Tuesday, June 8, 2010

Carbon fiber, tapered battens, and a slack leach

This project is a copy of an article originally published at Sail Delmarva, reprinted here with permission. This is not a large project, but it is a large post...
Before: different boat, but same model, same sails, identical problem.

I admit it; I'm cheap. And I enjoy fixing anything I can, to prove I can. If I have to fix it though re-design, that's even better, and if the repair method is unusual... better still. I'm a tinkerer.

The main sail leach on my boat had been falling off to leeward in a terrible way, as much in light winds as stronger stuff. No just an inch or so - a regular S-shape as much 12" to leeward. Another PDQ sailor mentioned that thicker battens had been installed in his main by a sail maker and had worked for him. Being cheap and willing to make a simple thing more complicated, I decided that tapering the battens by stiffening the back 60% with carbon fiber TOW (tension oriented weave or unidirectional fiber) would do the trick and not thicken them so much that they would not fit the pockets. After all, most beach cat battens are tapered. The draft of a sail is tapered - more in the front, and less as you move aft. And it was also a fix and improvement I could finish for the next weekend!
Batten stiffness is rated by the weight required to deflect a 40" sample by 4". The original battens were 3 pounds for the upper 2, and 4 pounds for the lower 2. The lamination changed this to 8 pounds on uppers and 10 pounds on the lowers, exactly what my back-of-the-envelope calculations using internet carbon stiffness values predicted. Unidirectional carbon works for this project because it is that it is not just stronger than glass, but also much stiffer. If you were to substitute glass it would take many layers and the battens would get too thick to fit the existing pockets. Unidirectional fiber is required; more of the fibers are oriented in the correct direction and they are not deflected into a serpentine path by the cross weave.The forward part of the batten remains nicely flexible. It seems so obvious; the battens on my Prindle were tapered, and in proportion to the PDQ, much stiffer at the leach that the stock PDQ battens.
A simple job, really, easily accomplished:
  • Sand the batten.
  • Cut the TOWs to size.
  • Laminate one layer of 1" wide by 11 oz. graphite to each side. Taper fibers to avoid a hard spot. I did this by ending the fibers about 6" differently on opposing sides.
  • Sand lightly and add a second coat of epoxy.
  • Sand lightly again, by hand, 220 grit or finer, to get any sharp spots.
  • Epoxy again. Optional; I didn't.
The carbon tow came from West Marine, though it is not in the catalog. Defender Marine has a 3" x 50' roll (http://www.defender.com/product.jsp?path=-1|10918|16458|309345&id=12428), though I think I bought a `15' roll, and that was just enough. I split the 3" roll to long 1" strips with sharp scissors - delicate because it tries to fall apart, but not difficult. There were no tricks to the laminating, other than starting at the leach end of each batten, adding resin to only about 1-foot of each batten and letting that soak while I added resin to the first 1-foot of the others, and then going back to the first to finish the full length. Doing it this way keeps the fiber in place, rather than having it creep as you brush the resin toward the luff end.

I am still going to re-cut the sail. I believe I need to remove ~ 3/16" out of 2 seams, 50-70% of the way up. The "after" photo shows some residual sag. But the stiffer battens were a huge help on the sag and treated some vertical wrinkles as well. I will be happy until fall.
An easy project for any boat with full length, un-tapered battens. Like so many engineering projects, it is invisible when well done... but I appreciate it whenever I look up, I found myself able to walk away from a couple of well-sailed 45-footers (all sail boats heading the same way are racing), and my pocketbook appreciates receiving a few more miles from a good sail. The speed difference is appreciable.

After: not perfect, but much better!

A typically thorough engineering treatment from Drew.

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