crossbeam flexibility

Discussion in 'Multihulls' started by pitbull, May 30, 2020.

  1. pitbull
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    pitbull Junior Member

    I've been reading this: https://www.sailingcatamarans.com/index.php/faqs/15-general-questions/88-crossbeam-design. And trying to work out the amount of movement to expect in the catamaran under load.
    The two torque load cases I have considered are "half the displacement x half the LOA x g" ie. what I think would be the effect of propping up each corner of the catamaran and then removing the prop under one bow. And also a torque I calculated based on this explanation: https://www.academia.edu/24915656/Scantling_calculations_in_catamarans of the lloyds Rules and Regulations for the Classification of Special Service Craft.

    Both of these give me a torque in Nm that I can apply to the crossbeams and calculate how much the bows will move relative to one another out of plane. Strength is one thing, but when it comes to designing for flexibility the only guidance I can see is that under load the bow should not drop off "appreciably" and of course that the cross beams should not break !

    Can someone help put a number on it, for a 12m catamaran what would "appreciably" be, how far do catamaran typically hulls move out of alignment when sailed hard ?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    There is an article all about this in Professional Boatbuilder Magazine, Dec-Jan 2020.

    Suitable limits are shown here:

    upload_2020-5-31_8-27-48.png
     
    bajansailor likes this.
  3. pitbull
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    pitbull Junior Member

    Thank you, that's really helpful.

    Do you happen to know if that is for the "3-point support" test ? and if so are these tests are done with the boat in "light displacement" (tanks empty, no-one aboard, no cargo etc.). Or was this deflection for some other derived design torque ? I'll have to see if I can get a copy of the back issue of the magazine.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    The deflection quoted is based upon the displacement from horizontal...to the max displacement and the location of the displacement is taken at the extreme ends.

    The loads assumed in the calculations are full load condition.
     
  5. pitbull
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    pitbull Junior Member

  6. Ad Hoc
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    Ad Hoc Naval Architect

    It all depends on the purpose of the beam's stiffness and the duty it must perform.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    BlueBell likes this.
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Were you able to get a back copy to view this article?
     
  9. pitbull
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    pitbull Junior Member

    I have emailed professional boatbuilder magazine but they have not replied yet. I think they are struggling like everyone else with the restrictions imposed with the virus.
     

  10. Ad Hoc
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    Ad Hoc Naval Architect

    If you send me your email address, Im happy to send you my digital copy...

    If you send me a PM.. Private Message or a "conversation" as it is is called now o_O... then you don't have to have your private email address posted in a public forum.
     
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