Last month we examined the technicalproperties of several fibre types now in usein performance sailing, and how the chem-ical composition and molecular structureof these fibres contribute to the uniquephysical properties of each. For manufac-turers of cordage and sailcloth this back-ground knowledge is key to fully under-standing the strengths and limitations ofhow the fibre can be used in various appli-cations; the challenge facing them is howto optimise these properties for their prod-ucts while being receptive to their users’feedback on how to develop and innovate.

This feedback can also get back to the fibremanufacturer, who by improving on theirfibre’s characteristics can in turn provide agreater range of options.

For cordage manufacturers the impetusfor improvement comes from a constantpressure to innovate as improvements aremade steadily in the design and construc-tion of all constituent parts of a perfor-mance sailing yacht: hulls and appendages,spars and rigging, deck hardware and sailsare all now carefully engineered and opti-mised for each application. The total of theincremental improvements made in each of

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Dobbs Davis looksat the practicalapplications – andconsiderations – of the latestDyneema fibres

The (new) miracle

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these areas has contributed to the tremen-dous leap in performance for both racingand cruising yachts in the past decade, andthis would not have occurred without asteady pace of innovation throughoutevery field. Indeed, this innovation isnecessary: the best-designed and built sailswill not deliver their full potential withoutthere being a similar level of performancefrom the rigging, the spars, the boat designand, yes, even the crew (gulp!). The achiev-able gains in performance will always belimited by the weakest link in this chain.

Part 1 of this series in last month’s issue

also focused on the particular merits ofDyneema, an HMPE fibre manufacturedby DSM in Holland and now also inAmerica. Compared with other high-performance fibres, such as high-moduluspolyester, aramids, Vectran and PBO,Dyneema shows superior qualities inseveral respects: its low density, excellentresistance to damage from sunlight, veryhigh tenacity and relatively low elonga-tion. It retains its strength through ageingbetter than any of these fibres, and even increep, or non-recoverable stretch, DSM’snew Dyneema SK78 formulation places it

within easy reach of its claim of being the‘strongest fibre in the world’.

All these properties and claims of supe-rior performance have been made from theresults of rigorous testing of the fibresthemselves, but the real proof comes in theperformance of products made from thesefibres and their application in the realworld. Too many times the headlong rushto innovate within any given product linehas resulted in premature claims made ofmaterials untested in the circumstancesthat matter: PBO’s use in sailcloth somenine or so years ago comes to mind, where

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Pressure for a better product… look nofurther than the current America’s Cup inValencia where the top teams would alllike to be able to maintain microscopictolerances when monitoring the dynamicperformance of a critical control such asa runner tail or headsail sheet. This isBMW Oracle’s latest ACC design USA 98during early trials in Auckland. At fullpower on an ACC boat it’s just like anything else: no amount of sustainableheadstay tension will ever be enough…

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its superior qualities compared to thearamids conveniently overlooked the factthat its strength is severely compromisedby exposure to sunlight.

Good cordage manufacturers will steeraround the known limitations of the fibresand endeavour to create lines that meet thecustomer demand for strength, durability,size and price. Through use of differentmaterials in cover and core, and varyingweave pattern and style for each, the linecan thus be engineered to fit a specific rolein the rigging package and should requirelittle customisation for the end user.

It is interesting to consider, however,that unlike many other constituent parts ofa performance yacht, where the spar,standing rigging, sails, hull and deck, andsometimes even hardware, are fabricatedspecifically for the design, the runningrigging customisation options are nothingother than splices, stripped covers and thelike: the actual composition and size of the

line are not varied from what is availablefrom the manufacturers. If a 9mm linewould be perfect, why do we have only8mm or 10mm choices?

Because there is some variation in thetwist of yarns, weave patterns in cores andcovers, and the finishes and resins used ineach, lines made by different manufactur-ers of the same materials can have variedcharacteristics that cannot be generalisedaccording to fibre type alone. Having saidthis, however, the differences are oftenmore of feel and ease of splicing than ofmore essential properties like strength.

Liros have been at the forefront ofdeveloping quality ropes for use on perfor-mance sailing craft as a significant part oftheir 150-year-old cordage business inLichtenburg, Germany. They work closelywith fibre suppliers such as DSMDyneema not only to create the productsto satisfy customer demand, but also toutilise a thorough regimen of testing at

their in-house facility to examine theproperties of their ropes and the fibres.This test data is a valuable source of infor-mation for riggers, hardware suppliers,sailmakers and anyone else who needs tospecify how the loads from the sail forcesare transferred to the boat.

One example is the measured breakingstrength of 10mm rope made from variousfibre types, from high-tenacity polyester(PET) to PBO. Note the lines made fromDyneema SK75 and SK78 break at loadsthat are twice that of aramids and 20-30%higher than LCP (known usually as Vec-tran), but about 10-20% less than PBO.

Since we rarely take lines to their break-ing strength, a more relevant measure ofstrength is to take 30m of 10mm line, loadit and measure the elongation. Whetherthe applied load is 20% of its mean break-ing load (MBL) or one tonne (1,000kg),Liros’s tests show that Dyneema is as lowin elongation as PBO, and lower than

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Vectran or the aramids. To investigatebending fatigue Liros set up their in-housetest bench with line wrapped around smallsheaves, then loading it through numerouscycles at 20% of MBL and 1,000kg. Againtest results show that Dyneema performedthe best of all fibres, especially outlastingthe aramids, Vectran and PET at thehigher loads.

In abrasion resistance the lines arefolded around an emery cloth-coveredroller and cycled back and forth untilfailure. Dyneema and PET prove similar intheir resistance to chafe, while boastingmore than six times the abrasion life ofPBO, Vectran or the aramids.

These results from the test bench wouldseem to indicate Dyneema’s qualities asbeing near ideal, with its only drawbackbeing non-recoverable elongation, orcreep. As shown in Part 1, these measur-able quantities of creep occur only oversignificant periods of time, which may notbe relevant to most sailboat racers.

If we take, for example, the test data andexamine its effect on the real-life perfor-mance of a 40ft inshore racer, we see that for15m of 10mm halyard made of DyneemaSK75 loaded to 1,000kg there is no measur-able degree of creep after eight hours ofsailing. After a full week of sailing the linelength has increased by 0.2%, or 3cm, andafter a full month the total is just 0.5% or7.5 cm. This compares favourably with

typical shrinkage figures for laminate sails.Because creep is a time-dependent qual-

ity it is perhaps more relevant to examineits effect on an offshore racing platform,where lines can be continuously loaded fordays or even weeks. Using a Farr-designedOpen 60 as an example, if a 20m-long14mm halyard is loaded to 2,500kg for 93days (2,232 hours), it will see creep of4.6% or 92 cm if it is made from DyneemaSK75, but this drops dramatically to 1.3%or just 26 cm if made from SK78.

Given this is an extreme amount of timefor a halyard to be loaded without change,this level of creep would appear accept-able. It’s no wonder SK78 was chosen by

the Pirates of the Caribbean VO 70 teamand is currently being used by at least threeAmerica’s Cup teams in Valencia.

However, because not every programmeis as price insensitive as a major America’sCup team with respect to its ropeperformance, it is useful to examine cost fordifferent fibre types; here Dyneema SK75compares favourably with its rivals, withSK78 being twice the cost… but still far lessthan PBO. But because the same size line isnot the same strength, depending upon thefibre, a plot of retail price versus strengthshows Dyneema SK75 being the best valueamong the available product types: a 16mmline made from PET is equivalent to a14mm line in aramid, a 12mm in Vectran,10mm in Dyneema and 8mm in PBO.

While this discussion has focused almostsolely on use of Dyneema in cordage, itspositive qualities lend themselves for otherapplications as well, the most notable beingsailcloth. And while the low-temperaturemelting point and slippery feel of the fibreseem a bar to its use in most current lami-nation techniques, both North Sails Clothand Dimension Polyant have expressed aninterest in exploring the potential futureuse of Dyneema in non-woven sailcloths.

In conclusion, the innovations ofavailable fibre types outlined in these twoarticles must be considered in connectionwith similar advances in the sails and thesail-handling systems onboard any modern

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INSHORE RACERS

Typical example:� 40ft inshore racer (above)� 15m 10mm halyard� 5,000kg break load� Average load: 1,000kg� Average temperature: 25�C

Creep elongation using Liros Racer 2001(Dyneema SK75):� 1 day (8 hours) of sailing: 0.0%� 1 week (56 hours) of sailing: 0.2%

(3cm)� 1 month (240 hours) of sailing: 0.5%

(7.5cm)

‘Barred-up’: if you are primarily going tobe racing inshore then you do not need(or need to pay for) the degree of longer-term creep resistance desirable on a long-distance racer such as a Volvo Open 70

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sailing yacht to fully appreciate an increasein performance. The use of a high-modu-lus, low-stretch line is meaningless if thehardware breaks under the load, the sparbends too much, the sails stretch withevery puff, and so forth.

For all the effort and expense placed ondeveloping these systems, the justificationfor use of the best possible line to comple-ment their performance is easy indeed.Calculations undertaken by Mani Frer’sdesign team at Victory Challenge found a13.75kg saving in running rigging weightusing Dyneema, equating to a 15-secondgain around an America’s Cup course…Cheap at twice the price! ❒

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OFFSHORE RACERS

Extreme example:� 60ft IMOCA Open 60 (above)� 20m 14mm halyard� 12,000kg break load� Average load: 2,500kg� Average temperature: 25�C� 25,000 miles around the world� 93 days (2,232 hours) of sailing

Irreversible elongation:� Rope incorporating Dyneema SK75:

4.6% (92cm)� Rope incorporating Dyneema SK78:

1.3% (26cm)

Pre-tension is the key, as another batch ofDyneema SK75 is woven into line formarine use at the Liros factory in Germany

While the rig loads on an Open 60 are ultimately well below those on, say, anACC boat, resistance to line creep is a keyin minimising the need for ‘maintenancetrimming’ by the always-tired solo skipper

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