Saturday, January 14, 2006

Selecting a Strong Hitch for Spectra/Dyneema Rope


The Problem


A knotting correspondent and acquaintance of mine has inquired about a suitable knot for attaching a rope to a ring or a shackle. This means that the rope is to be attached to a bearing surface with a small diameter. He described it as “a problem of termination of a braided rope made of Spectra/Dyneema” which cannot be spliced.

He wants to find “knots that may reduce the breaking strength less the most” because using a hard-laid hi tech rope such is this is worthwhile only if you can attach the rope to something without loosing the extra strength bought with the high tech fiber. He notes further that “the rope is very hard laid and so does not snug down easily.”

He comments that “a loop knot or a noose would do,” and suggests an angling knot such as a Palomar Knot.

Dyneema and Spectra

According to Wikipedia, “Dyneema® is a registered trademark of Royal DSM NV (The Netherlands)” and “Spectra is the brand name of the chemically identical product developed independently by Allied Signal (now Honeywell) in the USA. Though the production details will undoubtedly be different, the result is comparable to Dyneema.” Wikipedia also notes that “Dyneema will creep, meaning it will deform when placed under any stress.”

In this paper, I will refer to both Dyneema and Spectra as Spectra.

The Challenge of this Problem

What we are looking for is a suitable knot to attach a rope to a ring or a shackle. The is hard-laid and does not snug easily and it cannot be spliced. And we want to find a secure and stable knot that will not compromise the strength of the rope. This problem is no doubt faced by many as climbers, rescue persons, and boat riggers, where special conditions create a demand for high performance knots.

Part of the problem of Dyneema is that like all high tech ropes, it does not easily tolerate severe curves. Knotting reduces its strength more than other types of rope.

If the knot is to be attached to a carabiner or a shackle, it can be cast, that is, tied on the bight. If it is to be attached to a ring, it must be built, that is, tied with the end of the rope.

Allaboutknots.com has not investigated the problems of tying hitches to attach a rope to a ring, nor to using a high-performance artificial fiber such as Spectra. But finding a knot to fill these specifications presents the kind of challenge that structural analysis was designed to study and help solve. This question provides an opportunity to apply concepts of knot performance to a real-life knotting problem.

The Three Performance Characteristics Required in the Knot

To meet the challenge of these specifications, we need a knot that combines three performance qualities: a high level of security, stability, and strength.

  1. Security: A secure knot can be arranged and tightened so that it will not slip apart and fail under a normal load. A secure knot provides enough friction to keep it from slipping apart.
  2. Stability: A stable knot will keep its shape under an excessive or abnormal load; it will not deform into a less secure form.
  3. Strength: Under a heavy load, a strong knot can hold up without breaking.
In addition, we need to consider the factors that determine the breaking point of a knot.

We need to find a knot that will match the requirements in this special rope used in demanding applications and perhaps under adverse conditions. We will be looking for a suitable hitch, fixed loop, or adjustable loop that is to be used in a fixed installation such as some parts a ship’s rigging but also a knot that can be quickly and easily tied and untied several times during a climb or rescue operation.

All of the terms and concepts used here, such as wrap and first curve, are explained in full in the various papers in allaboutknots.com. References to discussions of various knots in The Ashley Book of Knots are given by page number and the knot number in parentheses, such as such the modified Clove Hitch, 293 (#1693).

How to Increase a Knot’s Security

A knot can be made more secure by several structural devices: 1) turns around the ring; 2) a tuck-under-turn device; 3) a core-and-wrap construction.

1) Make Several Turns Around the Ring or Shackle

To increase the security of the knot, double or triple the turns (wraps) of the hitch around the ring. As Ashley commented more than once, “There is unsuspected virtue in a few turns of line” (77).

It is often easy to pass the rope two or three times through a carabiner, a shackle, or a large ring, but the eyes of blocks and sails are often large enough for only a single pass of the rope.

2) Increase Knot Security with a Tuck-Under-Turn

In an Anchor Bend, for example, the end is wrapped twice around a shackle, then tucked between the wraps and the shackle. This is what I call a “tuck-under-turn.” The end can then be further secured by a couple of half hitches. The problem here, again, is the small aperture in blocks and sails.

3) Increase Knot Security with a Suitable Hitch

Some variety of two half hitches immediately suggest themselves. Also a fixed loop with a core-and-wrap structure. Secure the nub to the standing part with a Grapevine or half of a Double Fisherman's Knot.

How to Increase a Knot’s Stability

Knot stability can be increased by using the devices mentioned for knot security: several turns around the ring, the tuck-under-turn, or core-and-wrap.

How to Increase the Strength of a Knot

The factors that affect knot security and stability are fairly straightforward, but those that affect knot strength are more complex. To begin with, you need to recognize that knots usually break because an excessive load falls on the first curve; the severity of other curves in a knot do not affect its strength. You can increase the strength of a knot in two ways: 1) reduce the load on the first curve; 2) reduce the severity of the first curve. Either of these ways reduces the stress on the first curve. Increasing the number of round turns made through the ring or shackle does not increase the knot’s strength unless it also makes the first curve more gentle.

How to Reduce the Load on the First Curve

It is possible to reduce the load on the first curve only if the stem curves for the first time inside the nub of the knot, such as in a Double Fisherman's Knot.

A similar core-and-wrap structure can be used to create a hitch, but the core-and-wrap structure will be less effective in reducing the load because no great load falls on the wraps to draw them tight around the core.

How to Reduce the Severity of the First Curve

The second way to reduce the stress on the first curve is to make the first curve gentle. A knot that creates only a gentle curve at the point the stem first begins to curve will be strong. It is particularly important that the hitch does not make a severe curve as it enters the nub.

The obvious way to reduce the severity of the first curve is to use a knot in which the first curve occurs as the rope passes through its mooring, then to increase the diameter of the ring or shackle. This would reduce stress on the rope and thus increase its strength. You could do this by inserting a thimble in the ring or by increasing the diameter of the sheave in a block. Neither of these solutions may not be practical in the application specified here. In these tight situations, probably the best thing to do is to choose a knot in which the first curve is located at some point other than the place the rope passes around the ring.

To increase the breaking strength of a knot, the most effective procedure is to use a hitch that creates only a gentle first curve, whether that curve is at the entry point or further inside the nub.

Candidates Described in Standard Knotting Books

A survey of standard climbing, rescue, and sea-going knots in several books yields these candidates for this task:

Ashley
Ashley, Clifford W. The Ashley Book of Knots. Garden City, N.Y.: Doubleday & Company, Inc., 1944. Many later re-printings.

Ashley’s Chapter 21: Hitches to Spar and Rail (Right-Angle Pull) illustrates and describes several varieties of hitches.

In (#1693), Ashley shows a Clove Hitch modified with an additional wrap, turn, and tuck. His instructions tell how to create a collar that wraps around the standing part in an arc of 180°. “The result is a hitch,” he comments, “that is firm, strong, secure and easily untied once the load has been removed” (293). A whale symbol indicates that this knot is “probably original” with Ashley.

Inspection shows that the turns are held in place by the diagonal that crosses over them.

Security. This knot appears to be at least as secure as a Clove Hitch, and the additional twist probably adds more security.

Stability. I surmise that Ashley uses firm here to mean what I call stable. Even when the standing part is pulled back and forth in a plane perpendicular to the spar, the collar appears to stay in place and the knot remains quite snug and stable. But it is hard to say whether this knot tied in a small ring would be able to retain its form, particularly if the load was unsteady or intermittent.

As is the case with many knots, both its security and its stability could be increased by attaching the tail to the standing part with a backup knot such as two half hitches or half a grapevine.

Strength. The addition of the new twist to the Clove Hitch changes the location of the first curve and apparently increases the strength of the knot. In the usual Clove Hitch, the stem makes a severe first curve as it passes through the ring, but the addition of this new twist appears to reduce the severity of the first curve. With this modification, the stem in this cousin of the Clove Hitch curves more gently as it passes over the collar.

(#1720) Round Turn and Two Half Hitches. Ashley suggests that the round turn makes this knot preferable to the simpler Two Half Hitches (#1710) because it is stronger. He comments that “If a spar is small a round turn is preferable to a single turn. It makes a stronger knot and dissipates the wear.” Unfortunately, Ashley does not explain why the knot would be stronger or why it would dissipate the wear.

Assuming that the strength of a knot depends on the severity of the first curve (as argued in the paper on strength in allaboutknots.com), then this is a very strong knot because the first curve is quite gentle. It seems questionable, however, whether the first curve is gentler in the Round Turn and Two Half Hitches than in simpler Two Half Hitches

A correspondent has noted that Marlow Ropes resurrected the Buntline hitch, which is a variety of Two Half Hitches in which the nub is in the form of a Clove Hitch. They rate the Buntline Hitch at 53%.

(#1721) Two Round Turns and Two Half Hitches is “a strong, old-fashioned knot.”

Security. My analysis indicates that the addition of another round turn increases the security of this knot. Ashley says that the addition of a Round Turn in #1721 “dissipates the wear.” I suggest that this round turn distributes the load, which makes the knot more secure. As Ashley commented (p. 77), “There’s unsuspected virtue in a round turn.” `

Stability. Adding another half hitch may be an example of what Ashley calls redundancy, but it would also increase the knot’s stability.

Strength. As with the modified Clove Hitch (#1693), the first curve is very gentle.

My analysis shows that this knot, however old fashioned it may be (or whatever that term may have meant to Ashley), this hitch is secure, stable, and strong. In addition, it is easy to tie and easy to untie. And it is hard to get it wrong or to forget how to tie it.

(#1722 - #1724) Ashley shows several varieties of the Anchor Bend and marks some of them with an anchor to indicate that it is reliable.

(#1727) Ashley comments that the Jam Hitch “closes easily but does not tend to open.”

Other possible knots found in Ashley are various hitches (#1725 – #1726), 50 (#219), knots combined with a Grapevine (Half of Ashley’s #294), a variety of Becket Hitch or Sheet Bend (#297 – 301), Constrictor Knot (#1249), Ring Hitch 68 (#1859), Bale Sling Hitch (#1750), and Diamond Knot (#693).

Warner
Warner, Charles. A Fresh Approach to Knotting and Ropework: Knots Arranged According to their Structure. Picton, New South Wales 2571, Australia: Privately printed, 1992, 1993, 1995. ISBN 0 9592036 3 X.

Warner lists many of the same hitches, pages 244–245.

Biddlecombe
Biddlecombe, George. The Art of Rigging. Originally published by the Marine Research Society, Salem, Massachusetts, 1925. New York: Dover Publications, Inc., 1990.

In Plate VI, opposite page 49, Biddlecombe illustrates in Figure 7 an Anchor Shackle, but with no line attached, and in Figure 8 a hawser attached with a Fisherman’s Bend to a ring.

Budworth
Budworth, Geoffrey. The Complete Book of Sailing Knots. London: Hamlyn, an imprint of Octopus Publishing Group Limited, and New York: The Lyons Press, 2000.

In this book, Budworth shows a Camel Hitch (109), Round Turn and Two Half Hitches (111), and the Fisherman’s Bend (Anchor Bend).

Gerber
Gerber, Ham. Making Discoveries in Knots. Portland, Oregon: Binford & Mort Publishing, 1990.

Although Gerber makes numerous suggestions for ways to invent useful knots, he does not show a hitch that meets the present need.

Pawson
Pawson, Des. The Handbook of Knots. First American Edition. New York: DK Publishing, Inc., 1998.

Pawson shows a Round Turn and Two Half Hitches (132), a Fisherman’s or Anchor Bend (134), a Palomar Knot (154), and a knot for snelling a hook (156). The last two, although no doubt secure, stable, and strong, entail tying with a pullover maneuver, which would not be feasible for the present task.

Rosenow
Rosenow, Frank. Seagoing Knots. New York: W. W. Norton & company, Inc., 1990.

Rosenow shows a Clove Hitch secured by Two Half Hitches (38) and a Backhanded Hitch with Two Half Hitches (40).

Severn
Severn, Bill. The Book of Rope and Knots. New York: David McKay Company, Inc., 1960.

In this book, I do not find anything useful for the present purpose.

Smith
Smith, Hervey Garrett. The Arts of the Sailor: Knotting, Splicing and Ropework. First published by D. Van Nostrand Company, Inc., 1953. New York: Dover Publications, Inc., 1990.
Smith shows nothing of interest.

Vines and Hudson
Vines, Tom, and Steve Hudson. High Angle Rescue Techniques. First Edition. A Publication of the National Association for Search and Rescue. Dubuque, Iowa: Kendall/Hunt Publishing Company, 1989. Revised Edition, 1992. Second Edition. St. Louis, MO: Mosby, Inc., 1999.

Vines and Hudson (1999, 34 and 60) describe and illustrate clearly the effect of a severe curve, which they call a “sharp bend.” They note that “Strength loss in rope does not become significant until the rope has a bend less than four times the diameter of the rope. The 4:1 rule applies more to natural fiber and to a few synthetics than it does to nylon rope, where loss may not be significant until it gets below a 2:1 ratio.” (35). I have not found a comment that suggests the ratio appropriate for Spectra.

Luebben
Luebben, Craig., Knots for Climbers. How to Rock Climb Series. Evergreen, Colorado: Chockstone Press, 1995. Second Edition. Guilford, CT: Falcon: an Imprint of The Globe Pequot Press., 2002.

Luebben (page 20) describes and illustrates an “Equalizing Figure-8,” which is a Figure Eight Loop with the loop tied long so that it may be drawn back through the upper hitches of the 8 and form three points of attachment.

My Suggestions

Rejected Custom-Designed Knots

My attempts at custom-designed knots to meet the specifications have been less than successful. Interesting and informative, perhaps, but not successful. One of them, which I won’t show, probably meets the performance criteria, but it is bulky, awkward looking, hard to tie, and hard to make snug. In addition, it probably is not stable. My experiments lead me to conclude that it is better to select an off-the-shelf knot.

Standard Knots Described by Ashley

Ashley lists several standard knots that may fill the bill.

(#1720) Round Turn and Two Half Hitches. Ashley comments that “If a spar is small a round turn is preferable to a single turn. It makes a stronger knot and dissipates the wear. Unfortunately, Ashley does not explain why the knot would be stronger.

He suggests that the round turn makes this knot preferable to the simpler Two Half Hitches (#1710) because it is stronger. Assuming that the strength of a knot depends on the severity of the first curve, then this is a very strong knot because the first curve is quite gentle. It seems questionable, however, whether the first curve is gentler in the Round Turn and Two Half Hitches than in simpler Two Half Hitches

(#1721) Two Round Turns and Two Half Hitches is “a strong, old-fashioned knot.”

Security. My analysis indicates that the addition of another round turn increases the security of this knot. Ashley says that the addition of a Round Turn in #1720 “dissipates the wear.” I suggest that it distributes the load, which makes the knot more secure. As Ashley commented (p. 77), “There’s unsuspected virtue in a round turn.” `

Stability. Adding another half hitch may be an example of what Ashley calls redundancy, but it would also increase the knot’s stability.

Strength. As with the modified clove Hitch (#1693), the first curve is very gentle.

My analysis shows that this knot, however old fashioned it may be (or whatever that term may have meant to Ashley), this is a secure, stable, and strong hitch. In addition, it is easy to tie and easy to untie.

(#1722 - #1724) A variety of the Anchor Bend would be useful if the eye of the ring or sail is large enough to admit two passes of the rope.

Another standard knot is “Bunny Ears,” which Ashley calls a Double Figure Eight Loop (#1085). It is used for installing fixed ropes in indoor climbing walls, the practice areas used by rock climbers. The advantage of this knot over the Figure Eight Loop in fixed-rope applications is that the load is distributed between the two loops, reducing amount of wear on each loop at the point where it contacts the shackle or link. This is an important consideration in fixed tackle, where the same surface may bear a shifting load for a long time.

Luebben’s “Equalizing Figure-8”

A promising candidate is Luebben’s “Equalizing Figure-8” (page 18), a climber’s knot that appears to meet all of the criteria for performance and ease of use.

Luebben describes and illustrates an “Equalizing Figure-8,” which is a Figure Eight Loop with the loop tied long so that it may be drawn back through the upper hitches of the 8 and form three points of attachment.

This may be the best solution to the problem. The figure-eight form of the nub provides sufficient security and stability, while the three attachment loops provide, I would judge, something approaching three times the strength of a single loop. As Luebben says, this knot “spreads the load evenly” over the three loops. Apparently, the “Bunny Ears” does the same thing, but with just one loop. A possible disadvantage is that the knot uses a lot of line. But it is easy to tie and untie.

The problem with most of these knots is that they cannot be tied with the end of the rope, so cannot be used on a closed ring or the eye of a sail.

Two Half Hitches and a Grapevine

If the knot is to be attached to an eye with a small opening, such as a sail, block, or ring, the best thing I come up with is Two Half Hitches backed up with a Grapevine. And even for a knot to attach Spectra to a shackle or carabiner, those would probably be a good choice. Another choice would be the Figure Eight Loop, tied either in a bend or by the follow-through method. This is a standard loop used by climbers, and it is secure, stable, and strong. But it’s a tricky knot to learn to tie correctly, and it may be hard to tie under adverse conditions. So I’d go with the backed-up Two Half Hitches.


This may seem like a pretty lame conclusion after such a lengthy analysis. After all, I’ve settled for one of the simplest knots of all times, with a standard backup, but I am struck by two facts. First, none of the sources I have consulted specifically states the problem we’ve studied, and I have never found an inventory of knots suitable for meeting this challenge. Second, it seems to be the case that only sailors need to attach high-performance rope to a small-diameter eye. I surmise that sailors developed their kit of knots long ages ago and that for them there is very little, if anything, new under the sun. During the last century, I would guess that most of the innovative knot work has come from climbers, cavers, and rescue personnel.

END

8 Comments:

Blogger Brion Toss said...

Hello,
I very much enjoyed your exploration of candidates for knots in HM line. The only thing missing was destruction testing, which will almost certainly reveal something like a 60% to 70% reduction in line strength for any of the knots you suggested, with slippage likely to occur in at least some of them at as low as 10% load. I say this having conducted dozens of tests with many knots. Unfortunately, the only circumstance that seems to apply is one in which multiple turns between load and anchor are made, so that little load comes on the knots.
Fair leads,
Brion Toss

12:03 AM  
Anonymous HOLDWAY said...

Please consider, as a canidate, ABOK #1843. An additional modification should also be considered. The tail can be take another 'backward' wrap and by the 'follow-through' method worked back along the standing part for a neater finish (stopper optional).
It seems to satisfy all the requirements. It appears to be secure, stable and strong. It uses multiple turns, tuck-under-turn, core & wrap and gentle first turn.
It is not too difficult to tie or untie. It works well on small shackles and carabiners by effectively increasing the wrap diameter of the first two turns. It is also compact - requireing about the same amount of line as a triple overhand knot.
Respectfully,
JB Holdway

12:35 PM  
Anonymous Richard Clopton said...

All very interesting. I have contacted Samson tech support regarding a knot/eye solution for Spectra for use on a sailboat. Will forward whatever I find out. -- Richard Clopton

3:15 PM  
Anonymous Anonymous said...

Hi

I dont know what kind of dyneema rope you are talking about but the ropes I work with every day is 12 strand dyneema rope in dim 10 to 40 mm. Its very easy to splice in at least two ways.... This rope comes both with and without jacket. Normal splices is "1strand 6-3-3 splice", "tuck in splice", or "shoemakers splice". Other ways to make a eye would be just to tackle a long tackle.

7:42 PM  
Blogger bonerici said...

anon: about splices

first of all, you can't splice, well you can sleeve/splice with another rope, like using dacron sleeves etc, but you can't do a real splice with some of this cored dyneema and spectra because the outer core is often non-load bearing some kind of weak (but durable) polyester.

Second of all, you want a fid length that is 21 lengths the diameter of the rope, and the smallest loop possible is 10 lengths of the fid length so even for a 2mm 12 strand dyneema line that's one and a half feet.

If you are joining two things that are closer than that, you will either have to double your line just so the splice won't slip or . . . make a knot.

Actually this is exactly why I was reading this post, I need to attach a stainless steel coil to a fitting with some dyneema, about 1 inch apart, no way can I possibly use a slice so I'm looking at some knots.

9:08 PM  
OpenID simplesimons said...

A lot of work has been put into this question by the fishing community, which has been using spectra cord for some time. You might consider testing the Palomar or some variation of the Uni knot (with two round turns and > 5 wraps)

sources:
http://videofishingknots.com/alphabetical-index.html
http://www.fishingclub.com/fishing/nafc-video/articletype/categoryview/categoryid/1000/knot-wars-2009

8:53 AM  
Anonymous Anonymous said...

SPECTRA NOOSED
Despite my lack of experience with Spectra, I have a quick idea. I have read that the only bend recommended for use with Spectra is the triple fisherman's (grapevine) bend (ABOK#498 #1415--tripled). Well, the directly corresponding single-loop hitch is the Scaffold Noose (ABOK#1120), which is tied 'overhand'. It seems to me to be one of the strongest hitches (a hitch taking the load reduced as one leg of a loop, but with a knot more similar to fixed knots than to half-hitches). Further, IF with Spectra, first curve severity is more important than the support provided by the wraps, perhaps the Gallows Noose (ABOK#1121), which is tied 'underhand', would be even better. The Gallows Noose almost looks as if it was designed for the very purpose of reducing first curve severity.

With both nooses, adding two turns with the tail around the first curve in order to lessen curve severity works well, in that the tail-turns are held firmly in place. (The tail-turns also make the Scaffold Noose much easier to untie, which might be especially useful to the grapevine bend for its jamming problems).

3:27 PM  
Anonymous Anonymous said...

FIRST CURVE SEVERITY IN SINGLE-LOOP HITCHES
Thank you for your examination of knot strength and first curve severity. You are right to think that it should lead to some new ways of looking at knots. Despite (or because of) my ignorance, I have been encouraged by your blog to run a little with ways of reducing first curve severity, speculatively anticipating the needed research.

I am especially interested in single-loop Hitches, where the load on the knot is taken as one leg of the loop, and hence is about half the load of a fixed-knot loop where the load is taken directly. This is a huge initial advantage. The challenge then is to find ways to make the hitch's knot stronger.

The simplest way to reduce the severity of the first curve of many single-loop hitches is to turn the tail one or more times around the first curve -- but the results are usually clumsy (the tail-turns aren't held firmly in place, or conflict with the need to tug on the tail for adjustments, etc.).

My idea (which might be a new hitch -- I can't find it in Ashley) is this:

(a) loop the object, make three half-hitches on the standing: 1, 2, 3.
(b) spread open 1 a bit, push 1 over 2, so 1 now encircles the rope between 2 and 3.
(c) thread the tail through 1.
(d) center and tighten 1 between 2 and 3; tug the tail to tighten 2 and 3.

Try it and see -- it is very easy to make! (It may be one of those rare knots where verbal directions are clearer than photos). It expands the first curve from 1 rope diameter to 4 dia. It seems secure and stronger, etc. (on my 10mm climbing rope). (ABOK has nothing with three half-hitches (at most, he has two HH plus seizing or an extra turn). Although my idea is simple and (I think) clever, it makes the knot 'bulkier', something Ashley disliked, and here would find pointless. But he was not looking for ways to reduce first curve severity. I was. He would think my idea was already amply covered by other knots. But perhaps not -- if less severe first curves are important and deliberately searched for.)

What do you think (in general, and compared, say, to your choice of a round turn and two HH, i.e., one rope diameter first curve)?
(I welcome ANY comments on my 'maybe-new' hitch -- including news that it is not really new!)

3:29 PM  

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