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    David Drohan Preferred knots for use in canyons

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  • Автор: SuzanneDunn

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Bushwalkers Wilderness Rescue Squad

David Drohan
On behalf of the Bushwalker’s Wilderness Rescue Squad (BWRS) Rock Squad, the author is
conducting a series of tests in a voluntary capacity to determine the preferred knots that could be
used in recreational canyoning. This paper will be of interest to all abseilers who have to
retrieve their ropes. The project is planned for three stages. Stage one is now complete and this
paper focuses on the tensile strength and slippage of various knots. Cyclic loading and rope pull
down issues have also been investigated. 139 hours of actual testing has been conducted to date.
This time does not include the considerable time to plan, analyse and write up the report.
This paper was presented at the Outdoor Recreation Industry Council NSW conference in Sept
For further information regarding BWRS visit the web site at http://www.bwrs.org.au

Recreational canyoning groups are questioning the traditional knots to join tape or rope. It has
been argued that the traditional Double Fisherman’s Knot (Figure 1-a) to join ropes is very tight
to undo after use and often catches on obstacles during rope pull down. The Tape Knot (Figure
1-b) can be difficult to adjust and now some groups have started using unconventional knots
such as the Overhand Knot for joining rope or tape (Figure 1-c & d).
There is also evidence that some groups have been using smaller than usual size tape or cord for
anchors, in order to reduce cost. This is done on the pretext that their group will only use the
anchor sling once and they believe it is strong enough. It is commonly regarded that 50mm flat
tape or 25mm tube tape is acceptable for use in anchor slings. The project will explore if smaller
size slings could be used, such as flat 25mm or maybe even 19mm polyester tape.
The proposal for this project was outlined in March 2000. This paper discusses tests conducted
to determine various knot strengths, slippage and ease of rope pull down for various rope/tape

Literature Review
Existing Information on Ropes, Aging and Knot Strength/Slippage
The author has researched published information on the strength and slippage of knots in rope
and webbing. There is significant data on the rated strength of new tape and rope as indicated in

Bushwalkers Wilderness Rescue Squad

the sample of catalogues and specifications (see references). However only a couple of
references have been sighted that provide data on strength of knots tied in Kernmantle rope.
Warild (1990 p33&34) provides information on recommended and non-recommended knots
showing the static strength and falls taken for each knot. He states “the performance of most
knots is variable and depends on many factors, rope diameter, wet or dry, knot packing and to a
lesser extent temperature” (p33). He suggests the bulky knots appear to have a clear advantage
especially in 8 or 9 mm rope and the Overhand Loop (knot) performs inconsistently. Whilst
Warilds’ tables are very helpful, the current author has not been able to access the test data on
which they were based, and there are no test results for knot slippage. Luebben (1996, p7) also
provides some data on the strength of knots, but no supporting references are provided. Long
(1993, p54), mentions that the Water (Tape) knot should be checked frequently, as it has the
tendency to come untied.
Much testing has been conducted by the international organisation of alpine clubs, Union
Internationale des Association d’Alpinisme, (UIAA) on falling climbers and the equipment that
breaks their fall. It is important to understand what causes the severity of the fall and so
understand what is called a fall factor. Benk & Bram (Edelrid) describe the fall factor (FF) as
the proportion of fall and the length of rope run out. The fall factor describes the severity of the
fall and determines the load on the entire system. The most serious fall a climber could take in
normal circumstances is FF2, that is the length of free fall (say 20m) divided by the length of
rope paid out being (say 10 m) therefore 20/10 = 2. This means 10m of rope must absorb the fall
energy of a 20 m free fall. Abseilers do not take such extreme shock loads on their equipment.
Warild (1990 p 15) argues that the worst fall factor a caver (abseiler) could take if one of the two
anchor bolts snapped would only be FF0.6. The probability of a FF0.6 fall is extremely low and
the most abseilers should ever expect for a well rigged rope is FF0.3. Warild states the most
convincing evidence that caving (static) ropes are strong enough is the complete lack of
accidents due to ropes failing under shock loads from caving (or abseiling in canyons).
Warild (1990, p 17) mentions ropes could be damaged by mechanical deterioration by 10 FF0.1
minor shock loads, caused by prusiking or rough abseiling and suggests this is an avenue for
investigation. One of the future tests in Stage 2 will explore this issue, as old ropes (that are too
stiff to abseil on) are often used as back up anchor ropes.
Polyamide static ropes used for canyoning can be old. There is no “use by date” based only on
age. The Blue Water Technical Manual (2001) provides information on when to retire your
static rope, such as damage from sharp edges, glazing from fast abseils or soft hollow or lumpy
sections in the rope. Replacement is based on wear and tear. Age is not mentioned as a limiting
criteria. What has been observed for ropes over 10 years old is that they often become too stiff
to handle and abseil on. This is due to the mantle shrinking and so becomes less pliable. Blue
Water recommends the shelf life for one of their unused dynamic ropes as five years. Blue
Water admits there is no conclusive evidence from nylon manufacturers regarding aging of
unused ropes. Warild argues on p 17 that age does affect used ropes and gives data for a 4.5 year
old 9mm BWII rope that indicates it can only withstand four FF1 falls, where a new rope can
take 41 such shocks. AS4142.3 (1993) requires new 11mm static ropes used for rescue to
withstand two FF2 falls. Provided the abseiler does not intentionally shock load the rope the
way a climber could, age is not an issue, as polyamide static ropes can withstand some shock
loading. Even with the worst normal abseiling shock load of FF0.6 the old rope should survive
one such shock load, as Warilds tests indicated the 4.5 year old rope could withstand four of the
higher FF1 falls. It appears aging affects a ropes ability to survive shock. The five-year rule
only apples to dynamic ropes as they are designed for high shock loads. Static ropes are not
required to be condemned when they reach five years old as they are not intended to take high
shock loads.

Bushwalkers Wilderness Rescue Squad

Many rope manufacturers treat their rope with a “dry treatment”. This involves coating the
nylon fibres of the rope with either silicon or teflon. Details on the process are difficult to obtain
from manufacturers. This process is done so the rope will absorb less water when wet and
therefore maintain its strength. Warild (1990, p16) discusses that water absorption in nylon rope
makes it less abrasion resistant and reducing its static and shock strength by up to 30%. Some of
the Edelrid ropes tested in this study were “dry” treated.
For natural laid fibre rope Marks Engineering Handbook (1987, p88) states that the shorter the
bend in standing rope, the weaker the knot (based on Millers experiments 1900). However it
appears to be a different story for nylon ropes. A report (the author wishes to remain
anonymous), discovered that knots in Kernmantle (polyamide) rope failed at the point of
maximum compression due to the knot compressing one strand of the rope sufficiently that the
heat generated by friction caused the strand to become plastic and then fail.
Petzl (2000) (an outdoor gear manufacturer) provides some information on alternative knots on
their web page technical manual, such as recommending the Abnormal Figure 8 Knot to join two
abseil ropes together. There is no supporting data for this recommendation. There is also useful
data at this web site for UIAA limits and design criteria, such as that harnesses should not be
loaded to more than 15kN.
Delaney (2000) from the Australian School of Mountaineering states he is not aware of any
testing of aged tape and rope as typically found in canyons and suggests there is only limited
information on knot slippage, but he could not provide any supporting test data.
Manufacturer’s Rated Strength
The rated strength given by the product manufacturer is the minimum strength that the material
will fail at, normally given in kilonewtons (kN). Most of the older equipment was rated in
kilograms force (kg).
To convert to kg, multiply by 1000 (newtons) then divide by 10 (gravity rounded up).
For example; a karabiner is marked as 22kN.
22kN x 1000 = 22000(N) /10 = 2200 kg.
A quick simple rule is just multiply kN by 100 to get kg.
Adequate testing of the product has been conducted by the manufacturer to determine the mean
breaking strength. Therefore the minimum or “rated” breaking strength can be determined. US
and European companies that have extensive production undertake comprehensive testing of
large samples from many batches. Therefore an accurate statistical figure can be determined.
Some products like karabiners have a “Sigma 3” rating which is three standard deviations back
from the mean. However many overseas companies only use two standard deviations back from
the mean for rope and tape products.
To explain standard deviations (std dev), Freund (1988 p76) defines for the results that create a
normal (bell shaped) distribution as follows:
About 68% of the values will lie within one standard deviation of the mean, hence about
16% are outside the 1 std dev on the low side.
About 95% of the values will lie within two standard deviation of the mean, hence only
about 2.5% are outside the 2 std devs on the low side.
99.7% values will lie within three standard deviation of the mean, hence only about
0.15% are outside the 3 std devs on the low side.
Some Aust/NZ rope/tape manufacturers do not use statistical calculations to determine their
rated strength for the materials used in this study. Toomer (2000) has clarified that the rated

Bushwalkers Wilderness Rescue Squad

strength used by Australian rope manufactures is based on a number of batch tests, using the
lowest breaking specimen. Usually the rated strength is rounded down to the nearest 100 kg.
Small (2000) from Donaghys Industries (a New Zealand webbing manufacturer) stated the
webbing (WPM25-OPG63 &WPM50-OA900) tested in this project is uncertified. The only
testing conducted by the company on these products were by random sample and the “rated”
load stated in the specification sheet is based on the minimum strength from the random testing.
The random sample is based on three specimens from the end of a batch run. Batches were only
tested when the company had altered a part of the manufacturing process or for some other
reason. No statistical methods are employed to determine the minimum break load for random
sample tests.
Working Load Limit
The working load limit (WLL), sometimes referred to as the safe working load, is the maximum
static load that should be applied to a rated piece of equipment. Dividing the rated strength by
the Safety Factor (SF) will give the WLL for that piece of equipment. Our example of a
karabiner rated at 2200kg divided by SF5 for hardware will give a 440kg WLL.
Understanding Safety Factors
Most rope and hardware manufacturers give their product a rated strength. To determine the
WLL a Safety Factor (SF) is used. Jensen (1974) describes safety factor as the ratio of ultimate
stress (rated strength) to allowable stress (WLL). The safety factor is based on a number of
considerations including risk to human life, wear and tear of the product, aging and the type of
loading that may be encountered. To understand how SFs are determined, he gives examples
ranging from 2 to 10 depending on the machine and application. Engineers have agreed that 5 is
acceptable for lifting loads involving humans. SF5 has been adopted as the SF used for abseiling
equipment hardware. Bateman & Toomer’s (1990) Australian Lightweight Vertical Rescue
Instructors (ALVRI) verbal advice during the course as recorded by the author, discuss that SF5
is acceptable for hardware, however rope and tape must include a factor to account for loss of
strength due to the knot.
ALVRI use a strength loss of one-third (33%) for any knot used in rescue. That is 0.67 strength
remaining in the rope. The original rope strength with no knot is divided by the strength
remaining due to knot of 0.67. This gives a ratio of 1.49. Multiplying the SF5 by this ratio of
1.43 will give a figure of 7.46. This figure has been rounded up to give SF8.
Based on this rationale, AS 4142.3 (1993), notes the SF as not less than 8 is considered
appropriate. It is noted that the American Blue Water (2000) catalogue use the US Fire
department’s SF15.
An appropriate SF is important. An excessive SF may add a significant weight or volume
penalty to the equipment you have to carry if you wish to maintain the existing WLLs. A SF that
is too low may lead to equipment failure with possible loss of life.
What else needs to be done?
From the literature review it is clear there is still a lot to learn about knot strength and slippage in
rope and tape. The tests conducted in this study provide further data on these issues however are
not exhaustive. Such a study would require access to research databases covering strength of
rope materials to determine the extent of research already conducted on this subject and how best
to build on this knowledge.

Project Design
This project aimed to determine the best possible knot for joining ropes and slings together in a
canyon. A useful side benefit was to identify any hazards evident in alternative knots. A process

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of elimination determined the preferred knots. The knots being considered were eliminated in a
step by step process based on the results from tests of A to E (listed below). The preferred knot
(for each application) is the one that has the best results and has not been disregarded due to a
safety issue.
The first stage of the process examined the static forces involved. The definition of “static” in
this case, is load not subjected to dynamic forces.
Objectives: Stage One
To determine the tensile strength and slippage of standard knots in slings.
To determine the tensile strength and slippage of alternative knots that could be used to
join tape and rope.
To determine the tensile strength of certain single strand ropes without knots.
To determine if knots used to join slings or ropes slip under normal (cyclic) loading.
To determine the ease of double rope pull down using various rope joining knots.
Section A is used as a baseline for the strength of standard knots. Section B compares the
strength of the alternative knots to Section A. Section C attempted to confirm the strength of the
rope/tape used in Sections A and B, without the influence of the knot on rope strength. Any
knots that were found unsafe after completion of Sections A to C were deleted from the
remainder of the tests. Sections D and E are the final set of tests to examine the preferred knots
for canyon use.
A further two stages of the project are planned to consider the shock forces and aging process.
More information on these topics is provided under “Further Research” latter in this paper.

Testing Requirements
In order to maintain repeatability, reliability and validity of the tests the author has referred to
appropriate Australian standards. No standard could be found that gave a procedure for tensile
testing of endless loops, therefore the author has adopted the philosophy of AS 4143.1(1993) for
endless loop tests and has described the procedures followed in a later section of this paper.
For single strand rope testing AS 4143.1 (1993) is directly applicable and requires a gauge length
of one metre (the distance between bollards) at the required pre-tensioned load. This indicates a
test bed with a stroke of two to three metres would be required. Toomer (2000) from Spelean
(an Australian rope manufacturer) indicated that the one metre length is important in order to
have enough material between the gauge lengths when compared with the material wrapped
around the bollards. The machines that the author had access to only had a maximum stroke of
one metre. In an attempt to solve this problem the author noted that in AS 2001.2.3 (1988) which
is one of the standards for testing seat belts, the procedure only required a gauge length of
200mm. For this reason single strand tests using a gauge length of only 200mm were attempted.
There are no standards for conducting the cyclic and rope pull down tests in this project. Again
the author has described the procedures followed in a later section of this paper.
Clem (2000) (former chairman of the Life Safety Section of the Cordage Institute, USA)
provided useful advice on the testing requirements for tensile and dynamic testing of knots. His
findings indicate apart from obvious criteria such as rope material and diameter, that more
subjective issues can come into play. These include which side the tail comes out of the knot. ie
if the knot is tied right or left hand. The author has noted these concerns and attempts have been

Bushwalkers Wilderness Rescue Squad

made to record any unusual events during the tests by video recording a number of the tests and
taking individual notes on test records.
Clem advises the absolute minimum size of the sample would be six specimens of the same
material. It is acknowledged that the sample should be larger, however due to resource
constraints the author has chosen six specimens per sample based on Clem’s advice. Although a
sample of six is less than ideal, AS 4143.1 (1993) only requires a report based on two successful
test specimens. Therefore the authors sample is three times greater than the relevant standard
Units used.