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Standard Rigging: the Polyethylene alternative - Article by Chris Kinzel © 2006
Published in Boatpoint.com (New Zealand) / Features, March 2006
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here to view the original article
When Christopher Kinzel began building the rig for his 19m sailing catamaran, he wanted a lighter,
cheaper alternative to conventional steel rigging. He settled for high-density polyethylene
(HDPE) rope and, so far, it's worked a treat.
I first stumbled across the rope - Dynex 75 - at a fishing port near Nelson, where it is sold
by an Icelandic rope and net manufacturer - Hampidjan. It's reported to have half again the
breaking strength of steel of the same diameter. How much does it stretch? It doesn't, they
said. Could it be used for standing rigging?
My research expanded to other rope manufacturers, mast builders, riggers, engineers, composite
rigging builders, sailmakers and ocean racers - and I received a mixed response: "... wouldn't
use it for the danger of chaffe ... sure, we use it all the time ... the stuff creeps ... good
for rotating rigs, but not a fixed rig like yours ... to avoid creep you've got to over-spec
it and you'll be fine..."
The rope, I could see, offered a number of potential advantages. Costs for rope vs wire were
about the same, although fittings would be cheaper and the weight savings would be huge. The
creep issue seemed avoidable, albeit difficult to monitor, and there would be an improved safety
margin. Termination and tensioning were design issues I would enjoy solving. The installation
would be a bit time consuming, but I could do it myself. I decided to use it.
So what is HDPE? The base material is Ultra High Molecular Weight Polyethylene, refined by Netherlands
company DSM and marketed as Dyneema. Its characteristics are well documented, and it is sold
to rope makers, sail makers and fabric makers worldwide. Hampidjan braids the material (12-braid)
and adds a secondary heating/tensioning treatment - a UV protective and abrasion resistance
coating - and markets it as Dynex 75.
The rope, says Hampidjan, was developed as a replacement for steel when trawling, towing, and
mooring large objects. The company has carried out plenty of testing and comparisons with respect
to tenacity vs elongation, specific strength vs specific modulus, creep, bending fatigue, abrasion,
and UV resistance. It is the only Dyneema braider (of many), which is Lloyds certified.
Selecting the diameter
Our vessel - Augustina - makes its rig work. The loads are as follows: six tonnes at the cap
shrouds when flying a hull, 10 tons when you pitchpole.
If breaking strength was the only consideration, we could use a smaller diameter than a comparable
1x19 wire cable. However, the Achilles heel of HDPE seems to be cold creep: permanent deformation
from a continuous load, measured over time (months and years) at a given temperature.
Hampidjan has test data showing that a rope put under a continuous load of 50 percent of its
minimum breaking strength at 20oC will creep beyond 10 percent after seven months; not a very
useful length of time for holding up a mast.
However, when the sustained loads are at 20 percent of the minimum breaking strength at 20oC
for a year, 0.5 percent creep is encountered. A drop in temperature 10oC will improve this period
by a factor of three or four. Easing shroud tension when not in use for extended periods can
also help.
For Augustina, riggers and mast builders said I could get away with 12mm 1x19 SS wire, but recommended
14mm cap shrouds. For the rope alternative, I chose 16mm Dynex 75. This was based on the predicted
maximum sustained load while flying a hull (six tonnes) - some 23 percent of the minimum breaking
strength for the rope.
After three years of flying a hull in 20oC, we might see 1.5 percent creep (according to the
charts). That would give us a useful life of 18 years of hull flying until we reached the 10
percent elongation limit. We've been sailing about three months and have yet to sustain flying
a hull, so I'm guessing creep won't be an issue.
In terms of weight savings, the approximate difference between the rope and the SS cable I would
have used is:
* 125m x 16mm Dynex = 20kg
* 110m x 14mm SS wire = 106kg
* That's a weight saving of 86kg (or a factor of 5.25)
In an ultimate dynamic loading situation (diagonal pitchpole) when relying on the minimum breaking
strength of a shroud, the safety factor difference is:
* 14mm SS wire = 1.4 * 16mm Dynex 75 = 2.8
Termination design
The race is on among designers to come up with end fittings that accommodate a pin connection
for the rope. Some have tried the various wire end styles such as swageing, cone inserts and
cold casting - without success.
Instead, I have found three styles for eye-splicing 12-braid line. The weaving goes quite fast
once you get into it, and it can be undone and repositioned. The braid is loose enough to work
without tools, though the proper size tubular fid makes the job easy. Thanks to the UV protective
coating the fibers maintain their bundles well.
The simpler Brummel style takes minutes while the slightly stronger Tuck style can take about
an hour. Add at least another hour if you want a tapered finish (it offers the maximum strength
and elegance).
Eye splicing will reduce a line's minimum breaking strength by 5-10 percent, while a simple
bowline knot will reduce it by 20-30 percent. A description of the Brummel and Tuck styles can
be found on Hampidjan's web site (www.hampidjan.is).
Augustina's cap shrouds run continuous from the hounds to the chain plates and back up to the
upper spreader bases. The spreader tips are locked up and down by the diamonds, which run in
reinforced plastic hose over a molded groove.
The upper ends of the reverse diagonals are spliced into the cap shroud just above the spreader
tips. In fact, the upper diamond was added after the mast went up when we wanted more forward
push (mast camber); no need to call a crane or a swaging guy or machine shop.
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Tensioning
The rig was standing but the tensioning system needed improving. A major find was Precourt Systems,
a Canadian outfit working specifically on developing fittings for synthetic rigging (see www.precourt.ca).
It CNC-machines 6061 T6 anodised aluminum into a range of deadeyes and thimbles that accommodate
various stay sizes.
Individual holes prevent the lanyard parts from binding. Following the perimeter of the shape
with the lanyard hole pattern keeps the deadeye pointing in the desired direction (non-capsizing),
balancing the load on the lanyard parts. The machining, styling and finish is smart, and the
price very reasonable.
In our case, the nine-part lanyard (8mm Dynex, which amounts to a minimum breaking strength
of 60 tons) is led with snatch blocks to a winch. While tension is applied, a bit of prying
on the parts with a large screwdriver and fingers helps even up the load. Then with the sails
up it's easy to tack back and forth working the slack out of the leeward side.
The lanyard starts by splicing to the lower deadeye, and the tail finishes at the upper deadeye
by wrapping diagonally over the splice twice, and rolling hitches (at least four) around all
nine parts. Under full tension there is enough advantage and friction in the lanyard to untie
the hitches and still maintain the tension by hand (the hitches don't have to work hard).
The next improvement was to install a turnbuckle in series with ... the deadeye lanyards being
the gross adjustment and the turnbuckle doing the final tensioning. This has proved satisfactory
for keeping the mast in column, but we have never managed to get the headstay very straight.
That may be a function of the lack of a backstay and flex in the boat.
The Dyneema fibre does not stretch, but as a braided rope it gets longer when loaded. To date,
this has been the most important issue regarding Dynex 75 as standing rigging. We have tightened
and re-tightened the shrouds, the mast has stayed up, and now stays straight athwartships and
keeps a nice forward camber.
Measuring for length and splicing was done while the mast was horizontal. We started out with
a 60cm gap between deadeyes and are now down to about half that. It's been about a season of
sailing and indeed the tension seems to be stabilising. When we jump around in waves there appears
to be some give as the loads spike, which I believe is good to relieve stress. However, that
give must be recoiling as I'm not having to continuously tighten the shrouds anymore.
The good news is that Hampidjan has recently produced the next generation of Dyneema based rope;
called Dynex Dux. Through further heating under tension the 12-braided line is pre-stretched
and firmer or more compacted, is spliceable just the same and is stronger than the Dynex 75
by up to 40 percent.
Summary
Yes, replacing stainless wire with high-strength rope is possible, but not in all cases. The
potential for chaffe from hanks or furler gear would seem to rule out using it for headstays.
In rigs such as multihulls with rotating wing masts, and traditional sailing vessels some rigging
stretch is acceptable and a welcome relief for spike loads on the hull structure. Dynex 75 clearly
offers a good improvement over wire for those rigs.
For monohulls with short staying bases that require high shroud tension to keep the mast straight,
Dynex Dux will work as long as the continuous loads are less than 20 percent of the minimum
break strength. The bottom line is slightly more windage for less cost, and much less weight
aloft.
As for longevity, the trawlers are replacing their lines (UV protective coated but uncovered)
after five years of hard service. I imagine that typical sailing rigs work a lot less. If sized
and covered properly strong rope should easily out last stainless wire or rod as fatigue is
no issue. With time we hope to do further testing, know more, and will keep you posted.
Chris Kinzel © 2006
SV Augustina Specifications
LOA: 19m
Beam: 9m
Displ. Loaded: 11 t
Mast height off water: 27m
Rig style: fixed D section, double sweptback 30 spreaders (lowers almost full width of vessel)
Capshroud load
Flying a hull full sail: 6t
Flying a hull reefed: 8t
Diagonal pitchpole: 10t
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