Seahorse Article Dyneema® SK78 Part 2.pdf

The (new) miracle Dobbs Davis looks at the practical applications – and considerations – of the latest Dyneema fibres 42 SEAHORSE

Last month we examined the technical properties of several fibre types now in use in performance sailing, and how the chemical composition and molecular structure of these fibres contribute to the unique physical properties of each. For manufacturers of cordage and sailcloth this background knowledge is key to fully understanding the strengths and limitations of how the fibre can be used in various applications; the challenge facing them is how to optimise these properties for their products while being receptive to their users’ feedback on how to develop and innovate.

This feedback can also get back to the fibre manufacturer, who by improving on their fibre’s characteristics can in turn provide a greater range of options.

For cordage manufacturers the impetus for improvement comes from a constant pressure to innovate as improvements are made steadily in the design and construction of all constituent parts of a performance sailing yacht: hulls and appendages, spars and rigging, deck hardware and sails are all now carefully engineered and optimised for each application. The total of the incremental improvements made in each of

Pressure for a better product… look no further than the current America’s Cup in Valencia where the top teams would all like to be able to maintain microscopic tolerances when monitoring the dynamic performance of a critical control such as a runner tail or headsail sheet. This is BMW Oracle’s latest ACC design USA 98 during early trials in Auckland. At full power on an ACC boat it’s just like anything else: no amount of sustainable headstay tension will ever be enough…

cure? – Part II also focused on the particular merits of Dyneema, an HMPE fibre manufactured

by DSM in Holland and now also in America. Compared with other highperformance fibres, such as high-modulus polyester, aramids, Vectran and PBO, Dyneema shows superior qualities in several respects: its low density, excellent resistance to damage from sunlight, very high tenacity and relatively low elongation. It retains its strength through ageing better than any of these fibres, and even in creep, or non-recoverable stretch, DSM’s new 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 superior performance have been made from the results of rigorous testing of the fibres themselves, but the real proof comes in the performance of products made from these fibres and their application in the real world. Too many times the headlong rush to innovate within any given product line has resulted in premature claims made of materials untested in the circumstances that matter: PBO’s use in sailcloth some nine or so years ago comes to mind, where GILLES MARTIN-RAGET/BMWO

these areas has contributed to the tremendous leap in performance for both racing and cruising yachts in the past decade, and this would not have occurred without a steady pace of innovation throughout every field. Indeed, this innovation is necessary: the best-designed and built sails will not deliver their full potential without there being a similar level of performance from the rigging, the spars, the boat design and, yes, even the crew (gulp!). The achievable gains in performance will always be limited by the weakest link in this chain.

Part 1 of this series in last month’s issue

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its superior qualities compared to the aramids conveniently overlooked the fact that its strength is severely compromised by exposure to sunlight.

Good cordage manufacturers will steer around the known limitations of the fibres and endeavour to create lines that meet the customer demand for strength, durability, size and price. Through use of different materials in cover and core, and varying weave pattern and style for each, the line can thus be engineered to fit a specific role in the rigging package and should require little customisation for the end user.

It is interesting to consider, however, that unlike many other constituent parts of a performance yacht, where the spar, standing rigging, sails, hull and deck, and sometimes even hardware, are fabricated specifically for the design, the running rigging customisation options are nothing other than splices, stripped covers and the like: the actual composition and size of the 44 SEAHORSE

line are not varied from what is available from the manufacturers. If a 9mm line would be perfect, why do we have only 8mm or 10mm choices?

Because there is some variation in the twist of yarns, weave patterns in cores and covers, and the finishes and resins used in each, lines made by different manufacturers of the same materials can have varied characteristics that cannot be generalised according to fibre type alone. Having said this, however, the differences are often more of feel and ease of splicing than of more essential properties like strength.

Liros have been at the forefront of developing quality ropes for use on performance sailing craft as a significant part of their 150-year-old cordage business in Lichtenburg, Germany. They work closely with fibre suppliers such as DSM Dyneema not only to create the products to satisfy customer demand, but also to utilise a thorough regimen of testing at

their in-house facility to examine the properties of their ropes and the fibres.

This test data is a valuable source of information for riggers, hardware suppliers, sailmakers and anyone else who needs to specify how the loads from the sail forces are transferred to the boat.

One example is the measured breaking strength of 10mm rope made from various fibre types, from high-tenacity polyester (PET) to PBO. Note the lines made from Dyneema SK75 and SK78 break at loads that are twice that of aramids and 20-30% higher than LCP (known usually as Vectran), but about 10-20% less than PBO. Since we rarely take lines to their breaking strength, a more relevant measure of strength is to take 30m of 10mm line, load it and measure the elongation. Whether the applied load is 20% of its mean breaking load (MBL) or one tonne (1,000kg), Liros’s tests show that Dyneema is as low in elongation as PBO, and lower than

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

Vectran or the aramids. To investigate bending fatigue Liros set up their in-house test bench with line wrapped around small sheaves, then loading it through numerous cycles at 20% of MBL and 1,000kg. Again test results show that Dyneema performed the best of all fibres, especially outlasting the aramids, Vectran and PET at the higher loads.

In abrasion resistance the lines are folded around an emery cloth-covered roller and cycled back and forth until failure. Dyneema and PET prove similar in their resistance to chafe, while boasting more than six times the abrasion life of PBO, Vectran or the aramids.

These results from the test bench would seem to indicate Dyneema’s qualities as being near ideal, with its only drawback being non-recoverable elongation, or creep. As shown in Part 1, these measurable quantities of creep occur only over significant periods of time, which may not be relevant to most sailboat racers.

If we take, for example, the test data and examine its effect on the real-life performance of a 40ft inshore racer, we see that for 15m of 10mm halyard made of Dyneema SK75 loaded to 1,000kg there is no measurable degree of creep after eight hours of sailing. After a full week of sailing the line length has increased by 0.2%, or 3cm, and after a full month the total is just 0.5% or 7.5 cm. This compares favourably with 46 SEAHORSE

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)

typical shrinkage figures for laminate sails.

Because creep is a time-dependent quality it is perhaps more relevant to examine its effect on an offshore racing platform, where lines can be continuously loaded for days or even weeks. Using a Farr-designed Open 60 as an example, if a 20m-long 14mm halyard is loaded to 2,500kg for 93 days (2,232 hours), it will see creep of 4.6% or 92 cm if it is made from Dyneema SK75, but this drops dramatically to 1.3% or just 26 cm if made from SK78.

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

the Pirates of the Caribbean VO 70 team and is currently being used by at least three America’s Cup teams in Valencia.

However, because not every programme is as price insensitive as a major America’s Cup team with respect to its rope performance, it is useful to examine cost for different fibre types; here Dyneema SK75 compares favourably with its rivals, with SK78 being twice the cost… but still far less than PBO. But because the same size line is not the same strength, depending upon the fibre, a plot of retail price versus strength shows Dyneema SK75 being the best value among the available product types: a 16mm line made from PET is equivalent to a 14mm line in aramid, a 12mm in Vectran, 10mm in Dyneema and 8mm in PBO. While this discussion has focused almost solely on use of Dyneema in cordage, its positive qualities lend themselves for other applications as well, the most notable being sailcloth. And while the low-temperature melting point and slippery feel of the fibre seem a bar to its use in most current lamination techniques, both North Sails Cloth and Dimension Polyant have expressed an interest in exploring the potential future use of Dyneema in non-woven sailcloths.

In conclusion, the innovations of available fibre types outlined in these two articles must be considered in connection with similar advances in the sails and the sail-handling systems onboard any modern

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

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)

sailing yacht to fully appreciate an increase in performance. The use of a high-modulus, low-stretch line is meaningless if the hardware breaks under the load, the spar bends too much, the sails stretch with every puff, and so forth.

For all the effort and expense placed on developing these systems, the justification for use of the best possible line to complement their performance is easy indeed.

Calculations undertaken by Mani Frer’s design team at Victory Challenge found a 13.75kg saving in running rigging weight using Dyneema, equating to a 15-second gain around an America’s Cup course…

Cheap at twice the price! ❒

Pre-tension is the key, as another batch of Dyneema SK75 is woven into line for marine use at the Liros factory in Germany SEAHORSE 47