Beam Design.... Again!

Discussion in 'Multihulls' started by cookiesa, Jun 29, 2015.

  1. cookiesa
    Joined: Mar 2007
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    cookiesa Senior Member

    Thanks Phil, that's really what I guess I am chasing. I don't doubt the original alloy beams as per the plans will be suitable, more as sourcing them is an issue I am working on the timber/composite equivalent.

    It was cheaper to buy full rolls of cloth than part ones, so I have plenty of extra cloth I can use (the lamination is based on the composite mast beam)

    Can I ask what you mean by don't load the web to much? (Is this referring to fanning out the cloth on the bulkheads?)

    Cheers
     
  2. cookiesa
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    cookiesa Senior Member

    I'll bite,

    So what design would you use for the front beam to suit...

    25ft catamaran
    bridge deck
    cruising style
    Has a main mast beam that is composite
    Expected displacement weight 1600kgs

    ?
     
  3. MikeJohns
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    MikeJohns Senior Member

    If the plans specify an alloy tube and give it's dimensions then in this application you should match the characteristics for bending shear and torsion, depending on the core type and skin thickness you might also want to look at buckling too.

    You will get deflection from both shear stress, and bending moments. Also
    Torsional stiffness is going to be the really hard property to match and I suspect you will end up with a lot more weight trying to replace the alloy with composite unless you make the dimensions large.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Not sure what it is you are "biting" here???

    This is your boat and clearly your design. You seem very satisfied with your own methodology and the information supplied by others so far. Great. So why not demonstrate it...inform others, whom also follow the same path as your self, a list of:-

    1) Your assumptions
    2) A basic Structural GA to ascertain any other transverse members that may or may not contribute
    3) The life span it is designed too, 1,5, 10 or 20 years
    4) The amount of deflection how much do you get and are you are satisfied with the amount if not what mitigation have you taken
    5) The area of operation that feed in to #1,3 & 4.
    6) The material properties of the layup you are using...and how you validated this
    7) The attachment to the hull, how is this achieved, and thus what fixity you are assigning it
    8) The load paths from the beam(s) to the hull

    These are the basic questions that always need to be answered. Since you are happy with what you have done, you must have followed this or a very similar route.

    Thus the onus is on you (the designer/builder as you understand the terminology) to demonstrate to others how you arrived at the final arrangement you have come up with, i'm not the designer, you are!!
     
  5. catsketcher
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    catsketcher Senior Member

    Alternatively you may have done what most small cat designers have done.

    Build someone else's design.
    Build another one.
    Learn how things are built
    Look at lots of similar boats
    See how they are built. Learn about beams, hulls structures empirically.
    Use materials that are reasonably forgiving
    Design and build something new
    Suck and see.
    Trial inshore. Fix if necessary.

    This is how yacht design has been done for hundreds of years. It still needs to be heavily empirical so that one knows if the theory has you straying too far from the norm of design/building.

    I know Ad Hoc will say it isn't his way but in reality this is the main method the designers in Australia have designed most of the fab cats and tris that have been the mainstay of our historical fleet. A keen eye, lots of thieving, good boatbuilding prowess made Australian multis what they are today.

    There aint just one way.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Not at all. If one wishes to use that empirical method..fine. BUT...the only issues are you have no idea what assumptions or factors of safety are used and what loads are used nor the design allowable stresses and deflections in their calculations (if indeed they did any). Thus you could be copying a very over engineered beam, or one that is totally unsuitable and fail within a few trips.

    That's the point. If it is not 'designed' and fit-for-purpose (for that boat), a copy (from another) could be disastrous. You simply don't know.

    That's all.
     
  7. cookiesa
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    cookiesa Senior Member

    Wrong again Adhoc, not my design at all, hence I am comfortable using a substitute for a material not readily available locally. The designer is focused on other designs at the moment.

    Once again you fail to offer any actual substance, instead attacking those who do.

    Perhaps you could post 1 link to a boat you have designed that has been built? Or are you an armchair architect?

    Thanks again to those offering ideas and knowledge
     
  8. FMS
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    FMS Senior Member

    He designs commercial ships.

    You have a stock plan and want to make a change. You will probably over build and carry some extra weight to minimize risk if you substitute without completely re-engineering it.

    You may decide some level of risk of failure is acceptable to you and copy a part from a very similar design. There is a safety margin in all components. You probably don't even know what this is specifically for other components.

    You can also pay a naval architect to redesign it to minimize risk and minimize excessive weight. A relevant question would be what a naval architect would charge to redesign this component for you. Unfortunately you won't have the advantage of having all the calculations already done as you only have the plans meant to build the boat as designed.
     
  9. redreuben
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    redreuben redreuben

    Why don't we know what the design is ?
     
  10. MikeJohns
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    MikeJohns Senior Member

    I think the problem here is that your Opening post is a bit ambiguous where you said "The plan is to use..." instead of 'The plan specifies', it could be easily read that you are designing this from scratch.

    AdHoc really does have a wealth of practical specialist knowledge on multihull structures and loads. ;)
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Oh dear..there is so much wrong with this and you don’t even see it.

    So your initial question of:

    Fine you’re making them yourself.

    But..

    So you need assistance in the design of them. Fine, but then you seem to think it is merely a terminology thing??

    But then you say:

    Again,….this shows you don’t understand the “terminology” as you put it.

    Ohhhh...So you do have a designer... but is too busy to help.:confused::mad:
    And that does not strike you as odd?

    So either he wont work for nothing to help you…(why would he?) …or he also doesn’t understand how to design it either. Either way, sounds like you’re in a pickle.

    Ahhhh …and then this tiresome gem:

    So, from those replies that you found most favourable (because it aligns with what you want rather than what you need), I assume you shall be asking those to also provide evidence of their working designs to your satisfaction??

    Funny how those that come onto the forum seeking assistance, come armed with preconceived notions and ideas and will not entertain anything that deviates from their expectations and they all react in exactly the same way.
    Sad.. :(
     
  12. ThomD
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    ThomD Senior Member

    At the end of the day you are probably right, he doesn't know what he is doing which means that he can't provide the info you want. Meanwhile if he was stuck with a pooched alloy beam in a distant island and had to cobble something together out of available materials something would get made and it might well last.
     
  13. rxcomposite
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    rxcomposite Senior Member

    A chopped down coconut tree trunk might work as a crossbeam. :D
     
  14. Jetboy
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    Jetboy Senior Member

    IMO - the reality of beams is that no one really knows how strong to make them. You'll never get a good because most designers simply don't really know and they don't want you to know that they don't know. The people who do know hold the info pretty close. We use empirical data from experience and go from there. Transferring between different materials is relatively easy to calculate. Calculating the peak loading of a beam is both very difficult to do with any precision due to the extremely complex dynamic environment of ocean/wind etc. And it's not necessary to have any precision. Why would one want to calculate a load to more than 2 significant figures when you're likely going to double or triple it to add a factor of safety? The pursuit of precision doesn't make a lot of sense in this particular case.

    You'll note for example that the MIT engineers who designed the Motive 25 likely did such calculations only to find that their computer modeled boat didn't actually translate to a stiff platform in a real boat. They added water stays after the fact because it was too flexy. Might they have been better off just going for a sail on an Astus 22 and getting a feel for it? I'm not sure.

    That's basically what I did with mine. I did a calculation of the max displacement of the float and beam deflection formula. But it's just a ball park estimate of what will actually happen out on the water. The loading is never quite what it would seem on paper. So I got kinda close and then added a factor of safety and away I went. I'll see very soon how it sails. And if I need water stays I'll add them.

    So I guess my conclusion is to stop looking for some type of formula. Unless someone can produce one, I don't think one exists.
     

  15. cookiesa
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    cookiesa Senior Member

    Adhoc anyone can cut and paste single sections of a sentence to make it appear however they want, I won't insult everyone else's intelligence by playing that game.

    If you read the thread, someone with relatively rudimentary understanding of the English language can work out...

    The boat is to a design, not mine (something you implied)
    The design has a beam, they are alloy, not easily sourced where I am, one poster was most helpful in providing a possibility
    I have "built" most of the boat

    As for pre concived ideas, you are dribbling. I am yet to say how it should be done. Yes I made a suggestion as to what I believe could be done, but that is because I don't expect to be given answers without some input myself.

    There are several designers who have designed great boats who are no longer able to help. Some have passed on, doesn't make their design any less valid. Others have moved on to other areas and are busy with those, some had plans they no longer support as they are focused on production boats now and don't have the time to support home builders.

    But of course the great and knowing Adhoc would be aware of this.

    And you are yet to contribute anything positive.

    Perhaps you have something else you can go do to make yourself feel important, obviously you are not getting enough attention.

    Like the plan, fx composite, but not many palm trees this close to the Antarctic!

    Interestingly Jetboy, there certainly seems to be more "experience" involved in beam design than pure science, something a lot of the open designers infer.
     
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