assessment of the strength of rope splices and knots …
TRANSCRIPT
ASSESSMENT OF THE STRENGTH
OF ROPE SPLICES AND KNOTS IN A
SAILING ENVIRONMENT
BY MARK MCKAY
200233547
Department of Mechanical Engineering
MEng Mechanical Engineering
Supervisor: Dr A.J. McLaren
2
Abstract
Limited research has taken place in the behavioural aspects of using eye splices as
rope terminations in a sailing environment. Splicing performance ropes has a distinct
lack of study carried out and with these performance ropes being used more and more
frequently, as sailing technologies improve, the need for study in this area becomes
more and more relevant.
As the feeding splice method (see section 2.2) is the only known way of splicing
performance ropes, due to their complex fibre structure, this method will become the
basis for the experimental side of this study. Over 50 break tests were carried out on
the inner core of these performance ropes all with eye splice terminations
incorporated into their body. Various different conditions were applied to these
splices including different whipping numbers and positioning (see appendix 1), varied
splice feed lengths and the use of larks foot knots to attach the splice to the testing
shackle. Strength factors of the varied sample conditions were investigated using a
statistical approach.
Conclusions were able to be drawn from the results including increased splice
strength varying directly with increased whipping numbers, the failure mode change
over point and larks foot influences increasing the maximum load produced by the
splice.
3
Nomenclature
CN – Condition Number (see section 2.1)
SN – Sample Number
X – Mean maximum load (N)
σ - Standard deviation in maximum load results
4
1. Introduction
Within a sailing environment, eye splices are very important as they are made use of
as rope terminations that are attached to various parts of the sail rigging. Therefore
eye splices in practice must be able to handle large loads in such a way that the rope
itself would fail before the eye splice. The importance of this is related to the splice
being the termination of the rope end, and if it were to fail before the rope, it would
render the rope useless, as its end would not be fixed to anything. Sailing nowadays
makes use of high performance rope such as Dyneema and Kevlar. These
performance ropes are very costly per unit length and can cost as much as £2 per
meter of rope. This cost is a major factor to consider, as large amounts of rope are
required within the rigging.
Taking these important factors into account, it was realised that the aim of this study
is to determine the most efficient way of manufacturing these eye splice terminations
for high performance ropes. However at the same time, investigation into failure of
these splices must take place in-order to prevent this occurring in a sailing
environment.
For this study, the high performance rope that was used throughout was Dyneema.
There is only one method of splicing these performance ropes, which is the feeding
method. As these ropes consist of an inner hollow core and an outer covering, this
method involves feeding the core into itself and making use of frictional effects to
hold the eye splice in position.
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2. Sample Preparation
2.1 Experimental Sample Conditions
Throughout every experiment in this study, there were seven conditions for the
Dyneema rope core samples that were made use of. These are described below and
should be read in conjunction with sections 2.2 and appendix 7.
Condition one – Involves no eye splice termination just a simple Larks Foot knot
attached to the shackle.
Condition two – Makes use of an eye splice termination with no whipping and no
Larks Foot knot. The eye of the splice is placed directly on to the shackle.
Condition three – Makes use of an eye splice termination with whipping at the crotch
of the eye but no Larks Foot knot. The eye of the splice is placed directly on to the
shackle.
Condition four – Has an eye splice termination with whipping at the crotch of the eye
and at the end of the feed length. This condition has no Larks Foot knot. The eye of
the splice is placed directly on to the shackle.
Condition five – Makes use of a sample created using condition two but uses a Larks
Foot knot to attach the eye splice to the shackle.
Condition six – Makes use of a sample created using condition three but uses a Larks
Foot knot to attach the eye splice to the shackle.
Condition seven - Makes use of a sample created using condition four but uses a
Larks Foot knot to attach the eye splice to the shackle.
6
2.2 Splicing Procedure
1. Cut a length of rope 2 meters long.
2. Remove outer covering of the rope to reveal the core.
3. Choose one end of the rope core to be spliced.
4. From this end measure down the rope the amount of rope core that you wish to be
fed into its hollow centre, i.e. your feed length and put a mark. In this case the
feed length will be 10 cm. This mark will be known as position one (see appendix
2). A rough feed length can be determined from the equation shown below:
Diameter of rope in mm = Feed length in inches
5. From position one you then measure down the rope the eye length required to
achieve the desired eye diameter. This point should be marked as well and will be
known as position two (see appendix 3). In this case the eye length is 20 cm.
6. Slip the splicing end of the rope core into the feed needle (see appendix 4). The
feed needle contains a hook that grips the core as it is entered into the needle. This
allows the needle and rope core to be fed through the cores hollow centre as one.
7. Insert the needle into the core at position two and feed it through the core until
position one is entered into the core (see appendix 5).
8. Once position one has entered inside the core, push the needle back out the core
walls and unhook the rope end from the feed needle hook. 9. The aim now is to pull on the feed side of the eye splice so that position one meets
position two and there is no excess rope core emerging from the core walls.
7
3. Experiment procedures
All the experimental samples were tested using the Tinius Olsen machine. The free
end of the rope was rapped around the machines drum (see appendix 8) four times and
then held in a fixed position by a clamp and knot arrangement. The spliced end was
attached to the shackle arrangement either directly or with the use of a larks foot knot
(see appendix 7). The machine was then initiated and kept running until the sample
failed, at which point the maximum load reading was recorded. All samples tested
were manufactured using the procedure in section 2.2 and were applied under
conditions one to seven from section 2.1. Each experiment had varied numbers of
samples under specific conditions that allow the investigation specific areas of study,
which are as follows:
3.1 Experiment 1
Aims: Determine the effects of whipping, friction produced from larks foot knot and
getting to grips with splicing procedure (see section 2.2).
Samples: Eight - Two of conditions one, five, six and seven.
3.2 Experiment 2.1
Aims: Determine the effects of different whipping conditions, with and without larks
foot knot and how the splices behave with the use of a small diameter shackle.
Samples: Six – Under conditions two to seven.
8
3.3 Experiment 2.2
Aims: Determine the effects of different whipping conditions, with and without larks
foot knot and how the splices behave with the use of a larger diameter shackle.
Samples: Six – Under conditions two to seven.
3.4 Experiment 2.3 and 2.4
Aims: Further investigate the effects of larks foot knot usage and validation of the
results produced from experiments 2.1 and 2.2.
Samples: Eight – Four of conditions four and seven.
3.5 Experiment 3.1
Aims: Investigate the effects of varied splice feed length.
Samples: Six – All under condition two.
3.6 Experiment 3.2
Aims: Further investigate the effects of splice feed length, determine feed length at
which a failure mode change occurs and validate the results from experiment 3.1.
Samples: Ten – All under condition two.
9
3.7 Experiment 4
Aims: Investigate the effects of splice feed length under realistic sailing conditions
and determine the failure mode change over point for these realistic conditions.
Samples: Twelve – All under condition seven
10
4. Results
4.1 Experiment 1
The following results shown in figures 1 and 2 are in relation with sections 3.1
(experiment 1 procedure) and section 2.1 (sample conditions):
SN CN Maximum Load (N)
Splice Efficiency
X σ
1 1 2668.9 0.33 2 1 2446.5 0.31
2557.7 157.3
3 5 8006.8 1.00 4 5 8006.8 1.00
8006.8 0
5 6 7562 0.94 6 6 8006.8 1.00
7784.4 314.5
7 7 6894.7 0.86 8 7 7784.4 0.97
7339.6 629.1
Figure 1 – Table of Results from Experiment 1
Maximum Load Versus Sample Condition
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
9000.0
0 1 2 3 4 5
Condition Number
Max
imum
Loa
d (N
)
Condition 1
Condition 5
Condition 6
Condition 7
Figure 2 – Graph of Results from Experiment 1
5 1 6 7
11
4.2 Experiment 2
The following results shown in figures 3, 4, 5 and 6 include experiments 2.1, 2.2, 2.3
and 2.4, which are in relation to procedural sections 3.2, 3.3 and 3.4 respectively and
section 2.1 (sample conditions):
6mm Shackle Diameter:
SN CN Maximum Load (N)
Splice Efficiency
X σ
9 2 373.7 0.05 10 3 4337 0.54 11 4 5827.2 0.73 12 4 4328.1 0.54 13 4 3442.9 0.43
4532.7
1205.3
14 5 3184.9 0.40 15 6 6160.8 0.77 16 7 6814.7 0.85 17 7 6556.7 0.82 18 7 7037.1 0.88
6802.8
240.4
Figure 3 - Table of Results from Experiment 2
10mm Shackle Diameter:
SN CN Maximum Load (N)
Splice Efficiency
X σ
19 2 2491 0.31 20 3 3576.4 0.45 21 4 4804.1 0.60 22 4 4070.1 0.51 23 4 4417.1 0.55
4430.4
264.6
24 5 2513.2 0.31 25 6 7063.8 0.88 26 7 6739.1 0.84 27 7 6939.2 0.87 28 7 7117.2 0.89
6931.8
189.2
Figure 4 - Table of Results from Experiment 2
12
6mm Shackle Diameter:
Larks Foot Knot Versus Maximum Load
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
0.5 1 1.5 2 2.5
Larks Foot Knot
Max
imum
Loa
d (N
)
No WhippingOne WhippingTwo Whippings
Figure 5 - Graph of Results from Experiment 2
10mm Shackle Diameter:
Larks Foot Knot Versus Maximum Load
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
0.5 1 1.5 2 2.5
Larks Foot Knot
Max
imum
Loa
d (N
)
No WhippingOne WhippingTwo Whippings
Figure 6 - Graph of Results from Experiment 2
No Yes
No Yes
13
4.3 Experiment 3.1
The following results shown in figures 7 and 8 should be compared with procedural
section 3.5 and the condition numbers should be compared with section 2.1:
SN CN Shackle Diameter (mm)
Feed Length (cm)
Maximum Load (N)
Splice Efficiency
29 2 6 10 364.8 0.05 30 2 6 12 444.8 0.06 31 2 6 14 3914.4 0.49 32 2 6 16 7215 0.90 33 2 10 18 7895.6 0.99 34 2 10 20 7495.3 0.94
Figure 7 - Table of Results from Experiment 3.1
Figure 8 - Graph of Results from Experiment 3.1
Maximum Load Versus Feed Length
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
9000.0
8 10 12 14 16 18 20 22
Feed Length (cm)
Max
imum
Loa
d (N
)
6mm Shackle10mm Shackle
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4.4 Experiment 3.2
The results from experiment 3.2 shown in figures 9 and 10 should be compared with
procedural section 3.6 and the condition numbers should be compared with section
2.1:
SN CN Feed Length (cm)
Maximum Load (N)
Splice Efficiency
X σ
35 2 14 3247.2 0.41 36 2 14 4279.2 0.53 37 2 14 3807.7 0.48
3778
516.6
38 2 16 7192.8 0.90 39 2 16 4875.3 0.61 40 2 16 7108.3 0.89
6392.1
1314.3
41 2 18 7797.7 0.97 42 2 18 7464.1 0.93 33 2 18 7895.6 0.99
7719.1
226.2
43 2 20 6254.2 0.78 44 2 20 6672.3 0.83 34 2 20 7495.3 0.94
6807.3
631.5
Figure 9 - Table of Results from Experiment 3.2
Maximum Load Versus Feed Length
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
9000.0
12 14 16 18 20 22
Feed Length (cm)
Max
imum
Loa
d (N
)
Figure 10 - Graph of Results from Experiment 3.2
15
4.5 Experiment 4
The results from experiment 4 shown in figures 11 and 12 should be compared with
procedural section 3.7 and the condition numbers should be compared with section
2.1:
SN CN Feed Length (cm)
Maximum Load (N)
Splice Efficiency
X σ
45 7 6 5560.3 0.69 46 7 6 5782.7 0.72 47 7 6 6361 0.79
5901.3
413.3
48 7 8 6939.2 0.87 49 7 8 7473 0.93 50 7 8 6672.3 0.83
7028.2
407.7
51 7 10 7642 0.95 52 7 10 7117.2 0.89 53 7 10 7130.5 0.89
7296.6
229.2
54 7 14 7624.3 0.95 55 7 14 7557.5 0.94 56 7 14 7784.4 0.97
7655.4
116.6
Figure 11 - Table of Results from Experiment 4
Feed Length Versus Maximum Load
5000.0
5500.0
6000.0
6500.0
7000.0
7500.0
8000.0
4 6 8 10 12 14 16
Feed Length (cm)
Max
imum
Loa
d (N
)
Figure 12 - Graph of Results from Experiment 4
16
5. Discussion
5.1 Experiment 1
Little was known about how the eye splice terminations would behave with the use of
a larks foot knot, which is why all samples tested in this experiment made use of a
larks foot knot. A rather large shackle of 20mm diameter was used throughout
experiment 1. Taking one end of the rope core and attaching it to this shackle using a
larks foot knot, with no splice involved, resulted in a maximum load value of 2668.9N
and the repeat sample produced 2446.5N. These maximum load values were larger
than expected. In other words, the larks foot knot produced more frictional load than
expected. Furthermore, all the other samples involved in this experiment failed due to
the rope, not the eye splice termination. The addition of whipping to these samples
only succeeded in making the sample rope fail further away from the splice itself.
From the analysis of this somewhat unexpected information, it was realised that
perhaps the shackle diameter was too large in proportion to the eye circumference of
the splice. Almost the entire splice eye was involved in the larks foot knot when
attaching the sample termination to the 20mm shackle. Therefore the splice itself was
irrelevant to the test as it was the rope that was essentially being tested. To produce
relevant results for the general size of the eye splice terminations being used in the
experimental side of this study, a smaller diameter shackle would have to be
incorporated into the shackle arrangement. In doing so, greater splice length will be
achieved after the creation of the larks foot knot and as a result, the splice termination
will be tested as opposed to the rope core.
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5.2 Experiment 2
Experiment 2 involved investigating the effects of the use of a larks foot knot,
different whipping conditions and different shackle diameters. Shackle diameters of
6mm and 10mm were used due to the fact that the 20mm shackle used in experiment
1 was too large in comparison to the general size of the splice terminations on the
samples.
After analysing the graphs shown in figures 5 and 6 produced from the results of
experiment 2, it was determined that the use of a larks foot knot improves the stability
of the splices and in-turn results in larger maximum load values. This is due to the
larks foot knot distributing the load around more of the eye of the splice as opposed to
just at the top of the eye, when the splice is attached directly to the shackle.
Looking closely at the graph shown in figure 5, the maximum load value for the
sample with no larks foot, no whipping and attached to the 6mm shackle is very low.
This is due to the instability of samples with no whipping. Perhaps as the sample was
being set up on the Tinius Olsen machine (see appendix 8), some slippage occurred
resulting in a decrease in feed length. For future experimentation, a temporary locking
device could be used when setting up samples with no whipping, which can be
removed after the sample is fully set up. This device would be applied somewhere
along the feed length of the splice to ensure no pull out can occur.
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The results also show that the addition of whipping to the feed length (see appendices
1 and 6) of the splice increases the maximum load value produced. This is as a result
of the whipping locking the part of the rope core, fed into its hollow centre, in
position and adding stability and strength to the splice structure. When the whipping
itself starts to fail, it acts as additional friction between the inside walls of the hollow
core, rendering it another reason for the increase in maximum load. When comparing
the experiment 2 samples with one whipping at the crotch against those with two
whippings (one at the crotch and one at the end of the feed length, shown in appendix
1), most results show that using two whippings increases the maximum load value
slightly. This is due to increased frictional load and stability and strength in the splice
structure. However looking closely at figure 6, it can be seen that the sample with a
larks foot knot, one whipping and is attached to the 10mm shackle has the largest
maximum load value. This could be a freak result or perhaps during testing, the
hollow core got very tight around the fed part of the core giving larger than usual
frictional loads. The use of a larger shackle and the larks foot knot could be the reason
for this tightening. However, more repeat testing on this sample would be required to
confirm this.
Comparison figures 5 and 6 shows that little variance in the results is produced when
changing the shackle diameter for the samples that made use of a larks foot knot.
However, some variance is produced in the samples that were looped directly on to
the shackles. The reason for this variance is due to the angle at which the splice eye is
being pulled at is larger with the 10mm shackle in use. This increased angle puts
larger stresses on the splice resulting in smaller maximum load values than those
produced while using the 6mm shackle. As the use of a larks foot knot allows no
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variance in the angle at which the splices are being pulled apart, the results produced
should not vary much when the shackle diameter is changed, which is what the results
prove.
20
5.3 Experiment 3
Experiment 3.1 was the first study in the effects of varying the splice feed length. All
samples tested within this experiment had no whipping and were attached directly to
the shackle with no larks foot knot used. The graph shown in figure 8 proves that
increasing the splice feed length will increase the maximum load value produced by
the termination. After analysing the rope samples after testing, it was clear that the
splices failed with feed lengths up to 16cm. Increasing the feed length beyond 16cm
led to the rope failing.
It would therefore appear that a feed length of 16cm was the point at which the failure
mode changes from the splice failing to the rope and that increasing the feed length
would decrease splice efficiency due to rope wastage. However, the samples with feed
lengths between 10 and 16cm were tested using the 6mm shackle and the samples
with 18 and 20cm feed lengths were tested using the 10mm shackle. This was due to
the fact that the 6mm shackle failed under the loads produced by samples with feed
lengths larger than 16cm. Therefore the sudden change in shackle diameter meant that
the results from experiment 3.1 were not entirely valid. A repeat experiment was
therefore required maintaining a constant shackle diameter of 10mm.
Experiment 3.2 was the repeat experiment mentioned previously. As stated before, all
samples tested in this experiment had no whipping, no larks foot knot and were all
tested using the 10mm shackle to prevent any unwanted hardware failures. Only feed
lengths between 14 and 20cm were tested, as this range was the area of interest due to
the change over in failure mode being expected to occur within this range.
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The graph shown in figure 10 reveals a decrease in maximum load between feed
lengths of 18 and 20cm. However this is irrelevant, due to the fact that the rope fails
before the splice for both feed lengths and the graph can therefore be assumed to
reach a horizontal asymptote after a feed length of 18cm
Again the results from this experiment shown in figure 10 determine that the feed
length is directly proportional to the maximum load value and in-turn, the splice
efficiency. The samples were analysed after testing and they proved to have failed in
the same manner as experiment 3.1. Samples with feed lengths of 14 and 16cm failure
occurred at the splice termination, where as the rest failed on the rope itself. Therefore
a feed length of 18cm is the required length that should be applied to a splice with no
whipping and no larks foot knot, in-order to produce the most efficient splice using
this condition, material and rope diameter.
22
5.4 Experiment 4
In a realistic sailing environment, eye splice terminations usually have whipping
applied to them as well as the use of a larks foot knot to attach them to the rigging. It
is due to this that the samples used in the testing for experiment 4 all had two
whippings applied to them (one at the crotch and one at the end of the feed length)
and made use of a larks foot knot when attached to the shackle arrangement. The
reason for experiment 4 is the interest in how the splice terminations behave with
varied feed lengths under realistic sailing conditions. The feed length values used in
experiment 4 were smaller than that of experiment 3.2 as the use of whipping and
larks foot knots means that less feed length is required to make the rope fail before the
splice termination.
Samples with feed lengths ranging from 8cm to 14cm were tested and then analysed.
It was found that the splice terminations failed with feed lengths of 6cm and
sometimes 8cm. This meant that feed length values of 10cm and above, within the
sample conditions stated previously, would result in the rope failing before the splice.
It can therefore be stated that for samples under these conditions, those with a feed
length of 9cm will produce the greatest overall efficiency.
Appendices 9 and 10 show the results produced from the Tinius Olsen machine for
two samples tested during experiment 4. The graphs show the difference in shape for
the two different failure modes, namely splice failure (appendix 10) and rope failure
(appendix 9). It is interesting to see how the shape of the graph produced differs. As
can be seen from these figures, if the rope is the failure mode of the sample, a far
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more sudden failure occurs, whereas when the splice fails, there are several failure
points. This is due to a major failure occurring due to the whipping giving way,
followed by a number of slippage points of the feed length (appendix 10).
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6. Conclusions
Several useful conclusions were drawn from the results of this study. These
conclusions however can only be applied to the type and diameter of rope used
throughout to produce the experimental samples. However, it can be assumed that
some of these conclusions would apply to different performance ropes of different
diameter. These specific conclusions shall be highlighted
The following conclusions can be applied to any type of performance rope:
1. Adding whipping to the crotch of the eye splice (see appendix 1) will increase the
maximum load and in-turn the efficiency that eye splice can produce. The factor at
which this increase occurs cannot be determined due to varied amounts of
increase.
2. Adding further whipping to the end of the feed length of the splice will increase
the maximum load and efficiency values by a small amount. Pinpointing these
increased values to samples with and without larks foot knot cannot be achieved
due to inconsistencies in the results.
3. When maintaining other variables constant, making use of a larks foot knot
improves stability in the experimental samples, giving increased maximum load
and efficiency values. The gradient of this increase with each sample whipping
condition remains relatively constant in most cases; however further
experimentation would be required to determine a value for these gradients.
25
4. Varied shackle diameters only seem to affect the spread of results if no larks foot
knot is made use of. Trends of whether the maximum load values increase or
decrease with larger or smaller diameter shackles, for samples not using a larks
foot knot, could only be determined with further experimentation.
The following conclusions can only be applied to splices manufactured using rope of
the same type and diameter as the rope used throughout this study:
1. For splices with no whipping and no larks foot knot used to attach it to rigging,
the most efficient splice would have a feed length of 10cm when taking cost into
consideration.
2. For splices with whipping at both the crotch and the end of the feed length of the
splice, a feed length of 18cm would produce the greatest efficiency when cost is
taken in to consideration.
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Appendix
Eye Circumference
Feed Length
Whipping Positions
Crotch
Appendix 1 – Sample variables and whipping positioning
Appendix 2 – Measured splice feed length
Appendix 3 – Measured splice eye circumferance
Appendix 4 – Insertion of feed needle and rope core
Appendix 5 – Core feeding of feed needle and rope core
Position One
Position One
Position Two
Feed Needle
27
Appendix 6 – Application of whipping
Appendix 7 – Application of larks foot knot to shackle
Rigging Support
Drum
Required Shackle Arrangement
Moving Crosshead
Appendix 8 – Tinius Olsen Tensile Testing Machine
Shackle Diameter
Maximum Load Versus Extension
-1000
0
1000
2000
3000
4000
5000
6000
7000
8000
-20.00 0.00 20.00 40.00 60.00 80.00 100.00
Ex t e nsi on ( mm)
Maximum Load Versus Extension
-1000
0
1000
2000
3000
4000
5000
6000
7000
-20.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00
Ex t e nsi on ( mm)
Rope Failure
Whipping Failures
Slippage Points
Appendix 9 – Graph shape of rope failure mode
Appendix 10 – Graph shape of splice failure mode
28
Acknowledgements
The author would like to thank his supervisor, Dr. A.J McLaren, for his continual help
and guidance over the course of this study.
Thanks are extended to Mr. A. Crocket who helped to run the testing machinery
throughout the experimental side of this study.
29
References
[1] Pawson, D. (2001), “Pocket Guide to Knots and Splices”, Chartwell Books.
[2] Pawson, D. (2005), “Rope Yarns”, Yachts and Yachting, 1513, 42-46.
[3] James, H. (2003), “Splicing Modern Ropes”, Yachts and Yachting, 1460,
42-49.
[4] Leech, C.M., “Modelling Tension and Torque Properties of Fibre Ropes and
Splices”, Proceedings of the Third International Offshore and Polar
Engineering Conference Singapore, 6-11 June 1993.
[5] Hearle, J.W.S., Parsey, M.R., Overington, M.S., Banfield, S.J. (1993),
“Modelling the Long Term Performance of Fibre Ropes”, Third International
Offshore and Polar Engineering Conference Singapore.
[6] Harken, Innovative Sailing Solutions, online at www.harken.com.
[7] Tinius Olsen, High Force Electromechanical Materials Testing Machines,
online PDF file, online at www.tiniusolsen.com.
[8] Leech, C.M., “The Analysis of Splices Used in Large Synthetic Ropes”, PDF
file available online.
[9] McLaren, A.J., “Design and Performance of Ropes for Climbing and Sailing”.
[10] Marlow, “Guide for the Splicing of Sailing Ropes” available in catalogue or
online at www.marlowropes.com.
30