BREAKING STRENGTH 8 MM SHEET LINE 7.42 kN measured
Polypropylene, sheath-core construction. Five measurements, remarkably consistent results. Here are the figures.
The 8 mm sheet line in polypropylene broke at an average of 7.42 kN — equivalent to 757 kg — across five individual measurements.
Polypropylene is widely recognised in technical literature as the weakest common synthetic rope material — lower tensile strength than polyester, lower than nylon, and rated lower for safety-critical applications. That assessment is accurate. However, anyone relying on an 8 mm sheet line for operating sheets, trim lines, or blocks on board has a legitimate question: is that 7-point-something kN sufficient in practice — and how reliable is that figure?
Five measurements produced a spread of just 0.10 kN: lowest measurement 7.36 kN, highest 7.46 kN. Three of the five measurements came in at exactly 7.46 kN.
For a polypropylene rope in this diameter class, that is a notably tight spread.
A characteristic feature of the tests was also a perceptible elongation before the rope failed, a faint snapping sound just before the break, and a clean, single-event break — despite the sheath-core construction.
Material and construction
This rope has a braided core enclosed by a braided sheath — a construction that provides better core protection than a simple braided rope and typically achieves a higher efficiency factor than a twisted rope.
In theory, the sheath in a sheath-core construction can absorb 30–50% of the load; however, this did not result in stepped failure behaviour here: the rope broke in a single movement every time.
Polypropylene is the lightest common synthetic rope fibre and floats on water. UV resistance is the weakest property of this material — under prolonged outdoor exposure, degradation occurs faster than with polyester.
As a sheet line that is not permanently rigged outdoors but is regularly used and stowed, that risk is manageable.
This rope is part of our Boat Rope collection.
The method: How we test
The breaking strength tests are carried out in-house by Otto Tromm on a universal testing machine with a capacity of 50 kN.
For this machine there are 2 sets of specially developed rope clamps, selected based on the diameter and material of the rope.
These clamps allow rope to be tested without first requiring a splice.
All tests are carried out in accordance with the guidelines of ISO 2307:2019.
This includes testing 5 times and using a test speed of 20 mm per minute.
Each rope is secured in such a way that it is taut. No pre-tension is applied, as elongation is not measured in these tests.
Each test is filmed from 2 angles with a Sony camera on a tripod. The exact progression of the graph is also recorded separately on video.
This allows the load progression and break behaviour to be matched precisely with the graph from the breaking strength test machine.
For each of the 5 tests, observations are recorded in writing by the tester, capturing details such as the degree of elongation, the manner in which the rope breaks, and the loudness of the snap when the rope fails.
Just before the break, always that faint snapping sound — and then failure in one movement. No stepped failure, despite the sheath-core construction.— from the test notes
The figures: breaking strength 8 mm sheet line
Five measurements at a test speed of 20 mm/s produced the following results: average breaking strength 7.42 kN (757 kg), with a maximum spread of 0.10 kN between the lowest (7.36 kN) and highest measurement (7.46 kN).
Three of the five measurements fell exactly on 7.46 kN — a level of uniformity that is not a given for a braided PP rope in this segment.
Prior to failure, the rope consistently showed a clear elongation, followed by a faint snapping sound. The failure itself was always abrupt and complete in a single movement.
There was no stepped failure in which the core failed first and the sheath subsequently — or vice versa. This indicates a construction in which the core and sheath carry the load simultaneously and also fail simultaneously.
Tests were carried out dry, without pre-tension, in accordance with the methodology applied by Prorope for all breaking strength tests.
In perspective: Comparison with other 8 mm ropes
The 8 mm sheet line measures lower than the nylon and standard polypropylene variants tested previously.
The values below are measured breaking strengths, not stated MBL values.
The nylon braided with core reached 12.57 kN — 69% stronger than this sheet line. The nylon braided without core measured 12.07 kN.
Standard polypropylene 8 mm measured 10.57 kN, still 42% higher than the sheet line.
In practice: when is this sheet line suitable — and when is it not?
An average breaking strength of 7.42 kN (757 kg) is sufficient for dynamic, lightly loaded tasks on board small to medium-sized boats.
Allow for a standard safety factor of 5:1 for general use, which gives a working load of approximately 1.48 kN (150 kg).
✓ Suitable for
- Sheet lines and trim lines — the primary application; low to moderate load, dynamic use, regularly operated.
- Control lines and block halyards on small boats — light rigging where high peak loads do not occur.
- Auxiliary and spare lines — as an emergency solution or for temporary use on board.
- Internal guide lines — not permanently outdoors, which means the limited UV resistance of PP is less likely to become an issue.
✕ Not suitable for
- Anchor line or mooring line — for prolonged static loading, polyester or nylon (higher MBL, better UV resistance) is the appropriate choice.
- Permanent outdoor rigging — prolonged UV exposure degrades polypropylene faster than polyester; inspection intervals must be shortened accordingly.
- Safety-critical lifting applications — PP is not recommended for applications involving the lifting or supporting of persons.
- Heavy rigging on larger yachts — at higher peak loads, the measured 7.42 kN provides insufficient margin with an adequate safety factor.
Alternatives with higher breaking strength
Conclusion: 8 mm polypropylene sheet line
The 8 mm polypropylene sheet line achieves a measured average of 7.42 kN (757 kg) with a notably consistent spread of just 0.10 kN.
For the intended use — sheet lines, trim lines, and light control lines on small to medium-sized boats — that is a workable value, provided the application does not require high static or peak loads.
Where higher breaking strength is needed for the same diameter, or where the application involves prolonged outdoor exposure, the polyester braided 8 mm is the appropriate alternative.
8mm sheet line
View this rope on prorope.eu →Otto Tromm noted that three of the five measurements came in at exactly the same figure, checked the machine twice for a calibration issue, and concluded that the rope is simply consistent. He has kept the remaining length as evidence.