Beach Catamaran Beams from Aluminum to ply/composite?

Discussion in 'Multihulls' started by Burnside Style, Jul 16, 2021.

  1. Russell Brown
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    Russell Brown Senior Member

    Of course you can build good beams from plywood & lumber and not have to do a ton of engineering. If done well, the glue cleats in the 4 corners can take the loads and the plywood can make the box. Many thousands of good multihulls have plywood and lumber crossarms. Can you post images of the boat and recommended crossarms?
     
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  2. rxcomposite
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    rxcomposite Senior Member

    Here is how to build from lumber and plywood. 3 plywood for the sides to take care of shear and lumber for end caps (tension/compression). It will be heavy.

    Wood properties are hard to predict but the engineering is easy.
     

    Attached Files:

  3. Burnside Style
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    Burnside Style Junior Member

    Thank you for the links and attachments, I am reading over them, will post any questions that I have.

    My goal is more on the experiment in design, learn to work with exotic fibers side of things. Not necessarily into looking for weight reductions at any/all costs..

    afloat!.jpg ready+to+go.jpg new+tramp.jpg
    Above are images of the Duo425 and Duo480.
    The beams are specified as round 70mm od, 64mm id
    Or at some point the design changed to rectangular beams mounted in hull because the drawings show them being about 67mm x 94mm
    Screen Shot 2021-07-18 at 3.36.33 PM.png

    Long term, I am building this smaller bi-rigged catamaran as a test on the way towards building the larger Duo 800.
    I would like to see how this beam material swap goes on the smaller build first and learn to be able to scale it up and build the beams for the larger cat later on.
    Initially my head went to the type of beam that Russell was describing, and I have read about that type of beam with uni's top and bottom, wrapped in biaxial.
    Below the first image with the blue and yellow sails has the aluminum round beams (140mm round, 132mm id) The other two's beams are built differently.
    2-DUOreadygomedium.jpg Duo800.jpg DUO+800++stepping+the+mast.jpg
     
  4. guzzis3
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    guzzis3 Senior Member

    The critical element is stiffness not strength. Aluminium beams on any cat only fail in extraordinary circumstances, but underdesigned beams flex too much and as you might know all aluminium will fail eventually even subjected to the lightest loads.

    So the equivalent cross sectional area is adequate provided your fibre orientation is right, and because glass is that much less dense than Al the beam ends up lighter.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    Yup. Aluminum is hard to beat even in aircraft structure. The overall modulus of composites is lower than alloy thus tends to be bulkier (therefore heavier) to get the same stiffness.

    To beat aluminum, design with carbon fibers and glass prepregs, and cores.
     
    Last edited: Jul 23, 2021 at 2:20 AM
  6. fallguy
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    fallguy Senior Member

    -all aluminum will fail eventually even subject to the lightest loads-

    sort of a disheartening comment to a guy with massive aluminum crossbeams
     
  7. fallguy
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    fallguy Senior Member

    This was my point!

    I can't fathom a fiberglass beam to replicate the s830 sparcraft that is lighter...
     
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  8. guzzis3
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    guzzis3 Senior Member

    This boat is small so the weight saving probably isn't there but in bigger boats glass beams properly engineered are probably better than aluminium.

    -all aluminum will fail eventually even subject to the lightest loads-

    Yes they will fail eventually. The crystalline structure of aluminium is different to steel. One is center face cubic the other isn't. I can't remember. Decades of memory loss plus my recent strokes, I am sorry. But you have to take it in context. It might take 100 year for the aluminium to start cracking, it depends on the load and frequency. Boat designers use aluminium beams because they are relatively cheap, readily available and give decent service and strength to weight. They are tolerant also of mistakes in manufacture and things that happen in service, but if you have a larger boat properly designed and built you will probably get a lighter stiffer structure with glass, let alone carbon.
     
  9. rxcomposite
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    rxcomposite Senior Member

    For the same load and cycles, aluminum will fail first. Composites is much more gradual. About twice as much when aluminum fails.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    All materials will fail, if not designed correctly.
    One must check the deflections and fatigue of each material one is using...

    The BCC, FCC and CPH is only relevant when looking at the number of slip planes and directions.
     
  11. fallguy
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    fallguy Senior Member

    all good

    I have a problem more with the weight.

    If my beams start to fail, maybe I'll be over 80 by then and the boat will be in another's hands. If sooner, I'll need to use them as a plug for something carbon. Oh boy...
     
  12. rxcomposite
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    rxcomposite Senior Member

    That would be 10,000 cycles if you constantly load it to max stress.
     
    Last edited: Jul 23, 2021 at 10:46 AM
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  13. fallguy
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    fallguy Senior Member

    I think I am more worried about the beams operating beyond max stress already. It would be nice to understand it better, now that the can of worms is opened. Also, what happens in a beam failure. The boat has 3, but if one breaks that supports the cabin. It gets ugly. But, alas, this is not my thread.

    But is interesting to think if the beams are underdone for the loads I created; they could he remade in carbon. Not by me and at great cost likely.

    I think I'll pester the designer for the load limits and such.

    Actually, it is a simple calc of a beam under uniform load perhaps.
     
    Last edited: Jul 23, 2021 at 10:23 PM
  14. rxcomposite
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    rxcomposite Senior Member

    Though carbon has a higher modulus than aluminum and is lighter, it is problematic. It is brittle, has poor compression (0.6 of the Ultimate Tensile), zero to negative coefficient of thermal expansion, not compatible with other metals, ect. It cannot even be mixed with Eglass because of the low strain (1.1% against 2.5%). It will pull down the strength of the carbon and will create internal stress because of CTE. Meaning you cannot use glass to its full potential.

    Alone, carbon is king.
     
    Last edited: Jul 23, 2021 at 11:17 AM

  15. guzzis3
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    guzzis3 Senior Member

    Sigh.

    There is a point where the load is small enough so that steel cycles to failure become essentially infinite. There is no such lower limit for aluminium. Even if you tap it lightly with a stick it will eventually fail. Might take a billion cycles but it will fatigue and crack.

    In the real world there are plenty of cats 40 years old, from beach cats to large sailing boats, with aluminium beams that have not failed, but there are also plenty where fatigue has set in and the beams needed replacing. They don't go suddenly, they usually grow stress cracks, usually starting at stress concentrations.

    Any engineering materials text will explain the problem better than I can.

    In boats the life of steel is determined by corrosion. If the boat is properly engineered it should never fail by fatigue.

    I don't know enough about glass to say for sure, but I believe glass beams have a longer life than aluminium. Could be wrong though.
     
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