Pandora's box and homebuilt carbon fiber spars

Discussion in 'Sailboats' started by joeymarc, Mar 26, 2013.

  1. joeymarc
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    joeymarc New Member

    First, this is a question simply to gain knowledge for knowledge sake...I am pretty aware of my actual building limitations and I totally understand the risk and reluctance of NA's handing out "how-to's" on carbon fiber mast construction...but my curiosity and thirst to know is sometimes overwhelming.

    In a post I promptly lost track of, and cant seem to locate again, Mr Eric Sponberg gave a kind of rule of thumb regarding orientation of the fibers in a carbon mast. Can anyone post a link to that thread or perhaps Mr Sponberg could repost it here for me? Also, there were some cross section drawings of an elliptical rotating mast I would like to study again...

    Actually, I'd would love to read any practical design theory of regarding mast construction (esp rotating varieties)...any good books on the subject?

    Thanks for any input.
     
  2. michael pierzga
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    michael pierzga Senior Member

  3. joeymarc
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    joeymarc New Member

    Thanks for the reply Michael.

    It wasn't a hard and fast lamination schedule. It was a percentage of 0/45/90 degree fibers.

    I'm not actually looking to have a mast designed...and so am rather reluctant to contact an NA or engineer who would probably rather get paid than answer annoying questions from curious wanna-be builders.
     
  4. michael pierzga
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    michael pierzga Senior Member

  5. joeymarc
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    joeymarc New Member

    With such a high potential of catastrophic failure, I seriously doubt I would ever attempt to backyard build a carbon fiber mast...I would love to have those bragging rights, but let's be realistic. Anyway, for now, its strictly theoretical.

    I believe that the majority of fibers would run at 0 degrees to the long axis...but is there a need for any cloth/fiber at an off 0 degree axis?
     
  6. Skyak
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    Skyak Senior Member

    A mast is a beam that gets loaded in bending and compression. For compression, longitudinal orientation is most effective, but some radial component is needed to keep the fibers in column. In bending, the outer fibers are longitudinal (because they are in tension and compression) and in between fibers are at 45 degrees to longitudinal.
    like this llllXXXXllll

    But of course all this is theoretical. In reality you need to know how well the glue holds the fibers, and the glue to fiber ratio. Then there is the 'crimp' of woven fabric or mono directional...

    If you are just doing a tapered circular section you could just wind the fiber in alternating directions at 5 to 10 degrees like a golf club shaft.

    I recall seeing that same layup example by Eric. My recollection is that he alternated longitudinal mono direction with 45 degree fabric on the outside, and just 45 degree in the web.
     
  7. Andy P
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    Andy P Junior Member

    I've done a few booms that were OK ( for int Moth ), but for the same weight as commercial booms were bendier, or for the same stiffness were heavier.
    You just can't beat pre-preg, 4 ish x atm pressure, 130°C curing.
    Vaccing, heatshrink tape, wound peelply just can't match it.
    The material costs for a home build are quite high, and there's always the chance that it will not quite work out right. ( and so you've wasted half the cost of a proper one ) .

    However some commercial spars can't seem to do 0° - and these are the ones that break ;-) They look nice with the fancy weave at 5°, but it's better at 0°. ( and no rivets ! )
     
  8. joeymarc
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    joeymarc New Member

    Thanks all. That's getting more to the meat of my questions. I appreciate all the info.

    I'm seeing some mast concepts that are "composite" in nature...developed ply and such. Are these using fabrics, E-glass, S-glass, carbon? Are they coming in lighter and/or "stronger" than the Al benchmark.
     
  9. Skyak
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    Skyak Senior Member

    Carbon adds stiffness. I don't like the idea of carbon on wood because the carbon is so much stiffer than wood, when it flexes, all the strain is in the carbon, which will buckle and de-laminate from the wood. The pictures I have seen used 45 degree fiber angle, which would be less stiff.

    It would make sense to make a stiff carbon structure, and then use wood to make an aerodynamic or aesthetic shape over it.
     

  10. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Joey,

    I am sure I have written a number of times about the science of lay-ups for masts, and I haven't tried to find them again here on this forum. First, if you have not already, read my article on the State of the Art of Freestanding rigs on my website; that will give you a lot of basic information: http://www.sponbergyachtdesign.com/StateoftheArt.htm

    Basically, you need 0°, +/-45°, and 90° fibers in the laminate. For round tubes, you can have an 80/10/10 split. The middle 80% is the unidirectional (0°) fibers, and these should be sandwiched between layers of circumferential (90°) fibers, and outside of that on the very inside and outside layers, the double-bias (+/-45°) fibers. For non-round sections like wingmasts, the narrower they are (longer chord to smaller width), the more they have to have a split towards 60/20/20, following the same kind of lay-up with the unidirectional in the middle, and the double-bias on the outside. Correspondingly, as more and more off-axis fibers are included in favor of the unidirectionals, the overall strength and stiffness of the laminate goes down, and so you have to take that into account in the mast engineering.

    Also, typical hand lay-ups of carbon fiber laminates will be in the 50-60% fiber content by weight range, and better lay-ups under vacuum bag will be in the 60-70% range, 70% being just about the maximum you can achieve. Higher percentages and you just don't have enough resin to hold the fibers together. A really poor laminate will be in the 40-50% range. Again, strength and stiffness numbers go down with decreasing fiber content.

    I hope that helps.

    Eric
     
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