Beach Catamaran Beams from Aluminum to ply/composite?

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

  1. rxcomposite
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    rxcomposite Senior Member

    You are right. It's fatigue resistance is 2X better.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Correct, that is called the fatigue limit.
    This is nothing new, it has been known about for decades...

    And that's the point. Any material can fail.. it is the engineering design behind the material that makes the product, whatever it is, a success or a failure.
    Ignorance of deflections and fatigue will always make it fail sooner rather than later..again, no matter the material.

    That is just one of many possible failure scenarios.

    Kind of...but at the same time, in doing so, they tend to become cost prohibitive in their mitigation of such.
    Everything in design is a comprise.

    If one looks at the "pure" material under classical loading for mechanical fatigue properties, then yes.
    But if one looks at the "whole" design cycle, things are very different.

    Composites require a significantly greater input into the QA - this leads to a much greater tendency for the final product to fail, by whatever means, before its anticipated 'design life'. The QA side of design is where most product fails. The material and its properties are just one input into the long chain of events that are performed (design wise) in going from an idea on a piece of paper to the final finished product. One only has to read the threads on amateur boat building projects on this site to see the problems and issues many have with temperature and humidity etc of just the resins alone..that's before going deeper into layups and handling etc.

    Thus:
    And by 'design' I mean the total life cycle from inception of the idea to the finished product.
     
  3. guzzis3
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    guzzis3 Senior Member

    Now I'm not sure what your point is.

    I spent my working life as a design engineer. Much of it doing research, building experimental apparatus, but also in design in production environments.

    This thread started as a discussion of re-engineering aluminium beams in plywood or composite. The boat is a small catamaran.

    If you truly want to optimise the design and manufacture of a couple of small beams for a little cat that can become a vast undertaking. The suggestion I made is cheap and easy so you can just try it and see. If they are either too heavy or break you can just make some more for not too much money.

    There are good reasons why lots of cats have aluminium beams. They can be cut from mass produced standard sections making them cheap. They are tolerant of damage and give a long service life. But they aren't perfect. They corrode. They fatigue and crack. They need support around bolted attachments which can be problematic.

    If you look at every aspect of catamaran design you will find a range of good options. No one is right for every situation. This is why builds in ply, foam and solid glass are common. Few people build cold moulded catamarans anymore because they are easier options which deliver usually better results. There are always people experimenting, and good on them. That is part of what this forum is about. But when you're done experimenting and want a good cheap easy solution the parameters for these are well established.

    There are regular threads on this forum where someone new to multihulls has asked a question or is about to undertake an often committed mistake. They get a warning, usually offered with manners. This is often the "I plan to build an impossibly big bridgedeck cabin catamaran", or "I intend to spend a tremendous amount of money and time building something" that probably won't work. This is a small manageable project and failure won't be catastrophic. I hope the OP proceeds with it.
     
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  4. Ad Hoc
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    Ad Hoc Naval Architect

    It is this:

    This true for every material.
    It appears you have focused upon aluminium as being "the bad guy" and provided a list of attributes to look out for, as reasons why

    It is about good design, no matter the material, and such 'check lists' are common to all, it is not material dependent.

    Design does not focus on one attribute and assume the rest is taken care of, merely because, in this case, it is not aluminium.
    Every engineering design requires the designer to investigate the 'check list' of compliance for a safe and fit-for-purpose design, to ensure it does not suffer its known common issues, regardless what the materail is.
    And if it works - it works. Job done.

    Every material has good and bad characteristics... ergo, so what?
     
  5. Russell Brown
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    Russell Brown Senior Member

    The guy just wanted to build some beams for a little plywood cat. Why go down rabbit holes instead helping him with an easy solution?
     
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  6. TANSL
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    TANSL Senior Member

    Because they are not capable of giving it, they do not know it but they do know how to speak, speak and speak. The OP should say something along the lines of you: "OK gentlemen, stop telling me gruesome stories and give me a solution suited to my possibilities." (By the way, I don't have it either, sorry but I humbly acknowledge it. That is over my "fatigue limit", whatever that is).
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    That has already been given HERE for those that are interested.

    However, when said rabbit holes emerge, wherever they arise, contain misleading information, then it must be highlighted, to prevent those reading from being mislead.
     
  8. TANSL
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    TANSL Senior Member

    An easy solution, not a material resistance treaty. We can all refer to books, lectures and studies, that's easy!
     
  9. fallguy
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    fallguy Senior Member

    I'll take some credit for the rabbit holes.

    This is a place for the collection and sharing of knowledge. Sometimes, thread drift occurs, generally it is edifying.

    If the fellow wants new beams, he even gains some knowledge on aluminum it seams [sic]. And he did ask about how aluminum compares in the second part of his post.
     
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  10. Russell Brown
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    Russell Brown Senior Member

    Great response from everyone.
    I never go down rabbit holes. ****, my nose is growing...
     
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  11. garydierking
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    garydierking Senior Member

    Here is the crossbeam I use for the catamaran version of my 20' (6M) Tamanu design. 3/4" timber top and bottom, 1/4" ply on the sides.
    [​IMG]
     
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  12. Russell Brown
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    Russell Brown Senior Member

    That's my kind of cross beam!
     
  13. fallguy
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    fallguy Senior Member

    Is the beam glassed to extend life?
     
  14. garydierking
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    garydierking Senior Member

    Glassing is optional and not needed structurally , but will extend the life of the paint.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Why is it that everytime I take a look at this I fall into a rabbit's hole?

    And this is because it is a small boat and a sailing one at that.

    If this is a powerboat, with a cabin or superstructure, the crossbeam will be a simply supported beam. But this a small boat with a naked beam and the crew weight can be a substantial part of the load.

    If it is a single mast in the center of the crossbeam, The load will be a concentrated load in the center with the side stays pushing the mast down, like a drawn bow and arrow. The center part of the beam is thick. This stay tension force could equal or exceed the displacement force.

    Depending on where crew is, this becomes a constantly moving load. A live load.

    If this is unstayed mast, with one per hull, the moment or the center of pressure will be coming from about 1/3 of the height of the mast. If one hull lifts out of the water due to a gust of wind, this becomes a cantelever load. Not to mention that a crew could be hanging her/himself out of the hull. The thickest part of the beam will be where it connects to the hull.

    What if the hull is hit by a 45 degree wave? This is torsion as part of one hull lifts itself out of the water. More of combined load actually.

    And I'm still in the Load model analysis?

    It goes deeper and deeper when I choose the material to be used.
     
    Last edited: Jul 27, 2021
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