Carbon-Fiber Mainsail Battens
Peter and Deniz laying the pre pregged carbon into the aluminium mould.
Adding the absorbent scrim that soaks up any surplus resin.
Cutting the pre pregged kevlar into double bias strips for the portion of the batten that needs the most flexibility.
Kevlar and core pieces that determine the batten shape when loaded.
Ready to start the vacuum pump for the final kevlar lamination.
A little test, and yes!! The leech end stays straight and the batten bends at the desired 40% of cord length when given a little pressure.
According to the sailmaker at Doyles loft the sail shape looks pretty good.
Why we built our own mainsail battens
Since our sails were being built in New Zealand and had to be freighted to Turkey it was impractical for the sailmaker to supply the battens. We also wanted to save as much weight aloft as possible, so glass battens were not the best option. All of the commercially available carbon battens were hideously expensive and had a less than perfect record of reliability. In fact, many offshore racing classes have banned them for this reason. So we decided to manufacture our own.
The battens have proven to be very reliable having done 10,000 miles already, including a miserable squall ridden, beam sea, Atlantic crossing. The continued shock of slating sails repeatedly inverting, as the boat rolled underpowered, with the passing of every squall, caused the only remaining piece of the original cast aluminium gooseneck components to shatter two days out from Martinique. This despite the fact that the boom had been held rigidly in place with preventers.
CONSTRUCTION
The basic design was a unidirectional carbon fiber core, 30-40mm vertically, narrower for the top battens, wider for the lower and of varying thickness starting at about 3.5 mm and increasing for the lower battens. This provided the necessary compression strength but on its own would be too supple to provide any sail shaping. To get the battens both stiff enough and to bend in a manner that would give a flat leech and maximum draft at 40% of the chord length, a tapering light-weight core was glued to both sides of the carbon. Adding a kevlar skin to the outside of this, with the fibers arranged to give varying amounts of tensile resistance both completed the shaping of the batten and gave a wear resistant outer layer. Carbon fiber on the outside faces of a batten rapidly marks the sail.
To make the carbon core a wet-layup of unidirectional material was tightly stretched inside an aluminium “U” sectioned extrusion lined with peel ply and absorbent scrim. A similarly covered box sectioned extrusion fitted inside the “U” on top of the carbon finished the sandwich and was mechanically clamped in place. Lots of large clamps can produce more than the 14psi of a vacuum bag if placed close enough together.
After gluing the tapered wedges that would alter the stiffness of the batten in place the Kevlar outer reinforcement was added. For the section of the batten we wanted to bend this was cut as a 45/45 degree double bias and for the section 70%-100% of chord length (leech) was laid parallel to the batten. This cloth was quite light so vacuum bagging it on was the easiest way to make it form around the batten.
Stretching the carbon so it is absolutely straight is most important in compression structures. Analysis of failed wind-turbine blades has shown that the slightest waviness in the carbon layup leads to catastrophic failure. We used a carbon cloth specifically produced for the wind turbine industry. Unlike most unidirectional cloth which is woven with transverse fibers that introduce the exact wave form that precipitates failure, this cloth is instead held together with a glue that dissolves in epoxy resin.
Where the batten fits into its holder additional carbon and kevlar was used to strengthen and stiffen it.