Catamaran composite beam design

Discussion in 'Multihulls' started by groper, Apr 29, 2012.

  1. groper
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    groper Senior Member

    All valid points, i often wondered the same and it occurred to me that perhaps you have the exact opposite problem of not being able to transfer the shear from the tension and compression caps if they are too wide/far away from the shear web...?

    In addition, the deck and bridgedeck no doubt take some of the load, but id rather design it without these taken into consideration for teh primary structure - i take these for additional strength for an additional safety margin.

    End of the day, many things work, the best way is whatever makes my life easier when it comes to building it!... and the more you can infuse on the table with less messy handling of wet tapes/tabbing the better IMHO...
     
  2. catsketcher
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    catsketcher Senior Member

    Don't forget straight unis

    When I was talking to Peter Ullrich about building Ellen Macarthur's B and Q he was very insistent on a point he called fibre tension. He said that it is not much use using unidirectional fibres of they are not pulled tight during application. He doesn't even like female moulds as he can't pull the fibres tight.

    Put the unis on the flanges and pull tight. Vac Bag them too if you like. You can easily bag a secondary joint if it is nice and flat.

    BTW Why does the uni take an angle and run deep into the hulls? Won't this produce a stress concentration. The stress concentration is dependent on radius of load path. A gentle or no curve with tapered and feathered layup seems better. Why have all the uni end at one point - makes a stress concentration again.

    Make a model of the bulkhead out of perspex and bend it under polarised film with glasses on. The angle will be highly loaded. The bulkhead will be feeding out load all along its bonding edge with the hull so the loads should be directed to the edges along its edges - not just to one point.

    Dissect a chicken leg and look at the tendon attachment to the bone. Nicely spread out and low stress. I have been reading "Biomimicry" again.

    cheers

    Phil
     
  3. groper
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    groper Senior Member

    Nicely spotted... i was "fishing" with that drawing... :D im surprised noone else has said anything about it...? Its difficult to feather out the fibres in practice to such an angle with the size beam dimensions im dealing with. The UNI tapes dont like going around corners when laid flat - even more reason not to do the rear beam this way.

    I considered making a truss from the UD tapes in this corner area, but the overall beam height is by far the most influential element on the main design problem of controlling deflection...
     

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  4. groper
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    groper Senior Member

    Another observation;

    Ultimate strength doesnt seem to be the issue, it always comes back to stiffness as a beam that is stiff enough always seems to have MILES of strength... So i played around with a few different beam sizes trying to reduce the deflection to a more desirable number using rectangular hollow section modulus and noticed in the formula the materials youngs modulus... i was already using 42GPa for UD glass and decided to look up the UD carbon modulus.... i took the conservative compression value of 115GPa from the material data and plugged it in the beam equation... using the same beam geometry, this time in carbon, it reduced the deflection from 29.2mm down to 11.4mm! The stress in the material is still only around 10% of max yeild...

    So, Ill have to ask my supplier what the price of carbon is these days... but last i checked, the UD carbon was actually quite cheap as opposed to the woven fabrics... :idea:
     
  5. BobBill
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    BobBill Senior Member

    I would suggest, as someone noted somewhere, that plain marine ply might remain the best overall choice...unless weight is the main factor. I have been mulling these choices over for some time, and always return to the simplicity, low cost and long term durability of ply.
     
  6. groper
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    groper Senior Member

    Im considering using carbon in the spar caps only, the extra cost would be minimal and the complexity doesnt change as the entire boat is foam cored epoxy laminate.

    The most prudent question i have from here is;

    what loading condition should i design for? AdHoc?

    Currently there is no definitive design loading for catamaran bridge structures...??? Im sure the classification societies have their own rules governing what these structures should be designed to handle, but these are as arbitrary as anything else IMHO... Im looking for a load condition that is not overly conservative like a classification society would impose, as this is a relatively small home built private vessel, and i want it to be very light.

    I tried to find Art Edmonds guideline without success, and the Dinsenbacher criteria is probably too conservative for my needs...

    So, what loading condition should i design for?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect


    You seem to have answered your own question.

    If you elect not to use Classification rules, what else are you going to use in its place as a guide. Not just a guide but one that will provide you with confidence that the structure you calculate and design shall be safe in the most harsh conditions you expect to encounter? And that alternative how widely used is it and has it been proven to be an acceptable alternative?

    Anyone can come up with their own “hhmmm..i reckon the loads should be XX or YY”. But, in doing so, you have to understand how Class arrive at their rules. It is over many decades of trial and error and endless R&D from a vast database. In other words, confidence! If you elect to use a different route for establishing your loadings, does it give you and others, i.e. if you decide to sell her, confidence that the scantlings are more than adequate for any role the vessel has been designed for?

    If you want it “very light”….you only have 2 options.

    1) Ignore all the evidence and R&D from Class rules which are for safe structures and longevity
    or
    2) use another material that yields a lighter structure weight whilst satisfying the structural requirements.

    Or a kind of third option…build what YOU want as light, and pack an EPRIB, Flares, lifejacket/rafts inform the coastguard of your every move and give a drill to all those that accompany you every time you sail. As this is your own “back-up” plan when things go wrong..as you’ll never know when a failure will occur nor where. As Richard said....its your boat, do what you like.

    As for UDs in shear webs…this is wrong. They should be biaxial +/- 45 degrees fibres.
     
  8. groper
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    groper Senior Member

    For the record, the idea was to include the caps on the sides edges, top and bottom, of the shear web... not make the shear web from UD exlusively...

    And AdHoc,

    What have i said that makes you reply in such a negative, supercilious almost condescending flavour everytime? If your not going to be helpful, why write anything at all?

    What im looking for, is a load condition specified by the LEAST conservative method that IS STILL PROVEN strong/stiff enough to last... my english fails me sometimes, i dont always convey a clear meaning...

    So, i ask the question again, if i may, what load condition, class society specification, or "other method" will be the least conservative in estimating the bridge structure of a catamaran?

    Eric Sponberg beleives Art Edmonds guideline would be the best, however i cant find this online, can anyone point me in the right direction?
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Well I’m sorry you feel that way, none is intended I assure you, but please show me where my posts are negative? If you interpret them as being negative where none is implied, what can I do about that? There is little I can do about that I am afraid. I can only conclude you are taking my comments personally. This is not surprising when one is personally involved with a project of any nature, it occurs all the time, especially with ‘home builds’. Thus one must be rational and objective when designing, no emotive.

    My comments reflect my opinion…it is your choice whether you wish to listen to it or not. Doesn’t bother me at all. But you have to ask yourself the question…are you posting hoping to get a quick fix answer by someone who tells you exactly what you want to hear and make you happy...or someone who will tell you the hard unedited facts, objectively, in order to make you think?
     
  10. catsketcher
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    catsketcher Senior Member

  11. Richard Woods
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    Richard Woods Woods Designs

    You say
    But then you say "What im looking for, is a load condition specified by the LEAST conservative method"

    Which isn't the same at all

    To be honest I don't think you can expect anyone to tell you the answer, or even how to start without having a full set of plans available

    If you were doing an open deck catamaran or a trimaran and want a conservative result then it would be easy, but you have a full bridgedeck and want the lightest structure

    Anyone here who will give you an answer that will make you happy will have to spend quite some time thinking about it. I doubt if anyone will do that, sorry

    So I agree with catsketcher (and Ad Hoc). Reverse engineering is the first approach to try. Some people call that copying, some plagiarism but if it gets the result you want then do it

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  12. MikeJohns
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    MikeJohns Senior Member

    Groper
    As a professional I certainly wouldn't read it as condescending.
    An experienced professional multihull designer takes the time to give you his opinion and it's your reply that is actually hostile and condescending!

    You should try a peer review panel, as an designer you want critical peer review, you don't take negative comments personally they are nothing but helpful.
     
  13. groper
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    groper Senior Member

    I wont continue the personal argument, but still noone has answered, what i thought to be a simple question...Im not familiar with all the methods of estimating these loads and i understand its still not an exact science... i know there a few methods - all of which will produce a different result! Im looking for a METHOD of ESTIMATION, that wont be overly conservative for a 35ft composite catamaran of 4000kgs displacement?

    So, WHICH GUIDELINE or CLASS RULE or... - should i use - in order to estimate the loads? Is this a difficult question?

    And still noone has helped me find Art Edmonds Guideline for estimating these loads...? Although i have found this;
    Catsketcher,

    Ive already reverse engineered a slightly larger/ beamier boat which should be stronger than i need, and based my boat around the same engineering/structure... obviously i have developed my own lines and styling to go with it, making it unique... so i can already use these scantlings and still end up with a very light boat.. im just curious as to what result i could get if i engineered it from scratch and then compared it to other structures... im just curious and willing to learn, whilst at the same time building what im drawing... few have the luxury of being able to make changes on the fly...

    The thread you linked me to, is one of a couple that sparked my questioning on this subject... noone can tell you which rules or method is most accurate, but those with design experience should know which rules or guidelines produce overly conservative results and which allow lighter less conservative results i would have thought...? Im sure AdHoc has the answer im looking for, but refuses to give it to me for reasons unknown to me and thus my frustration...
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    To paraphrase and repeat what Richard has already noted to you.

    So where does that leave you?
    The basics are very simple. But in order to arrive at a solution, it cannot be achieved by a one liner and requires far more data and understanding. If YOU may think it is simple, that’s fine. But it should be obvious by now, from the responses by other processionals on this thread, whilst it may be simple, there is much more to it than you realise and appear willing to accept for some unknown reason. It does beg the question why ask, if you have your mind made up already..based upon what too?

    It is an exact science, it is called structural design and engineering.
    The only “unknowns” are 1) establishing what loads, in the magnitude sense, to apply; that’s where any Class rule will assist you (they all have their pro’s and con’s). And 2) then with composites, the quality control in order to ensure the material properties used in the calculations of the scantlings are maintained and consistent.

    The rest is “simple” as you put it.

    However, without knowing what loads you are going to subject your design to, from the layout and arrangement and characteristics of your vessel and what material properties you are using and can demonstrate this independently, as Richard said:

    With such incomplete data provide, what do you expect? No professional shall stick his/her neck out based upon such limited data. Careful and proper structural design is about material choice, arrangement of structure by identifying load paths and how to shirk loads and ensure that the structural arrangement does not yield excessive stresses and stress concentrations or displacements that are deleterious/dangerous to the final layout to satisfy the SOR.
     

  15. groper
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    groper Senior Member

    See there you go again... your not understanding what im typing...

    im not asking anyone to stick their neck out...
    im not asking anyone to engineer anything for me...
    Because, Im not asking anyone to tell me what the loads are or how to design for it!!!

    Im looking for a method of establishing the loads myself, then i can design for it...

    Seeing as class rules are the only thing you seem to understand and make reference to, which class rules would you use establish the loads for a 10m recreational composite sandwich bridgedeck catamaran used in paritally smooth waters?
     
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