Rule of Mixtures-Wingmast

I have been advised on the steamradio multihull blog to keep quiet when I see some preposterous assertions made there by famous designers. I always think that someone will say something; lots of the guys there seem to know things. But it rarely happens.

One really stuck in my mind though.  A few months ago Kelsall noted that he uses 30% carbon fiber in his wing masts. I kept waiting, but nobody questioned him on that. So maybe school has to be in session.

First, surely everybody knows that a long, slender composite part like a wing mast must have at least 70% of the fibers in 0 degree.  Ted van Dusen noted to me when he built a couple of critical masts for my boats, that had to be built with too small diameter, he went to 80% in 0 degree, just to be sure. 

OK, so if all the 30% carbon was in the 0 degree, that is still less than half the 0 degree fibers as carbon.

That is bad, bad, bad.   My readers all already know that the stretch to failure of carbon fiber is around 1%.  And that of E-glass is around 6%.  The carbon in the wing mast would have to fail completely before the glass even helped with 20% of the job.  I was dying to know; did he not believe in 70% zero degree? Or in rule of mixtures.  Getting everything possible right the first time is important.

Ron Reichard explained that with carbon in the 0 degree, glass can be used in the off-axis fibers. That lowers cost and even adds a bit of resilience.

I will have more on wingmasts next month, including a study that I co-authored with Dr. Paul Steinart on a FEA study of wingmasts, and composite hounds.



4 thoughts on “Rule of Mixtures-Wingmast”

  1. Maybe it is a stiffness not a strength issue. Do you think 30% carbon can add enough stiffness to increase the buckling resistance? I could not find the Kelsall reference to 30% carbon on his web site and have no idea what layup he was promoting.

    1. again the carbon would have to already fail before the glass added much to stiffness at all. It was a posting on the steamradio in January as I recall. I have it here if I look back.

  2. What—? You mean that those yellow threads that production monohull manufacturers love to include in their hull layups don’t make them bulletproof?

    The tri is not quite as red as she was while waiting for the drawbridge in St. Martin. Still the sexiest big cruising trimaran on the planet.

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