Catamaran beam construction

Discussion in 'Multihulls' started by rturbett, Jul 11, 2021.

  1. rturbett
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    rturbett Senior Member

    Rebuilding a Shark catamaran, and found that both the main and rear beams need to be replaced.

    We traditionally make these out of 7/8" thick mahogany- and I can go that rout. I'm curious though about creating a beam with a foam core , mahogany around the perimeter (because there are some bolt through hinges and bridge pieces screwed in) and then wrapping it in a carbon fiber layup.

    beams are just shy of 10 feet long, flat on bottom. Main is 12 inches tall in the center, arching down to 5" on each end. Rear beam is 8 inches tall in the center, 4 1/2" on the ends. These beams are cut in the center, as Sharks are designed to be folded. That means I am rally just making 5 foot beams.

    Is there a way to calculate how many layers of carbon fiber would be needed? I was thinking three on each side, with the middle layer weave pattern rotated by 45 degrees to add resistance to twisting.
    this will be hand laid, with overlapping at the top and bottom

    Any thoughts ? I was hoping to use US composites 5.7 oz materials
    Thanks,
    Rob
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Here is a link to the Shark Cat on Sailboat Data for reference.
    https://sailboatdata.com/sailboat/shark-catamaran

    Re the dimensions you give above, are these for the mahogany only beams?
    Re how they fold, is it just a simple hinge in the middle, with some type of locking device?
    Could you perhaps post any photos of typical beams?

    I am wondering if you need to go to the expense of using carbon fibre - I would have thought that an 'ordinary' glass / epoxy lay-up to create your sort of 'triangular top hat' shaped beams should be enough?
    The most loading will be in the centre, at the join.
     
  3. rturbett
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    rturbett Senior Member

    The dimensions are off the plans- I have rebuilt a few sharks already, and are very familiar with the mahogany beam construction.
    There is a hinge at the bottom center of each beam- the only lock is the bolt at the top of the rear beam, where we also attach our block.

    I could go glass, but there is an interest in the fleet in "modernizing" the boat. Also, the fibergalss hulled boats always come in over class weight, so I would love to shave pounds where I can.

    it looks like the carbon cloth- at roughly $20 yard- (60incheses wide) is not cost prohibitive. The foam seems like it will cost as much- anyone have a good source they wish to share?
     
  4. guzzis3
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    guzzis3 Senior Member

    Re engineering cats is never trivial and racing beach cats especially so. You would do well to read up on the experiments done on tornadoes over the years, for example the cedar cored hulls windrush (I think) built some years back..anyway..

    To make glass beams comparable on strength stiffness (more important) and weight you need to engineer them properly. Uniaxial top and bottom and 45/45 around the inside outside front and back, possibly with a light core front and back would be good. Woven cloth would be a waste of time. It is VERY easy to make glass or even carbon beams in that size as heavy or heavier than timber, which is why so many beach cats carry aluminium beams.

    The shape of your beams is unusual. It'll take some thinking. Just substituting carbon cloth for timber would be a disaster. I assume those hulls are double diagonal cedar ? Hard to beat in that size.
     
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  5. fallguy
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    fallguy Senior Member

    No cv here, but based on my builder knowledge, the insistence on mahogany exterior renders the rest of it rather silly.

    The gains in using foam are done by wet bagging or infusion and reducing resin content,

    If you need fastenings, you locate the fastening cores as needed where needed, not et al. And let's just say you use 9 yards of 6 oz cloth to keep it simple, if you wet bag, you cut resin use by about 18 ounces, so you are playing a game of ounces and each part of it must be done to gain much.

    Anyhow, I may be driving you back to timber, but if you get a part specified in carbon; the outer mahogany seems rather silly.

    I recommend you get out the excel spreadsheet and write up all the ways to make it lighter as option a,b,c.

    Somewhere, there needs to be a load calc done and I'd say that is a starting point using the mahogany beams as a minimum as well.
     
  6. rturbett
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    rturbett Senior Member

    Thanks fallguy- good advice.
    Fastening cores make sense, but putting the full strip takes the thinking out of it, and lets me put known strengths at the points I need it. (at least that's the logic I was telling myself)

    Perhaps the best way is to just put an outer layer of carbon fiber on- and scare the other competitors with my high tech beams!

    I did just pic up a piece of sapele to make a sample with- we'll see how it turns out- I can compare it to the beams I remove.

    Rob
     
  7. redreuben
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    redreuben redreuben

    The Golden Rule of high performance parts is;
    If it breaks its too light and,
    If it doesn’t break it’s too heavy
     
  8. guzzis3
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    guzzis3 Senior Member

    ROFLOL. So your real name is Colin Chapman ?
     
  9. rturbett
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    rturbett Senior Member

    Wow- just read up on the story of Colin Chapman- I think we have 3 or 4 of him in our Shark fleet!
     
  10. redreuben
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    redreuben redreuben

     
  11. redreuben
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    redreuben redreuben

    Oh the laughing emojis didn’t make it !
    So LOL
     
  12. rturbett
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    rturbett Senior Member

    I wish I was kidding...... FYI- I had nothing to do with this well engineered weight loss program
     

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  13. rxcomposite
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    rxcomposite Senior Member

  14. redreuben
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    redreuben redreuben


  15. rxcomposite
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    rxcomposite Senior Member

    "Hybrids using wood and advanced composites, including aramid, S-glass, and carbon fiber, are being EXPLORED".

    The book does not say so.
     
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