Strength of Knots

[GordMay]

Knot Strength:

Every rope has a "breaking strength," which means that if we place a heavy enough strain on a rope then it will eventually break. The Safe Working Load (SWL) of a rope is generally considered to be one-fifth (20%) of the rope's breaking strength.

Tying a knot in rope always reduces it’s strength - sometimes by as much as fifty percent (50%). It’s difficult to find definitive and consistent data on the strengths of the various knots that mariners use, because there are a number of factors that affect knotted rope strength, including:
- type of rope used
- how old or how worn the ropes are
- the skill with which the knot is tied, set, & dressed
- the temperature and humidity in the air (& is the rope wet or dry)
- how many times each test was done in order to get a good sampling, etc
- more ...

It should be obvious that a single number for the strength rating of a knot (such as 70 %) cannot realistically tell us how our particular ropes will stand up under the particular weather conditions of the moment (hot and dry, rainy and wet, freezing cold), under the particular strain which we are placing on the ropes (static or shock), with the particular way that we have tied that particular knot (properly tied or improperly tied), and so on. Therefore, the prudent mariner will interpret the common strength ratings of knots as (relative) general guidelines only.

Never overestimate the practical strength of a rope nor knot !!!

Type of knot, bend or hitch - Percentage of retained strength:
I suggest you use the lower emboldened figure in strength calculations

Alpine Butterfly or Lineman's Loop: 65 -75%

Bowline: 65 - 75 %

Bowline on a Bight: 65 - 70 %

Anchor Bend
- Over 5/8" dia. ring: 55 - 65 %
- Over 4" dia. post: 75 - 85 %

Clove Hitch: 60 - 70%

Two half hitches
-Over 5/8" dia. ring: 60 - 70* %
- Over 4" dia. post: 65 - 75* %

Timber Hitch: 60 - 70 %

Square (Reef) Knot: 45 - 60* %

Sheet Bend: 45 - 60* %

Fisherman's Knot: 55 - 65 %

Carrick Bend: 55 - 60 %

*Smaller sizes (particularly nylon) are liable to slip without breaking.

Eye Splice (Not a knot): 80 - 95 %


Some On-Line References & Resources:

New England Ropes - Common Knots
http://www.neropes.com/splice/knots.htm

Knots on the Web - by Peter Stuber
http://www.earlham.edu/~peters/knotlink.htm
Including extensive links to:
Knot Tying: http://www.earlham.edu/~peters/knotl...m#knot%20tying
Knot Theory: http://www.earlham.edu/~peters/knotl...#knot%20theory
Knot Art: http://www.earlham.edu/~peters/knotlink.htm#knot%20art
Knot Forums: http://www.earlham.edu/~peters/knotlink.htm#knot%20art

Ropers Knots Page:
http://www.realknots.com/knots/index.htm

Industrial Rope Access Investigations ... (166 page PDF)
http://www.hse.gov.uk/research/crr_p...1/crr01364.pdf

Rescue System Mechanics, Interim Report (9 page DF)
http://www.amrg.org/Rescue_System_Mechanics.pdf

HTH,
Gord May

[Alan Wheeler]

Plus there are a few interesting figures in there. I wonder if they are testing breakage at the knot itself, or at any place the rope breaks, which could be the created loop around what the rope has been harnesd to. i.e. the anchor and clover hitches are different over a ring and over a 4" post. But I would have thought the knot isteslf would have failed at the same pull, no matter what it was around.
Same comment for eye splice. That figure is very low and I would have expected it to be close to full strength.

[GordMay]

Wheels raises some good points.

Most knots, used by mariners, fail due to rope breakage at the knot, whereas knots used by climbers and fishermen often fail by “inversion” (flip / capsize / roll) or “slippage” (spill), and/or “friction” heating due to slippage.

In a knotted rope, energy (which is normally spread aver the entire length of rope) directs itself to the knot, causing compressive & shear stress concentrations, which crush the rope’s core. These sharp loops (in knots) result in some fibers being loaded much more than others, and a stress concentration occurs at that point. The rope fibres reach their ultimate load capability at this stress concentration, and fail. The adjacent fibres become more highly loaded, and are subject to failure. Eventually, the rope breaks at this point (adjacent to the knot). In effect, the knot, when tensioned, cuts the line.

See the failed bowline - page 7 - at: http://oberon.ses.nsw.gov.au/resourc...Wellington.pdf

The sharper the loop or bend (relative to the rope diameter) the greater these compressive forces. Hence, a hitch around a 4" post will be stronger than a hitch around a 5/8" ring.

Rope Strength Specifications:
Because the wide range of rope use, rope condition, exposure to the several factors affecting rope behavior, and the degree of risk and life and property involved, it is impossible to make blanket recommendations as to working loads.

The Minimum Tensile Strength (MTS) is based on test data from a significant number of break tests conducted on new and unused rope, and is a value two standard deviations below the mean. The rated strength is based on the rope having no knots in it, and the MTS is assumed to decrease once the rope is put into use.

Maximum Working Loads (MWL) [formerly Safe Working Load - SWL] are normally determined by dividing the MTS by the safety factor. The safety factor is a function of the physical properties of the rope, the age and history of the rope, the type of service in which it is to be used, and the risks involved if failure occurs. The safety factor varies from a minimum of a 3:1 ratio, where new rope being used in a static environment and failure would cause little or no risk to equipment or personnel, to as high as a 20:1 ratio, where severe conditions exists or where failure of the rope could cause severe risk to equipment and personnel.

A point to remember is that a rope may be severely overloaded, or shock loaded, without breaking. However, damage and strength loss (fatigue) may have occurred, without any visible indication. The next time the rope is used under normal working loads the acquired weakness causes it to break.

For an excellent discussion on Breaking Strengths (MTS) & Safe Working Loads (SWL) goto:
http://www.boatsafe.com/marlinespike/breaking.htm
and
http://www.boatsafe.com/marlinespike/safeload.htm

PS1: About Fabric Softeners & Rope:
Tests conducted in a study by Smith (1988) indicate that treatment with concentrated fabric softener reduced the strength of a new rope.
Frank (1989) showed that certain ropes treated with dilute softener (per manufacturer’s recommendations) were stronger than the same rope without softening, after aging and washing.
Frank reported that the likely mechanism at work explaining these results, is that the fiber lubricants contained in new rope are lost with age, allowing the fibers to cut one another. Fabric softener replaces some of the lubricants. Excess softening leaves the rope effectively wet, with the corresponding loss in strength.

PS2: About Splices:
A splice is stronger than a knot. The strongest way to join two ropes, or to make a sling or endless rope, is to splice it correctly. Even the most efficient knots will reduce rope strength to as much as 50%, while a carefully made splice may have up to 95% of the strength of the rope being spliced. A short splice gives the strongest coupling, however, it doubles the rope size, and is not suitable where rope must run through pulleys or sheaves on a block. For such purposes, a long splice, with up to 90% strength efficiency, is normally used.


Terminology:
Here is a brief explanation of a few knot terms:

bend - a knot used for joining two ends together

bight - a semi-circle of rope where the rope does not cross itself; also the part of the rope between the standing part and the end that can be used in tying the knot

dress - to remove slack in the knot by drawing up the knot neatly; to make sure the knot is tied correctly, that all parts are where they should be

end - the end of the rope

friction hitch - a knot tied directly to the standing part, another rope, or a cylindrical object that is adjustable (can be slid) when the knot itself is grabbed and moved, but otherwise stays put (from friction) when the load is on the standing part

hitch - a knot that attaches a rope directly to an object

loop - a circle of rope in which the rope crosses itself

set - to fully tighten a knot by pulling on all parts

slip - to use a bight of rope instead of the end when finishing tying a knot; used to make untying a knot easier

standing part - the part of the rope not used in the knot itself

stopper knot - a bulky knot that is used to prevent the rope from pulling through a hole or pulley

Aditional On-Line References:

Grog's Animated Knots Index: http://www.grogono.com/knot/

Cordage Institute - CI 2001 - “Fiber Rope Inspection and Retirement Criteria”
http://www.ropecord.com/cordage/publ.../CI%202001.pdf

HTH,
Gord May

[John Richard]

I recently found Gord's knot post on the Hunter Owners site. I immediately taught myself the Butterfly and then looked further into his links. I found one that showed how to construct a Monkey Fist.

I went down to my boat and took the four bitter ends of the lines that control my adjustable genoa car system and put in the Monkey Fists. I must say, they came out really nice. It took me a while to get the hang of the thing, but once done, I was able to move alone with good speed. They added a nice touch to the boat.





http://http://www.jacksplace-hunter410.com

[GordMay]

John:
The traditional use of a Monkey's Fist is as a weight for a heaving line. It's often classified as a covering knot, since the knot usually covers a weight. It’s also used as fancy knot for key-rings, necklaces and (thanks to John) as decorative stoppers on genoa track lines.
Anyway, they're fine to play with ...