BOWLINE IN HMPE: STRENGTH 60% retained — but the lowest measurement tells you more
A bowline costs this 5 mm HMPE rope an average of 40% of its breaking strength. The average, however, is not the figure to rely on.
A bowline in 5 mm HMPE blue/gold breaks at an average of 5.58 kN (569 kg) — 40% less than the unknotted breaking strength of 9.27 kN (945 kg), with spread ranging from 50% to 70% retention.
HMPE is known as the strongest rope material by weight. That holds for straight rope — but once a knot is tied, the picture changes substantially. The assumption that "a rope this strong can afford some loss" obscures how quickly and how unpredictably those losses can occur in practice. Five measurements with a bowline show that the average tells only part of the story.
The five measurements ranged from 4.66 kN to 6.53 kN — a spread of 1.87 kN. Expressed as retention: the lowest measurement retained only 50% of the base strength, the highest 70%. That is a range of 20 percentage points across five identical tests on the same rope.
In all five measurements the rope broke at the knot but did not separate completely. The residual strength after failure was measured once and was 10–20 kg — negligible for practical purposes. The reported values are the load at which the bowline failed, not the full tensile strength of the straight rope.
5 mm HMPE cord blue/gold with sheath — product characteristics
This 5 mm HMPE cord blue/gold has a braided construction with core and sheath. The core carries the majority of the load; the coloured sheath provides protection and identification. HMPE (high-modulus polyethylene) has extremely low elongation (<4% at break), does not absorb water, and retains its strength in wet conditions.
The downside of low elongation is low energy absorption — shock loads are barely dampened.
This rope is part of our HMPE rope collection.
How we test with a bowline
Breaking strength tests are carried out in-house by Otto Tromm on a universal testing machine with a capacity of 50 kN. Two sets of purpose-built rope clamps are available for this machine, selected based on the diameter and material of the rope. These clamps allow rope to be tested without first requiring an eye splice.
All tests are conducted in accordance with the ISO 2307:2019 guidelines. This includes testing 5 times and using a test speed of 20 mm per minute. Each rope is secured so that it sits taut. No pre-tension is applied, as elongation is not measured in these tests.
Two repeats of each test are filmed with a Sony camera on a tripod. The exact progression of the load curve is additionally recorded on video. This allows the stress profile and failure behaviour to be matched precisely to the graph from the breaking strength test machine. For each of the 5 tests, observations are also recorded in writing by the tester, capturing details such as the degree of elongation, the manner in which the rope breaks, and the intensity of the snap at the point of failure.
For this test the standard procedure was followed, with one deviation: a bowline was tied in the rope before it was placed in the clamps. The machine, clamps, test speed, and the number of 5 measurements are identical to a test without a knot, so that the difference in measured breaking strength can be attributed to the bowline. The reference value without a knot comes from an earlier test of the same rope (5 measurements, 9.27 kN).
The average is 60% — but the lowest measurement is at 50%. With a knot you do not get the average; you get what the knot decides to do that day.— from the test context
Test results: bowline in 5 mm HMPE blue/gold
Five measurements with a bowline on the same rope type give an average breaking strength of 5.58 kN (569 kg). The base breaking strength of this rope without a knot is 9.27 kN (945 kg) over five measurements (measured on 18 December 2025). Average retention is therefore 60% — a loss of 40%. The spread in retention runs from 50% to 70%.
The fact that failure occurred at the knot in every case, and that the rope did not separate completely, is relevant for interpretation: the reported values are the load at knot failure, not the breaking strength of the straight rope. The residual strength after failure, measured once (10–20 kg), confirms that the rope is functionally unusable after knot failure.
For safety-critical applications: calculate with the lowest measured retention of 50%, not the average of 60%. The lowest measurement was 4.66 kN (475 kg). Working load calculations based on the average overestimate the reliable lower bound by a margin that is practically significant.
Comparison: the same rope in three states
The comparison bar shows the same 5 mm HMPE blue/gold rope in three measured states: unknotted, previously tested with a knot (type not recorded, 18 December), and the current bowline test. These are explicitly the same product in different configurations — not different ropes.
Without knot: 9.27 kN (945 kg). Earlier knot test (knot type not recorded): 4.83 kN (492 kg), a retention of 52%. Bowline (current test): 5.58 kN (569 kg), retention 60%. The bowline performs marginally better on average than the previously tested knot, but the spread within the bowline test is large enough to warrant caution. The difference between the two knot tests (52% vs. 60%) also illustrates how sensitive HMPE is to exact knot geometry.
Conclusion: a bowline in HMPE costs an average of 40% — plan for 50%
The bowline in 5 mm HMPE blue/gold breaks at an average of 5.58 kN (569 kg), corresponding to a retention of 60% of the unknotted base breaking strength of 9.27 kN (945 kg). The spread, however, runs from 50% to 70% — and for safety applications the lower bound is the governing figure. Anyone using a bowline in this rope should work from 4.66 kN (475 kg) as the conservative minimum. According to the Handbook of Fibre Rope Technology (McKenna, Hearle & O'Hear), knots generally reduce breaking strength by 40–60%; the average retention of this rope falls within that range, but the lowest measurement touches its lower end. The bowline is suitable for temporary, non-critical connections where the safety factor applied to the lowest knot strength is sufficiently generous. For structural or safety-critical connections, an eye splice remains the appropriate choice.
Otto Tromm tied a bowline five times for this test, watched the rope fail at it five times, and measured the residual strength once — after which he concluded that 10 to 20 kg was not worth repeating. He records this as a scientific decision and not a time constraint.