend, figure-of-eight bend, double fisherman’s bend, two consecutive flat overhand bends, and double overhand bend (Fig. 4).

Fig. 4: The knots tested.

Method

The knots were cleanly created (without crossing over) with tails of at least 30 cm and pulled tight on all four ends individually. The knots were tensioned between two sheaves with a diameter of 180 mm and a speed of 1,000 mm/min. The force that caused the knot to roll for the first time was determined (‘1st roll’). If the knot rolled off the ends, the highest force that occurred while it was rolling was determined (‘rolling’). If the rope snapped, the ultimate tensile strength prior to the rupture was specified (‘UTS’). The values indicated are the mean values calculated on the basis of three tests.

Results for knots for connecting ropes with the SAME DIAMETERS

SWIFT 48 PRO DRY 8.9 mm diameter; single, half, and twin rope certified to EN 892 with an initial strength (unknotted) of 18.1 kN::

Fig. 5: Knot strengths with a SWIFT 48 PRO DRY 8.9 mm diameter; single, half, and twin rope certified to EN 892 with an initial strength (unknotted) of 18.1 kN

CANARY PRO DRY 8.6 mm diameter; single, half, and twin rope certified to EN 892 with an initial strength (unknotted) of 16.8 kN:

Used CANARY PRO DRY 8.6 mm diameter; single, half, and twin rope certified to EN 892 with an initial strength (unknotted) of 17.6 kN:

APUS PRO DRY 7.9 mm diameter certified to EN 892; half and twin rope with an initial strength (unknotted) of 15.1 kN:

Used APUS PRO DRY 7.9 mm diameter certified to EN 892; half and twin rope with an initial strength (unknotted) of 14.7 kN:

SKIMMER PRO DRY 7.1 mm diameter; half and twin rope certified to EN 892 with an initial strength (unknotted) of 13.3 kN:

RAPLINE PROTECT PRO DRY 6.0 mm diameter certified to EN 564 with an initial strength (unknotted) of 11.0 kN:

PROSTATIC SYNC TEC 10.5 mm diameter certified to EN 1891 Type A with an initial strength (unknotted) of 34.5 kN:

Results for knots for connecting ropes with DIFFERENT DIAMETERS

Discussion

With regard to strength, the double fisherman’s bend achieves the highest values, still reaching 6.9 kN even when two RAPLINES are connected. The figure-of-eight bend starts to roll under the lowest forces, which is not ideal. The flat overhand bend rolls off the end of most materials, but is the smallest knot and has an asymmetrical form, thereby offering the lowest risk of snagging. The double overhand bend rolls little and its strength lies between that of the figure-of-eight bend and the fishermen’s bend, but it is relatively large. 
Impact of usage: The used ropes tested had been used with average frequency to climb on rock for about five years. The usage made the rope sheath coarser than that of the new ropes as the impregnation has worn away and the sheath is slightly fuzzy. Knots tend to roll less. This effect is regarded as normal for ropes that are being used. The strength of the knots was not noticeably reduced with these ropes. However, the reduction in strength depends on the type of use and the precise condition of a rope, so the results do not allow any universally valid conclusions.
To provide an assessment, the question arises as to how much a knot that connects ropes has to withstand. The following requirements can be derived from the results of the rappelling tests: When rappelling/lowering a person weighing 120 kg on a single strand, knots for connecting ropes that do not yet start to roll at 2 kN and do not yet break at 5 kN still offer a worst-case-scenario safety buffer with a factor of approx. 1.5. In double-strand usage, the load on the knot is halved and knots for connecting ropes that do not yet start to roll at 1 kN and do not yet break at 2.5 kN also offer a worst-case-scenario safety buffer with a factor of approx. 1.5.
In the tests on single ropes, all knots for connecting ropes complied with the requirements for a person weighing 120 kg on a single strand. When connecting a single rope and a RAPLINE PROTECT, the requirements are only just met. In the case of half ropes, when using an flat overhand bend, the requirements for rappelling on a double strand are met but too little buffer is offered for working on a single strand.

Conclusion

The double fisherman’s bend is the safest option in terms of strength alone. However, when looking at the rappelling process as a whole, this bend poses major risks and disadvantages with regard to the rope snagging. As a result, the flat overhand bend can be justifiably used to connect ropes. It offers enough strength when rappelling/lowering on a double strand and is less prone to snagging. When rappelling/lowering on a single strand with thin, smooth half ropes, the flat overhand bend should be secured against rolling off the end of the rope with a second such bend.
With all knots for connecting ropes, it is extremely important to leave sufficiently long tails (approx. 30 cm) and to properly tighten the knot by pulling all four strands individually.