Plywood Power Catamaran Scantlings

Discussion in 'Boat Design' started by Paulo, Aug 3, 2016.

  1. Paulo
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    Paulo Junior Member

    I am a great fan of plywood and all it is possible to be achieved with it in boatbuilding. When plywood is combined with epoxy, then the sky is the limit. However, when it comes to scantlings for catamarans built with plywood and epoxy I have hit a wall. My question to you is: What is the best scantling rules for catamaran built with plywood and epoxy?

    I know what many of you are thinking: it would be nice to have specifics about the catamanran to be build. I don’t have a finished project. What I have is a good start:
    - Offshore Cruising Power Catamaran
    - LOA: 13.5m - BOA: 6.0m - Draught: 0.85m - Displacement: 7000kg
    - Diesel inboard on shaft - cruising speed: 15knts - max speed: 20 – 23knts
    - Hull skin: plywood with epoxy bonding and glass sheathing (plus glass for reenforcement?)
    - Hull structure: plywood with epoxy bonding - crossbeam spacing: 1050mm - frame spacing: 1050mm (??)
    Obs.: Longitudinal stiffening can be incorporated into the hull form – hard chine, inverse chine and deck to hull bonding – so that no panel is wider then 750mm, and 1000mm in length.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    In the absence of any verifiable rules, your best best would be to make up some samples of what you are proposing. Then test them to establish the strength. Then use software such as LR's SSC rules or similar to establish the pressures that could be expected. Then design the structure based upon the pressures from the SSC rules and knowing your E and I you can arrive at a sensible set of scantlings.
     
  3. Rurudyne
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    Rurudyne Senior Member

    I seem to recall a recent thread where catamaran scantlings were referenced.
     
  4. Paulo
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    Paulo Junior Member

    Ad Hoc,
    From what you are seing, am I to understand that there is an awful lot of guestimating to be done before I can arrive to any kind of sensible results. (I will look into testing panels and how I can achieve so kind of useable results without testing machinery - to which I don't have access to).
    I worry how the insurance companies and certifying agencies will view my experimental results . . .
    I was kind of hoping for a simple table like the good old Lloyds rules, you know? :cool:

    I wonder, if I could approach these panels as composite panels with plywood cores and go from there . . . ?

    And

    Rurudyne,
    I am actually not surprised. I have read a few about this subject and they were about GRP or Aluminium boats. From what I could gather, there is an awful lot of doubt about this subject for those materials as well. Especially because the ISO rules for multihulls is still to be completed - no publication dates in sight!
     
  5. Richard Woods
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    Richard Woods Woods Designs

    There are lots of successful wooden powercats around. Tennant, Kelsall, Schionning, Hill, Oram, even I have a few. So you could get some plans of those existing designs to help you. Or do it all from first principles as AdHoc suggests.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. Rurudyne
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    Rurudyne Senior Member

    If it wasn't for those darn bridge decks they'd be simple.

    If you think about it, it shouldn't surprise either.

    Imagine you wrote scantlings rules for multihull construction and a bunch of amateur designer/builders applied them ...

    ... now imagine something went wrong and even though there was nothing wrong with your rules, maybe even if they were used incorrectly, you still found yourself staring down a bunch of lawyers just because they were used?

    If I sound cynical consider ...

    Back in school one professor of mine described a consultant as usually someone from 50 or more miles away, who had a beard, who will take a week and several thousand dollars to tell you something you could have looked up in your engineering library in maybe an hour ... but who signs some paper that spreads the liability around a bit.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    That's part of it yes. Since what data is out there...and then once you find said data, what is the sample base for verification of that database? Minimal at best..

    Then once you decide ok, if i need insurance etc...how best to achieve and arrive at a conclusion that satisfies these insurance companies?

    Hence the approach noted above.

    Insurance company would go:

    1) Where did you arrive or calculate your loads from?....LR SSC Rules....check :)

    2) Where or how do you know what your material properties are.....built and tested them at an approved and independent test house.....check

    3) How did you arrive at the scantlings....used LR SSC rules by inputting the material properties from #2 which calculates the load as noted in #1 and then provides me with a set of scantlings, all based upon LR rules.....check.

    That's about all you can do..
     
  8. Paulo
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    Paulo Junior Member

    Richard,
    thank you for your post. I am aware of your achievements as those of the other designers you mention and I would include Brady as well. I admire your work very much.
    Preciselly because I do I would like to understand more about plywood construction specifically aplied to catamarans.


    Rurudyne,
    I am not sure if you are serious or you are making fun of me. The american sence of humour excapes me. Either way, I thank you for your post.
    Rules are writen by classification societies around the world, so that rules writen by people like yourself are not required by any kind of boaty people.
    I am a professional boatbuilder and as such I like to understand the materials I am working with. I build composite power boats and I have a check system to ensure that what is specified in a project is correct and that I am not building any mistakes.


    AdHoc,
    I understand what you are saying and I thank you.
    I don't have access to testing facilities locally, but I get it.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    You would have got a different response if you had added that paragraph to your first post!

    Where are you based?

    RW
     
  10. Paulo
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    Paulo Junior Member

    Richard,
    I am based in Brazil.
    I am a Southampton Institute graduate in YMM.
    (I didn't know it matters this much.)
     
  11. TANSL
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    TANSL Senior Member

    Here many people are obsessed with knowing the title/degree you have to respect you or not.
     
  12. Richard Woods
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    Richard Woods Woods Designs

    a lot of people reading/posting here have no knowledge of boatbuilding or design at all.

    So regular posters, like me, Adhoc etc always assume the OP knows nothing. Clearly not the case with you

    BTW I did the yacht design course at Southampton, graduating in 1978

    Richard Woods
     
  13. Paulo
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    Paulo Junior Member

    I was in the last ever Yacht Manufacturer and Management Class and graduated in 2001!
    I always consult the old Lloyds Rules to ensure there are no mistakes built into a new project or to find out what I should have in a repair or rebuild project. Particularly with old style wooden boats.
    Don't have the same luck with catamarans though, hence the question posted here.
     
  14. Rurudyne
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    Rurudyne Senior Member

    I was most definitely NOT making fun of you. Please don't think that I was.

    Also I wasn't meaning to indicate that I'd written any scantling rules, though I might have been thinking about folks like Gerr. I was also not talking about professional builders such as yourself misusing published rules but amateurs.

    Again, my intention was not to in any way mock you and I'm profusely sorry if you thought I was.

    The post was intended to merely be conversational in tone, nothing more.
     

  15. bergwerk
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    bergwerk Junior Member

    Richard, I think Bob Oram may no longer be active, his websites have been taken down.
     
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