David Jones

I see my friend and my structural engineer David Jones is a rock star in Pro Boat magazine this issue. Cool. I’ve known David since the 80s at MACAM conferences.  David always understood that my customers wanted the engineering without having to pay for it. So I would do the calcs in Excel typically, send to him, and he would agree or not. See issue #149.  David knows things.


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.



Make that Mast Fit in a Container

The mast on my Geko was 60′. When Haiko bought that boat, the mast had to go into the container. Haiko is a master machinist, so I watched him do it. He recently topped up my recollection with the procedure.

“As you said: the mast was indeed welded together from 2 pieces. It was done with 2 inner-plates made from the same mast profile on port and starboard.  By sanding the paint you could see the different structure of the welded aluminum.
Three rows of holes measuring about 3/4 of an inch were drilled in the outer profiles and to the welded  plates.
So we had to drill holes of 30 mm through and  around them as the weld is always bigger than the initial hole if done properly.
The mast was cut very attentive (carefully) 3 mm under the weld, to know sure it would part while trying not to touch the inner-plates.
Back in Holland 3 different series of props (nuts? I will find out)where milled 3d for inside as well as for outside.
After welding we still had to do some sanding on the inside. The mast is now demountable in two parts for legal transportation on the road.”

Mast Splice 006_copy

Fiber Orientation

By now every boat builder knows that fiber orientation is critical to laminate strength.  And I’m sure everybody knows that 34 oz triaxial for example has a bending strength of some 65,000 psi in the 0 degree, and about 20,000 psi in the 90 degree orientation, hand layup. And unidirectional fabric will have some 100,000 psi bending in the 0 degree and maybe 3,000 psi in the 90 degree.

But that is not all there is to it.  There has to be off axis fiber even if it performs less well.  I first learned about that in Tsai’s composites class in ’90 I think it was.  In tension it would not matter, but in compression and bending, the hard-working 0 degree fibers have to be kept in column, as it were.  The trick is to  have no more than 3 zero degree layers stacked before adding an off axis one.  Then you can add three more.

I have visited mulithull build sites where I have seen plans from famous designers with like, 10 or 20 layers of 0 degree stacked up, and no off axis fibers.  Don’t they know?  And they charge many times what I do.

Was Out Much of This Week

I had rare good weather to mix epoxy. Supposed to rain next week so will be back in. I have thought of many topics for here for the coming week.
Fiber orientation update
Fiber mix examples
Taking a mast joint apart
Upgrading bad details from other designs
Psychology and window shapes

See, I think when I am stirring epoxy.

Skype and Big Files

Prospective customer Anton showed me that Skype can be used to send big files without setting up an FTP site. Lets try if someone wants the e-catalog, or composite construction manual or the CM construction manual. I will look into the DVD later as it is so much bigger. $25 for each? I will get something set up on the website. For now just use cards? For now I would rather not use Paypal.  My Skype is khughes.multihulldesigns
Hit me and I can send files back via Skype.