Safety. What does 100% mean? Rope and Fishing Line obviously vary in strength from one sample to the next. And expectations vary too. If you were planning to trust your own weight to a rope you would certainly want a “Safety Margin” and a generous one as well. For you, and for other critical loads, a 12:1 safety margin is the standard. If you were standing right under a heavy load, you might need the same margin. But, as you moved further away, or the loads became less critical, the safety margin would be less and fall somewhere between 5:1 to 12:1. When in any doubt use stronger rope.
Average of Several Tests. As a practical guide, breakage at 100% means that the rope will break at the strength rated by the manufacturer. It is far better for the strength to be determined as the average of a series of tests performed right then, right there and under the test conditions.
This critical preliminary test for a specific knot or splice is all too frequently omitted. Fishing line manufacturers often underestimate the breaking strength; some fishing lines have actually broken at more than double their manufacturer-rated strength.
The rope shown in this picture has a breaking strain well in excess of 1000 tonnes. Nevertheless it still requires testing.
How is the strength of rope or fishing line measured? An ideal method might be to wind each end of the line on to a large diameter smooth drum. In actual practice a spliced loop often suffices secured around a thick pin , e.g., three times the rope diameter.
Method. Gradual stretching applies strain without initial friction, damage, or heating. The “Breaking Strain” for a piece of line is the average of several tests, often five. The risk of course is that, in real life, your piece may be damaged, have an imperfection, or be subject to sudden strain, heating, or abrasion – any of which may reduce the strength below the published 100%. Rope is often used with spliced ends – so that is the way it is commonly tested.
Fishing Line Variation. In addition, particularly for fishing line there may be a significant discrepancy between the Published and Actual Line Strength with actual strength often exceeding the label. The result is that lines are hard to compare and that people fish with a much stronger line than they need or may be suitable for a particular type of fishing or technique.
Knots weaken rope. They just do. It is sensible to assume that a knot weakens rope by 50% and this can be true! Dave Richards measured Knot Break Strength vs Rope Break Strength and found that weakening is more commonly in the range 60 to 70%. He also found some knots just simply slide undone at higher loads. This knowledge reinforces the search for the perfect knot. There are a few rope knots that may achieve 100% strength. e.g., the Tensionless Hitch. It consists of many wraps around a suitable bollard, post or tree. Before friction against a rough surface has a chance to chafe and wear the rope, 100% strength might be expected – at least for a while.
What makes a rope strong? Several factors underlie the final strength of a rope. Natural fibers were quite short and twisting them into tight bundles was required to achieve strength. Hemp retained more strength when the yarn twisted to the right. This resulted in a left hand strand, and a right hand rope. It wasn’t until modern times that microscopic examination of the plant illustrated that the structure of the plant has a natural twist that causes this effect.” However, a tightly twisted rope carries a penalty: some of the fibers are not in the line of strain – they don’t contribute to strength.
Extrusion. Modern fibers are extruded and spooled to any desired length; there is no need to twist them tightly together to achieve strength. Indeed, quite the reverse! Minimal fiber-angle makes each fiber take the strain at a better direction – nearly in line with the pull. The new long fiber length also allows rope manufacturers to use new rope designs. In a double-braided rope, the sheath protects the central fibers which may then be better aligned to the strain. And, loosely woven hollow braid ropes are wonderful for a different reason. Splicing merely requires feeding the end along through the rope’s hollow center; the end is then gripped as though by a Chinese finger grip. And, when the rope is High Modulus Polyethylene (HMPE) its strength and lightness allowed it to replace steel.
Fisherman’s Interest. The Line Strength is sometimes less than the weight of the fish; it may ultimately be the fish’s exhaustion that makes the final landing possible. Occasionally there will be strong fish that take with them some line as well as the Terminal Tackle. This partly explains the fisherman’s huge interest in completing the main line with a knot that has a very high rated strength like the Bimini Twist or the Australian Braid. Both of these knots take time to tie but their structure gradually transfers the load from the line to the terminal loop and both are claimed to retain close to 100% of the line strength.
Can It Be Done In Rope? Can knots be tied in Rope without weakening it? The quick answer has to be “Knots? No! Splices? Certainly!” Any knot in any rope creates increased load in some of the fibers and these fibers are prone to break first. Tests claiming strength greater than the manufacturer’s rating almost certainly lack a preliminary test under the applicable conditions. However, in high modulus loosely woven single braid ropes, a properly made Long Bury Splice does not weaken the rope. Even a Brummel Eye Splice in HMPE (Dyneema or Spectra) can achieve close to 100% of the rated breaking strain. The final strength of both the Long Bury and the Brummel depends on having a long enough buried end that has been appropriately tapered to minimize stress points. Even the more familiar Double Braid Class I Eye Splice should retain at least 90% of the rated breaking strain.