Changing Catamaran Beams from Wood to Composite.

Discussion in 'Multihulls' started by Mark O Hara, Aug 4, 2020.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks for the data.
    The basic transverse bending moment of the 2 hulls rotating away or towards each other, is a simple enough case and 6 beams will be adequate to overcome the bending moment.
    The problem you will face is that only the 2 extreme end beams will be effective when subjected to a torsional pitch connecting moment.

    From your data ive run a few numbers and each beam, at the ends, will require a modulus of roughly 4100cm3. Using a beam of just 146mm wide and 335mm deep, and using a low modulus material like E-glass, this is not easy to satisfy..... you need more width and depth to the extreme end beams. For example, if the laminate thickness ends up say 10mm thick, each beam only has a modulus of 1121cm3. That is roughly 4 times less than you neeed.

    The issue is the flexibility. You need to satisfy a basic deflection check. Since running these numbers, the stress check is not overly tricky to satisfy... it is the deflection that drives the required stiffness to significantly higher values.
    The last thing you want is the deck flexing as you go over waves!

    Unless you glass the whole deck between one I-beam to the next I-beam. Then you have massive amount of stiffness. Something like this:

    upload_2020-8-6_6-51-46.png

    upload_2020-8-6_6-54-27.png
     
    Mark O Hara likes this.
  2. fallguy
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    fallguy Senior Member

    Perhaps you could go up above the recesses for greater height? Some socket fun I suppose.

    All this is mostly fun for me to watch at this point.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    So, as I understand it, the new beams have to replicate the existing dimensions and shape, so as to fit the rest of the structure ? So you are left with a choice of materials, that will satisfy the mechanical properties, with that shape, and within a weight difference/increase that is tolerable ? I am inclined to think an "I" beam fabricated from alloy, with inserts that "pad it out" to the required profile, where required. Probably a purist's nightmare of inaesthetics.
     
  4. catsketcher
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    catsketcher Senior Member

    That does sound like a huge amount of movement. I am not sure it is required as no other catamarans use such a system. Of course you could get a layup out of uni and core that would do a reasonable job. I have also seen some truly good structures built from inferior timbers - wing masts out of clear radiata was one example.

    I think you can get around the lashings wearing away at the beam ends with composite unidirectional chainplates - like most custom multis nowdays. You can wrap the lashing areas with unitow or uni strips leading to composite deadeyes on the bottom of the beams. Then the lashings would go from the hull to the beam bottoms. The composite chainplates are easy to make. If your beams are repairable then this method would make the current beams usable and rot resistant.
     
  5. Mark O Hara
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    Mark O Hara Junior Member

    Yes, I want the new beams to look identical as I don't want to re-model the whole boat, if it can't be done in composite where it comes in at the same size and weight/or less than, then I'll have to go back to wood, reluctantly.
     
  6. Mark O Hara
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    Mark O Hara Junior Member

    Many thanks for your input, much appreciated. In your professional opinion is it doable in composite or not, without having to change the appearance and lay out of the boat?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, I think I would explore the option of a structural element (Alloy I-beam), "dummied" over with a semi-cosmetic overlay, that has sufficient mechanical properties to withstand compression, e,g., but is not the thing that is supplying all the structural requirements, that thing being the I-beam. As I understand Ad Hoc, your dimensions are marginal at best, for "normal" composites, but there may be specialty materials that can meet it, which is what I took from what Alik said. But, I may be wrong on both counts ! :eek:
     
  8. Rumars
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    Rumars Senior Member

    Use better wood then red lauan, Kwila or Yakal. Change the lashings from 3 strand rope to prestreched flat webbing, it distributes loads better.
     
  9. Mark O Hara
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    Mark O Hara Junior Member

    Kwila is hard to come by in this province kiln-dried, "Ipil" they call it here and usually it's logged illegally, as for Yakal, brilliant stuff, impervious to rot! There are bits of Yakal in this yard that are older than I am, only problem is it weighs a bloody ton! If I replaced all the solid wood on the beams with Yakal I'd increase the waterline somewhat! :D
     
    Last edited: Aug 6, 2020
  10. Mark O Hara
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    Mark O Hara Junior Member

    I fear you may be right! Aluminium may be the most sensible answer and composite may be a far flung fantasy, we'll have to wait and see what the NA's say.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Simple answer - yes. See above suggestion.
    But this requires compromises to be made - which - may not be palatable.
     
  12. catsketcher
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    catsketcher Senior Member

    I am interested in this thread but I don't get why the current design is not updated. I did this on a Newick trimaran that we removed some weight from by using unidirectional glass to remove weight from without reducing strength.
    I have mentioned the problem with alloy tubes but alloy welded trusses would be very problematic. Welding and aluminium is a problem and retempering is helpful to get full strength. Modern cat design is all about reducing metal use - not increasing it. Few (none?) modern composite cats use aluminium, especially welded trusses. My mast maker does not like welding forebeams for cats as he worries about the welds.
    The issue with the rope wearing through the beam ends will not be solved by going composite. The current beams could be rebuilt and a layer similar to the composite beams could be laminated on. It will still be glass being sawn away - one glass laminate has wood underneath, the other honeycomb - there is no difference in abrasion resistance.
    Making composite chainplates is very easy and pretty foolproof. They are often made from E glass uni and don't have to be carbon. No wearing issues then.
    I don't quite get why timber is hard to source but honeycomb, resin, glass and maybe welded alloy trusses would be easier. Gougeon techniques, addition of unis and other stitched fabrics and lamination allow the use of inferior timber. My mate with an 46ft Oro rebuilt his beams and they are still going strong 20 years later with good epoxy techniques.

    Some advice from the experts

    Wood/Epoxy Longevity https://www.epoxyworks.com/index.php/woodepoxy-longevity/

    This guy had the same issue

    Mast and Crossbeam Heroics https://www.sailblogs.com/member/kaimusailing/459756
     
  13. catsketcher
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    catsketcher Senior Member

    Interesting - more research shows that this is a common issue - do Wharram builders glass the beams and epoxy saturate all holes?
     
  14. fallguy
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    fallguy Senior Member

    I'm basically a worthless idiot at this point. Could you increase the top height of the two end beams? All I see pictured is a forward walkway.

    Aft might be trouble.

    ...going back to my spot on the wall now

    D647524C-FD0E-4E99-A470-DD467500340F.png
     

  15. Mark O Hara
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    Mark O Hara Junior Member

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