Excerpt from the yacht report on Kokomo that appeared in issue 114 (June 2010) of The Yacht Report.
Doyle Sails NZ
Kokomo hosts a complete inventory of custom designed Doyle Stratis membranes and boasts numerous records in sail construction and sizing.
At 2,230m2 (24,200ft2), the gennaker is thought to be the largest sail ever constructed. The working inventory comprises a total of 3,038m2 (32,688ft2) of Stratis membrane sails, and many innovations in sail handling systems and detailing.
The boom furling system for the mainsail and self-loading batten car system, for example, was jointly designed and tested by Doyle and Southern Spars over an 18-month period inan attempt to resolve many of the weak points of boom furling systems and offer the advantages of full battens in a yacht of this size.
Building and designing Stratis membranes for this size of boat required a new approach to engineering and safety margins, along with a new custom-built 6,500m2 (21,325ft2) Stratis lamination and sailmaking facility in Auckland. Over a period of 12 months we utilised Relax 2, a fabric structural design program, to accurately model potential loads and redesign the previously used structural load paths to better address the enormous ﬁgures that this boat generates. The investment and lessons learnt from this new generation of superyacht places has put us at the leading edge of this market and the future of these high-performance luxury yachts.
The results after initial sea trials have proved to be very successful for Doyle NZ, with the many hours of R&D and structural analysis paying dividends. The upwind boat performance
in a fully loaded full sail conﬁguration with 20- plus knots of wind were impressive, to say the least, in what is essentially a performance cruising yacht.
One of the requests in the Kokomo blade sail was that when the sail is sheeted on the wind the leech comes as close as possible to the spreader. Only through accurate modelling for shape optimisation can this be achieved with a perfect result. Base sail shape moulds were used for certain geometries and applications and then adjusted for the requirements of the actual sail. For Kokomo more focus on the headsails was for upwind performance, so these sails were designed and engineered to be optimal at these angles, rather than reaching angles.
Once the geometry was established, we then analysed the loads on the sail and how the sail would react and change under load. This is especially important with custom ﬁbre laid sails, as a sail is engineered to stretch a certain amount, rather than to a percentage of its ultimate breaking strength.
This dictates how much ﬁbre is required in any area of the sail. During this process of establishing the correct ﬁbre alignment and density, different combinations of ﬁbres were used to get the best balance between weight and overall durability.
Kokomo’s sails use a blend of Vectran and carbon ﬁbre in the primary load directions (for example clew to head, tack to head, clew to tack) and a composite of carbon and twaron in the sub-structure (to deal with any loads or reinforcements from battens, impact areas, furling loads, etc.).
(Ed’s Note: Twaron is a paraaramid; a heat-resistant and strong synthetic ﬁbre developed in the early 1970s by AKZO. Its research name was Fiber X and then was ﬁrst sold as Arenka; its uses are similar to those of Kevlar.) The ﬁnal process in the membrane manufacture is the application of the top surface: a ﬁlm sheet, often with a polyester taffeta on the outside, pre-coated with glue. This is ﬁnally vacuum bagged to the table and the laminator, which uses infrared heat lamps and 13,000kg of downward pressure on a 2-metre-wide roller. The laminator will then make computer controlled passes, varying heat and speed based on ﬁbre type and density, over the membrane to activate the glue and expel any remaining air in the laminate. The membrane is left to cure for several days before being moved to the sail loft ﬂoor for ﬁnishing.