Changing Catamaran Beams from Wood to Composite.

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

  1. Mark O Hara
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    Mark O Hara Junior Member

    I have built a Ply/Epoxy/Solidwood Pahi 63 (Build License # 36) and after a good few years rot is really beginning to hammer the beams which means breaking the whole boat down taking the beams out, repair and reassemble. I'm really beginning to think about replacing the wooden beams altogether with composite alternatives, hopefully making them look identical with a plain white finish (see below), just as strong, but a lot lighter and a lot more impervious to rot. Any ideas? 101_0905.JPG
     
  2. fallguy
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    fallguy Senior Member

    Coosa and glass. But they have to be spec'd by a naval architect for that boat.

    or aquaplas

    or any number of high density foams
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Aluminium.
     
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  4. fallguy
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    fallguy Senior Member

    Yeah..if you can get it to fit;fab to fit..great option
     
  5. Mark O Hara
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    Mark O Hara Junior Member

    Thanks but not a fan, I build in wood or composite and I can repair both. This is my own boat so I'd like to be able to repair it if and when I have to, regardless of what remote pacific island I end up on.
     
  6. fallguy
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    fallguy Senior Member

    You could use the old beam as a plug and build a open mould. I'd still get a na to spec it out and help plan the layup and how to close it.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Noted.

    But it will weigh much more.. as you'll need to ensure the beams don't deflect too much in a large sea. As composite is a low modulus material.
    So long as these issues are understood from the outset... shouldn't be a problem.
     
  8. catsketcher
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    catsketcher Senior Member

    Looks like a nice boat. I reckon you could redo the beams, in wood composite, but this time make the beam so it can't rot. This means, no or reduced fasteners, all beams glassed, extra glass on abrasion surfaces.

    I would steer away from alloy tube beams, of course it can be done but there are also cats with alloy beams which need to be replaced after a decade or two (or three). For us mortals, we have to use alloy tube we can find, which can be really hard. Then we often have to use over specced alloy beams instead of the correct engineered beam because the chance of you finding a tube of the exact correct dimensions is pretty much impossible - so you have to go heavier than needed. Then there is the problem of not being able to reinforce the ends without welding. Alloy beams have been used for decades but for fast and strong beams wood composite is used before alloy today. No racers I can think of use alloy anymore. Even alloy forebeams are rare nowadays on custom cruisers because composite ones are better integrated, easier to reinforce where needed and built for an exact fit. You can then integrate your mast hardware, tangs, tracks, walkways etc which you have to add on the alloy tube.

    On top of this you then have to adjust your beam sockets to fit the alloy beam. So I would not do alloy.

    How did the beams rot? - I know a Wharram owner said that a Wharram he knew of had issues with rot and I think the beam shelves and the notches in the beams could hold water but when done well there should be no entry point for the water into the timber. I would suggest remaking the beams of the same construction, maybe updated slightly with double bias glass and unis, but have the whole totally encapsulated in glass with only composite fittings - no bolts and fasteners ever will ensure you don't get rot again. But where they rotted is very interesting and very important to know how to fix the problem.

    Then when you want to sell your boat is a better Wharram but not a Frankenstein with a questionable beam system.
     
  9. fallguy
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    fallguy Senior Member

    I do have one question. If a wooden beam is encapsulated well in glass n epoxy; as long as the timbers are dry at the encapsulation; really, no rot should occur, right?

    I mean, going away from timber for rot issues is a bit of a false idea.

    I wouldn't forego wood unless you can't source dry lumber or you can't encapsulate it well, etc.

    This from a guy building a foam boat...

    I see catsketcher posed similar.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    I suspect that the wood used was not properly air dried to begin with and maybe even had termites or other organism infested in it from the start.

    Wood.. when properly dried, is a very versatile material.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    What species are the beams ? The obvious choice seems to be select more durable timber that has the necessary strength as well.
     
  12. Mark O Hara
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    Mark O Hara Junior Member

    Well is is a Wharram so the whole thing is designed to move and flex, general wear and tear of the protective coating, under the tropical heat the expansion and contraction of the wood, even a hairline crack is enough to let the warm tropical rains in and then you're ringing the dinner bell for rot, fungus and all manner of wood boring insects. Every rainy season here is like mushroom season on my boat! The occasional repair on the hulls and the deck-pods I can tolerate but the beams means taking down the masts dismantling the whole boat to get access to them. Here is the original beam plan,

    The drawing shows the original plans where in the cross section the brown is solid wood and the yellow is ply, the ply faired out a lot better than the solid wood, the solid wood was Red Lauan which is okay. Calantas would be ideal but very hard to get and bloody expensive!
    Pahi 63 Beam Plan.jpg
     
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  13. Mark O Hara
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    Mark O Hara Junior Member

    I was thinking along those lines. It would give me the benefit of a gel-coat finish too. You're right I think I'd need an NA for the laminating schedule. Here is what I have come up with:
    Pahi Composite Beam.jpg
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Hell's bells, that is a complicated beam, I think I'd prefer a simpler solution.
     

  15. Burger
    Joined: Sep 2017
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    Burger Junior Member

    Getting a qualified expert design/layup schedule will likely cost more than a new set of as-designed in wood, but they will give you lifetime fit-and-forget beams.
     
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