Load Paths in a Catamaran

Discussion in 'Multihulls' started by AndrewK, Feb 24, 2009.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Rick

    It is very basic. For a moment to be applied, be it bending moment or torsional moment, it is force times a lever. If the lever is zero the moment is zero. It remains a simple shear force.

    Opss, sorry in my haste to reply...just noticed above i wrote pure bending, should've written pure shear (didn't read it before i sent it). Since the example above, the applied load is directly above the beam, if single beam at midships. The lever is zero. Only your hulls will experience a moment. The lever being from the support block, in red, to the applied load at midships!

    I have been happily designing multihulls for 20 years. I am more than happy with the structural solutions, as are my clients.
     
  2. Guest625101138

    Guest625101138 Previous Member

    You should provide a list of the boats you have designed because it will be a warning to others not to go near them.

    The moment that has to be carried by the interconnecting structure as I have sketched it is 2T by 6m. That is 120kNm - significantly above zero.

    This interchange should clearly show anyone contemplating buying a catamaran to understand the load cases and challenge the designer how they are resolved in the structure.

    Not understanding the torsional requirements is a common failing (with mono and catamaran or any structure for that matter) and the reason the structures crack around window frames and other openings. If the forces are not properly resolved in main structure then the stresses show up elsewhere and things crack.

    Rick W
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I refer you to my previous reply...i rest my case.
     
  4. Guest625101138

    Guest625101138 Previous Member

    I can now see why people like you choose to use silly names on this forum. You run a real risk of losing business if people knew how little you actually know about structural design.

    It should serve to any readers that they need to challenge so-called designers and press to get the right answers. Don't be fobbed off by claims that they know what they are talking about. If they cannot show calculations and explain it in simple terms then they do not know what they are doing. Keep clear and find a professional who can explain it.

    Rick W
     
  5. sigurd
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    sigurd Pompuous Pangolin

    Ad Hoc,
    I didn 't know the I-beam would be as strong in transmitting torque as a box/round beam. Interesting.
    When it comes to the single beam applied on the sketch of Rick W, I think he is right it transmits torque, I see the two hulls as two cranks on a shaft (the beam), each being cranked in opposite directions - maybe I didn't understand your explanation. I too am very interested in seeing the calculations of such a case.

    Marshmat,
    You explained what I tried to say much better than what I did with the greased round beams.
     
  6. Guest625101138

    Guest625101138 Previous Member

    The torque is 120kNm and has to be resoloved some how. Ad Hoc has decreed it as zero and can be neglected. You cannot just decree such things no matter who you are. Also 120kNm is not a moment to trifle with. This is only a 4T boat so a huge moment relative to the size of boat. This is of the same order as the maximum healing moment.

    If this moment is not resolved adequately it will cause all sorts of structural problems. Such problems are common due to poor understanding and result in the sort of problems Stefano describes in post #15 on this thread.

    Rick W
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Sigurd

    "...Ad Hoc,
    I didn 't know the I-beam would be as strong in transmitting torque as a box/round beam. Interesting..."

    You have mis-read, or misunderstood, please read again.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    "...It should serve to any readers that they need to challenge so-called designers and press to get the right answers. Don't be fobbed off by claims that they know what they are talking about..."

    I could not agree more. Those that wish to "talk numbers" rather than discuss the structural philosophy and principals clearly have no understanding of the aforementioned. My number is bigger than your number....so what??!!!...without context it is totally meaningless.
     
  9. sigurd
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    sigurd Pompuous Pangolin

    Maybe it is one of the words I misinterpret... When you say it is not stronger, then what do you mean exactly, if it is not that it is weaker or the same?
     
  10. sigurd
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    sigurd Pompuous Pangolin

    "without context it is totally meaningless."

    Rick already described the context he wished to discuss?
    And you say there is zero torque transmitted by the single beam, if this is right then I have no idea what the term torque means!
     
  11. Guest625101138

    Guest625101138 Previous Member

    In the context I have described the moment to resolve is 120kNm. You say it is zero. Your philosophy and principles are giving you a seriously WRONG answer. Having the numbers provides clarity on how poor your understanding is.

    If you have conviction in your philosophy and principles provide your name and list the boats you have designed. It will then be up to those wishing to select a boat whether they give your designs a wide berth or not. Let them be a judge of your philosophy and principles.

    Rick W
     
  12. Guest625101138

    Guest625101138 Previous Member

    Matt
    If the x-beams are in the same plane they do not resolve the moment I have described.

    Like I said initially, I am yet to see something as neat as a single torsional beam located around midships. Everything else results in something more difficult to analyse and I think less efficient. But I am prepared to be convinced otherwise by proper analysis.

    I know I am repeating this but I have seen too many structures crack up because of poorly resolved moments and cannot overstate its importance in structural analysis.

    Rick W
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Sigurd

    "..And you say there is zero torque transmitted by the single beam, if this is right then I have no idea what the term torque means!.."

    May be you need to read my explanation above maybe several times, if English is not your native language.

    I shall elucidate.

    The assumptions shown in the picture has a load applied at midships, a single beam also at midships.

    If you apply a load directly above a beam, remember we are discussing structural theory here, then there is no moment. The applied load passes directly into the beam as a vertical shear force. It is not an eccentric load case. The beam, as in the theory is assumed to be a small structure member, sufficient to apply the load.

    The case shown in the picture, whilst slightly analogous to a dynamic case, is not strictly correct. Since the picture shows a static load case. The load is applied in a structurally "simply supported" manner. The vessel at sea is a "free-free beam" and behaves differently.

    With respect to "..Rick already described the context he wished to discuss?.."

    No, Rick just wants to discuss numbers as that is palpable to him, rather than discuss the structural theory behind his "assumptions". Since numbers do not equate to a theory, numbers can be just plucked out of a hat. One can only concluded a lack of understanding of the structural principals, as his unwillingness to discuss the theory.

    Theory is devoid of numbers and lay ups or material properties or scantlings etc, it is simply that, the theory of what where and why. I don't know about you, but I was never taught "numbers" when doing structural theory at university nor do I when i lecture post graduate students structural theory and practical analysis too.
     
  14. grob
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    grob www.windknife.com

    I’m interested in finding out where Rick is going with this so I will post some numbers to further the discussion,

    Using the 12m x 6m Cat example, with two beams effectively acting as cantilevers positioned at the bow and stern, Aluminium beams have OD 300mm ID 280mm, if the load at the centre is 2000kg then the load at the bow and stern is 1000kg, a cantilevered beam experiencing 6000kgm of bending has a stress of 93MPa and a deflection of 110mm.

    A beam in torsion of similar mass would have an OD 600mm and ID 580mm, the torque would be 12000kgm, this would have a max stress of 22Mpa and the deflection at the bow and stern would be 103mm.

    It is the deflection that is important here not the stress and to my mind the deflection is pretty similar in both cases, Rick do your numbers give a different story to mine? If so could you present them.

    The caveat here is that there are other loads to take into consideration and while a single beam may be ok to handle these pure torsion loads it is not the best solution in my opinion for other loads mentioned in this thread.

    All the best

    Gareth
     
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  15. sigurd
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    sigurd Pompuous Pangolin

    did you add the torsion resistance of each of the two beams, or is it insignificant?
     
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