When life’s (not) a drag
Put together everything that’s been achieved to date and Future Fibres believe that the best of all worlds is now steadily drawing within reach
When carbon fibre standing rigging became a viable option for raceboat spar packages, most of us got very excited. This was the last frontier to pursue in raceboat technology where steel could finally be eliminated and huge gains in performance could be realised quickly from the many kilos of weight saved aloft in the spar system. Stability would go up, pitch gyradius down, and the boats equipped with this new technology would be measurably faster as a result. Enter Future Fibres. While most of the features of this new technology were positive, there were a few downsides. These included the complicated details of attachment to the boat and spar, replacement options when a new section may be needed, transport logistics from the fabrication facility to the commissioning site, potential damage from impacts to the rigging, and the larger diameter of carbon rigging that creates additional windage in the spar system compared with steel rod rigging. Future Fibres addressed some of these issues in its ECsix product, which consists of small carbon strands bound together in a unidirectional bundle that gives both flexibility for easy transport when coiled and multi- directional strength since the loads are distributed among the strands. In addition, this design can more readily absorb impact energy by deflecting when hit compared with a solid carbon rod. ECsix can lose 25 per cent of its carbon strands but still keep the spar upright and to date it has had zero failures. Yet solid carbon rigging such as Future Fibres’ Razr product does have its own advantages: the amount
72 SEAHORSE
of load-bearing carbon is in the smallest possible area when in a solid cross section; it has lower windage than multi-strand rigging and often minimal weight as well. The downside of solid carbon is that it’s very difficult, if not impossible, to coil, making transport logistics a significant challenge, particularly for very large rig plans. It is also much more likely to break in compression due to its monolithic structure, as well as being infamous for the noise it makes due to vibration. Given these issues, it’s useful to study the pros and cons in a less intuitive, more numerical manner. For weight, that’s relatively simple: the weights of the parts are added and positions in the rig plan mapped, and the weight and centre of gravity can be calculated for the package. Alternatively, the spar and rigging can be weighed and balanced to determine these figures as part of an IMS-defined measurement necessary for ORCi or ORCsy certificate data. However, for windage estimations, more research is needed. This topic had first been explored a few decades ago with wind tunnel tests on shaped steel and titanium rigging, demonstrating the windage advantages of an aerofoil-shaped rod, compared with circular shapes, in the 12 Metres raced in the America’s Cup. But the cost and complication of fabricating these sections kept it from widespread use. Scroll forward to October 2018 at the Yacht Racing Forum in Lorient, France, where Future Fibres’ Jonathan Duval, head of research and development, presented research on the topic of windage in composite rigging and its ancillary
Above: the new AeroSix rigging from Future Fibres: aerodynamic, elliptically shaped
rigging made of bundled carbon rods in a chafe resistant sheath.
The result is claimed to be the best of both worlds: a flexible
rigging cable that can be coiled for
transport and doesn’t suffer from excess vibration, with lower drag than
regular ECSix cables.
AeroSix has been trialled in recent months
aboard early adopters such as the Reichel/Pugh Wallycento Magic Carpet3
effects. His findings were interesting and revealed another issue among composite rigging types: vibration and its effects.
On windage drag, a study was performed at the University of Auckland’s Twisted Flow Wind Tunnel that examined three shape options with with the same cross-sectional area (CSA) of 314 mm2
: round
(aspect ratio 1:1), elliptical (2:1 ratio) and an extended ellipse (2.7:1 ratio). Using the same material for each, at the same air density and velocity (about 22kts) the drag was measured in orthogonal X and Y reference axes at 10° intervals from 0-90° wind angle.
The results were interesting: as expected, the round shape had its highest Fx drag measured at 0°, ending at zero Fx drag at 90°. The ellipse shapes were very low at 0- 20°, then rose to about half that of the round shape for the ellipse and about a third for the extended ellipse. Along the Y axis, the Fys measured were zero at 0°, then rose rapidly for the extended ellipse before falling at 10° to less than the round and ellipse shapes until at 30° rising above the round shape until being about 30 per cent more at 90°. Combining these results suggested lower drag overall for the elliptical and even less for the extended elliptical shapes. With help from North Design Services, Future Fibres then put these numerical results into a VPP study that varied the windage drag coefficients from the wind tunnel study on rigging shapes. This study found for a generic 100ft superyacht sloop with a four-spreader spar that the gains in VMG upwind speed for
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