Why knot strength percentages disagree, and why none is a working load

Published knot strength percentages disagree, and no single figure should go into a pitch plan. The numbers depend on the material, the construction, the diameter, whether the rope is wet, how the knot was dressed and set, how fast it was loaded and how the test was run. None of them is a working load. A manufacturer breaking strength is measured on new rope under specified test conditions, and a working load limit is a separate figure set by the manufacturer for a stated use.

Why a knot weakens a line

A knot bends the line sharply and shares the load unevenly along its length. Tight bends and uneven loading are the general reasons a knotted line is weaker than the same line kept straight. How much weaker depends on the knot, the material and how the knot was set, so no one number covers every case.

This applies to a stopper such as the figure-eight stopper as well as to a tie-off such as round turn and two half hitches. The reduction is not a fixed penalty you can subtract from a rope's rating. It is a property of the knot, the rope and how the two were used together.

Why published percentages disagree

A percentage describes one knot in one rope under one set of conditions. Change any of these and the figure changes:

• The material, such as nylon, polyester or UHMWPE.
• The construction, such as a single braid or a core and sheath.
• The diameter of the rope.
• Whether the rope is wet or dry at the time of the test.
• How the knot was dressed and set.
• The rate at which the load was applied, and the test method.
• Whether the rope was new or had already been used.

Before you quote a percentage, ask what it was measured on. A figure from one test on one rope does not transfer to a different rope, tied in a different way.

Breaking strength is not a working load

Breaking strength is the force at which a new rope failed under the test conditions a manufacturer specified. It describes a test, not a limit for use. Rope maker Samson notes that its published strengths are measured on new, unused rope with spliced ends, and that splicing keeps the specified strength in most cases while knots can significantly decrease it. A knotted line therefore starts below the published figure before any wear is counted. A working load limit is a different figure. It is set by the manufacturer for a stated use, and it is not something a knot percentage turns into a safe load for a tied line.

This site does not calculate a working load for any knot, and it does not prescribe a retention figure. The Knot bench multiplies the breaking strength you enter by the retention you assume, and shows that product as a hypothetical. The product is arithmetic, not a finding about your knot.

What the Knot bench retention figure does

The Knot bench asks for a manufacturer breaking strength in newtons and an assumed retained strength between 10% and 100%. It multiplies the two. You type the retained figure in. The tool does not measure it, and it does not look up a retention figure for the knot you select.

Because the retention field stops at 100%, the tool cannot show a result above the breaking strength you entered. That limit describes the arithmetic only. It says nothing about how much a real knot keeps.

hypothetical result = 1,000 N × 60% = 1,000 N × 0.60 = 600 N

The result is 600 N. It is what the arithmetic gives when the assumed 60% is typed in. It is not a rating for any knot, and it does not tell you what the line can carry in use.

Why the hypothetical result is not a rating

The 600 N figure looks precise, so it is worth listing what it leaves out:

• It is not measured for your rope, your knot or the way you tie it.
• It is not a safety factor, and it does not state what share of strength any knot keeps.
• It does not account for rope age, wear, wetting, sudden loads or the way the line is used.
• It is not a working load, so it is not a number to hang weight from.

If you want to compare two ropes, compare their stated figures on the same basis: the same kind of figure, the same test and the same knot. Set aside any figure that lacks those details, because a comparison built on mismatched figures says more about the sources than about the ropes.

Questions to ask about any published figure

Before you use a figure from a manufacturer or a test report, check what it describes. Four questions cover most of the differences set out above:

• Is the figure a breaking strength, a working load, or a percentage of a breaking strength?
• Was it measured on a straight line, a knotted line or both, and with which knot?
• Was the rope new, and was it wet or dry?
• How was the test run, and at what rate was the load applied?

If a figure does not answer those questions, it does not tell you enough to use it for a knot. A figure for a straight rope is a different measurement from a figure for a knotted rope, and the two should not be swapped.

What to do instead of quoting a percentage

Use the manufacturer's stated figures for your exact rope, and note what each figure was measured on. Read the material comparison as a set of trade-offs, not a strength ranking. The nylon and UHMWPE comparison sets out stretch, slipperiness and termination without a knot percentage. The guyline cord materials guide covers the cord itself.

Inspect the rope before each use. Look for cuts, abrasion and a sheath that has separated from the core. Practise the knot on spare cord, and ask an instructor to check your tying. For a tarp, the question is usually whether the guylines hold the pitch in wind, which is a different question from what share of strength a knot keeps. For how much line a pitch needs, see how much guyline a tarp needs.