Baseline testing for lightweight rope rescue

Baseline testing for lightweight rope rescue

Making sure we have sufficient safety factor for 10mm rope and 7mm cord

This article is about getting some hard data to back up a change in practice – using lightweight rope rescue systems. That is going from 11mm to 10mm rope for backcountry rescue and the associated change in Prusik cord size from 8mm to 7mm. Who would want to carry more gear that’s heavier when lightweight may well do the job. Check out what we found out with a bonus download and testing video at the end.

Materials used

Rope

• Kordas Iris 10mm 26kN semi-static (Type A EN1891)
• PMI Classic 10mm 26.5kN static (CI1801).

Cord

• Edelrid Powerloc 7mm 14kN accessory cord (EN564).

Knots and bends tested

Knots tested with 10mm rope

• Figure-8 on the bight
• Bowline
• Alpine butterfly

Alpine butterfly

Bends tested with 7mm cord

• Double fisherman’s
• Overhand rethread

Testing procedure

A repeatable test procedure was set up.

• A 20t testbed was used at the Chainman Ltd, 1 Cass Street, Sydenham, Christchurch.
• New rope and cord were used for testing.
• All 10mm rope tests used a figure-8 knot on the bight at one end.
• The 7mm cord was tied as a loop – 14kN x2 = 28kN.
• 10cm tails were used as standard on the knots and 4cm on the bends.
• All knots and bends had hand tension.

Slow pull testing results

Fig-8 to Fig-8 knots – Kordas Iris 10mm (26kN)

• Test 1 17.79 (68%)
• Test 2 17.93 (69%)
• Average 17.86 (69%)
• Broke at the fig-8 knot

Fig-8 to Fig-8 knots – PMI Classic 10mm (26.5kN)

• Test 3 19.26kN (73%)
• Test 4 20.94 (79%)
• Average 20.10 (76%)
• Broke at the fig-8 knot

Fig-8 to Bowline knots – Kordas Iris 10mm (26kN)

• Test 5 15.57kN (60%)
• Test 6 18.22kN (70%)
• Average 16.9 (65%)
• Broke at bowline knot

Fig-8 to Alpine Butterfly knots – Kordas Iris 10mm (26kN)

• Test 7 17.41kN (67%)
• Test 8 17.52 kN (67%)
• Average 17.47 (67%)
• Broke at Alpine Butterfly knot

Loop tied with double fisherman’s bend – Edelrid Powerloc 7mm (14kN x 2 = 28kN)

• Test 9 20.96kN (75%)
• Test 10 22.62 (81%)
• Average 21.79 (78%)
• Both bends broke at double fisherman’s bend

Test 9: Double fisherman’s bend – Edelrid

Test 10: Double fisherman’s bend – Edelrid

Loop tied with overhand rethread bend – Edelrid Powerloc 7mm (14kN x 2 = 28kN)

• Test 11 17.87kN (64%)
• Test 12 19.91 (71%)
• Average 18.89 (67%)
• Both bends broke at overhand rethread bend

Static safety factors

A static safety factor is a load divided into the breaking strength derived from testing using slow pull. The lowest breaking strength has used and rounded to the nearest 1kN.

Fig-8 to Fig-8 knots – Kordas Iris 10mm (26kN)

• Lowest value 17.79 (68%)
• Safety factor 1kN load 18:1
• Safety factor 2kN load 9:1

Fig-8 to Fig-8 knots – PMI Classic 10mm (26.5kN)

• Lowest value 19.26kN (73%)
• Safety factor 1kN load 19:1
• Safety factor 2kN load 9.5:1

Fig-8 to Bowline knots – Kordas Iris 10mm (26kN)

• Lowest value 15.57kN (60%)
• Safety factor 1kN load 16:1
• Safety factor 2kN load 8:1

Fig-8 to Alpine Butterfly knots – Kordas Iris 10mm (26kN)

• Lowest value 17.41kN (67%)
• Safety factor 1kN load 17:1
• Safety factor 2kN load 8.5:1

Loop tied with double fisherman’s bend – Edelrid Powerloc 7mm (14kN x2 = 28kN)

• Lowest value 20.96kN (75%)
• Safety factor 1kN load 21:1
• Safety factor 2kN load 10.5:1

Loop tied with overhand rethread bend – Edelrid Powerloc 7mm (14kN x2 = 28kN)

• Lowest value 17.87kN (64%)
• Safety factor 1kN load 18:1
• Safety factor 2kN load 9:1

Final thoughts

This testing was undertaken to assist us in making informed decisions about using lightweight rope rescue. Could we keep safety margins acceptable for the core rope rescue activities in the backcountry?

• For a 1kN load, the safety factors are well beyond 10:1 for a single 10mm rope.
• For a 2kN load, the safety factors are under 10:1 for a single rope 10mm rope and range from 8:1 to 9.5:1.
• The safety factors for the 7mm loops meet or exceed the single 10mm rope and range from 9:1 to 10.5:1.
• For a two-tensioned rope rescue, the safety factors from a system point of view would exceed 10:1 if 1kN was shared evenly on each of the two ropes.
• We have only done a couple of tests in each type of knot and bend so this a small sample size. As always more testing will provide more reliable data.

Check out the video of the testing here:

Check out the other articles in this series:

• Double fisherman’s vs. overhand rethread
• Let’s lighten the load. Testing and analysis of a lighter weight rope rescue system.

Grant Prattley at Over The Edge Rescue