SAILMAKER'S UPDATE, DECEMBER 2003

December 16, 2003: With her mast scheduled to be stepped on December 28th, progress on Mirabella V continues apace.  The six sections of the mainsail along with the six battens are scheduled to leave Boston bound for Southampton on December 23rd.  The battens, too long to be shipped via air, will be transported by containership. The mainsail and three headsails will be shipped via air from Marblehead, MA and Auckland, NZ, respectively.

Following is an excerpt from a paper presented at the Royal Institute of Naval Architects The Modern Yacht Conference on September 17, 2003. To read the full paper, click here.

MIRABELLA V BATTEN TECHNOLOGY

For batten development and construction, Doyle Sailmakers turned to Ted van Dusen of Composite Engineering, a Concord, Massachusetts-based company.  Ted’s experience includes battens for Team Adventure and the mast and boom for Bruce Schwab’s Open 60 Ocean Planet, a competitor in the Around Alone Race.

BATTEN MATERIAL: A high roach mainsail presents a challenge for almost any size sail; a superyacht mainsail with a roach as large as Mirabella V’s presents a very special challenge.

The batten development for Mirabella V was largely a question of optimization: at the required batten stiffness, what is the maximum toughness that could be achieved while minimizing weight aloft? To answer this question, Doyle Sailmakers and Composite Engineering surveyed the different material options available: standard pultruded fiberglass, S-glass and carbon fiber.  Although carbon fiber is renowned for its stiffness and low weight, S-glass, a tri-axially woven fiberglass with a reinforcing carbon layer (~15% of fiber content), has a bending radius before break that is three times greater than carbon.  It is for this reason that the top four battens, which will experience the most flogging, highest loads and least support, were specified as S-glass.  The bottom two battens – the largest and heaviest – will be constructed of carbon fiber, providing the greatest weight savings for the battens that require the least toughness.   The savings in weight from switching from all S-glass to an S-glass/carbon fiber combination is 145 kg. 

BATTEN END FITTINGS: The inboard and outboard batten end fittings secure the sail segments and provide a means to tension the foot and head of each segment along the batten (see Figure 7).  In keeping with the low-weight, soft construction concept of these sails, a high strength Vectran line will run along the luff and leech of each segment and form a loop at the top and bottom corners. The ropes will loop around the battens and provide vertical load transfer up the luff and leech. Webbing loops on the inboard and outboard ends will be lashed forward to the batten end fittings to provide horizontal tension. 

COMPRESSION RELEASE BATTENS: The batten materials selected clearly demonstrate Doyle Sailmakers’ commitment to durability and toughness.  However, regardless of material selection, certain conditions will stress any batten to break.  In particular, tacking and gybing often cause the sail to flog or “snap” across the centerline of the boat, loading the upper battens and cars significantly.   In order to minimize the risk of breaking battens aboard Mirabella V, Doyle Sailmakers has developed the Compression Spring Batten. The CSB utilizes a spring integrated into the inboard batten end to relieve this compressive load.  It will be implemented on Mirabella’s second and third battens down, which are located along the most aggressive curvature of the roach.  These battens see the highest loads and are generally the most likely to fracture. With the implementation of the CSB, the batten will compress under high loads, reducing the overall length. This reduces the load on the batten, which minimizes the likelihood that the batten will break. 

Concept Testing: Compression Release Batten testing began on a local X-3/4 Ton in winter 2001.  The first stage of testing involved a basic concept analysis.   During these tests the breaking mode of small wooden battens was analyzed to determine which events were most likely to lead to breakage.  The findings supported Doyle’s previous experience as described above.

Second stage testing involved the introduction of a relatively low load gas spring on the inboard end of the batten.  As with the wooden dowel battens, the compressive load was found to be highest when tacking.  Sailing upwind did not compress the spring. However, as the sail crossed the centerline of the boat during a tack it tended to take on an S-shape to pass through the shorter distance between the straight mast and the leech, compressing the spring with the inboard end load.  Interestingly, only the strongest of the various springs exerted enough force to re-extend after initial compression.

Design: The working components of the Compression Spring Batten are located at the batten’s inboard end, between the luff car and the standard batten. The dynamic portion of the CSB consists of a carbon/S-glass plunger that slides inside the batten. Housed inside of the plunger is a nitrogen-gas spring that allows the batten to respond to the extreme compression loads that could cause fracturing (Figures 9 & 10). 

The interface between the plunger and the batten occurs through a nylatron bushing that is secured to the inner wall of the batten. This self-lubricating plastic allows the batten to slide along the outside of the plunger without the need for fragile bearings that are more susceptible to failure. The plunger acts as a rigid structural member that resists the torque in the batten, making sure that the batten remains rigid in the dynamic region of the CSB. By acting as a structural member, there is no transverse loading of the gas spring, which can lead to the failure of the spring’s seals. By housing the spring inside the plunger, the spring is also protected from water and salt spray. The end of the plunger will have a hole from which the spring’s rod will protrude. The other end of the rod will rest against a retaining wall in the batten that is protected by a stainless steel plate to resist wear. The back end of the spring is restrained in the plunger by the end fitting that attaches to the luff car and will slide inside the plunger. By enclosing the spring inside the plunger, the plunger unit can be easily separated from the batten, allowing for easy removal or replacement of the spring if necessary.

Spring Selection: After exploring mechanical and gas springs, a gas spring was chosen for the CSB. The reasons are three-fold: First, the gas spring provides a choice of initial load at which the spring will begin to compress. This allows the batten to behave normally until the end loads are equal to the initial load required to compress the spring. A mechanical spring does not share this behavior, instead deforming under much smaller loads, leading to a dynamic batten in all conditions. Second, the same amount of travel can be achieved at similar loads with a gas spring that is physically far smaller than a mechanical spring. The smaller size allows the gas spring to be housed inside the plunger portion of the batten instead of inside the batten itself (see Figure 9). By housing the spring in the plunger, the length of the batten that must remain rigid to keep the spring, batten, and plunger properly aligned is minimized, allowing as much of the batten to retain the designed characteristics as possible. Finally, the use of a gas spring allows for some fine adjustment of the loads that cause initial and maximum deflection. By varying the cylinder pressure the same spring can work over a range of loads.  

The spring chosen for the Mirabella V project is a custom, stainless steel spring that is designed to begin compressing at approximately 8,000N and reach its maximum deflection of 200mm at 12,750N. Standard gas springs have a ratio between the initial load required to deflect the spring and the maximum load around 1.4. The Mirabella V project, however, requires a custom spring with a load ratio of almost 1.6 in order to achieve the desired behavior. The design loads for the spring were determined from the buckling loads of the battens. Batten designer Ted Van Dusen estimates the batten end loads to be three times the buckling loads of the batten column, which range from 2,250N – 4,500N, producing end loads over the range of 6,750N-13,500N. By allowing the batten to begin compression at an end load of 8,000N, it is anticipated that the loads will be reduced so that the end loads will not reach the upper end of the load range, where fracturing the battens becomes possible. 

To read the full paper presented at the Royal Institute of Naval Architects The Modern Yacht Conference on September 17, 2003, click here.  

PUSHING THE BOUNDARIES

Mirabella V, the latest of Joe Vittoria’s Mirabella yachts, is pushing the boundaries of modern boat building and sail construction. At 75 m, Mirabella V will be the world’s largest sloop, and, with a reacher measuring 1,900 m 2 , Doyle Sailmakers will be building the world’s largest sail.

The full inventory consists of a mainsail, staysail, working jib and UPS (Utility Power Sail), or reacher. All told, the inventory weighs in at an astounding 1,500 kg - without hardware!

To read the full extract from the Yacht Design story, click here for the PDF file.

COMPOSITE COLOSSUS

The £33 million Mirabella V superyacht is presenting unique challenges to its designers and material suppliers.

October 22, 2003: By this time next year, the largest single masted yacht in the world – the Mirabella V – should be making its way to the Mediterranean from the port of Southampton in the UK.

The yacht is currently being constructed – at an estimated cost of £33 million – by UK warship builder Vosper Thornycroft (VT) for US businessman Joseph Vittoria. Mr Vittoria, former chairman and CEO of Avis car rental, has made plans to build Mirabella V largely from fibre-reinforced plastic and the project is requiring everything from new sailcloth and developments in rigging to captive hydraulic winches to be custom designed and built.

To read the full extract from the Future Materials story, click here for the PDF file.

SAILMAKER'S UPDATE, JANUARY 2002

January 25, 2002: The "it can't fail" criteria of the Mirabella V Project presents two distinct challenges to the sailmaker. The first is to develop material to withstand enormous loads that is resistant to breakdown from abrasion, flogging, flex, mildew and delamination. The second is to design sails resistant to wear from battens, lazy jacks, and routine handling. The scale of Mirabella V's sails requires novel engineering that will take full advantage of Doyle's extensive experience with superyachts.

Materials
Doyle Sailmakers has partnered with a special applications fabric mill in New England to develop OceanWeave®, an extremely durable woven sailcloth utilizing the most advanced synthetic fiber and textile technology available. In a patent-pending process, OceanWeave combines a tightly woven, warp-oriented Vectran x Vectran core with a specially woven and applied taffeta. The result is a low stretch material that does not rely on film, which can delaminate, and does not sacrifice warp or bias stretch performance. The absence of film also enhances the material's ability to withstand flexing, crunching, and flogging and to resist mildew far more effectively than a standard laminate.

Construction
Significant advances have been made in construction methods. We have filed for a provisional patent governing the construction of a "Segmented Mainsail". In this design, the mainsail is composed of seven separate, yet interdependent, sections. Six full-length battens join the segments at the top and bottom edges to form the complete sail. Building the sail in these segments makes construction, transport and service significantly easier, as the largest of the sections will not exceed 315 m2. As part of the testing phase, we have constructed a prototype 42 m2 X-3/4 Tonner segmented mainsail.

Batten Development
For batten development and construction, Doyle Sailmakers is working with Ted van Dusen of Composite Engineering, a Massachusetts-based company. So far, we have conducted the first two stages of batten testing on Kaos, an X-3/4 Tonner.

During these tests we carefully analyzed the breaking mode of wooden battens and progressed to a more in-depth study of the "Compression Batten" concept. The Compression Batten utilizes a hydraulic spring attached to the inboard end of a segmenter batten. The spring relieves the compression load that often leads to broken battens during tacks and gybes.

Surprisingly, the compressive load was found to be highest when tacking and only the strongest of the various springs tested exerted enough force to extend after compression. 

Future steps include the addition of strain gauges to record load while sailing, tacking, gybing, and flogging and the use of significantly stronger springs that will compress only when shock loaded. Combination compressed air/mechanical springs will also be considered. On-water testing will resume in the spring of 2002.

From our preliminary sail quotation to the present stage of development, the Mirabella V Project has proven to be an exciting design, engineering and construction challenge. It has been a pleasure to work with the Mirabella V team of Ron Holland Design, Vosper-Thornycroft and Mr. And Mrs. Vittoria, all experienced and knowledgeable professionals. We look forward to pushing the boundaries of modern sail materials and sailmaking with the entire Mirabella V crew.