drag calculations for large catamaran

Discussion in 'Boat Design' started by fredscat, Oct 2, 2010.

  1. fredscat
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    fredscat Junior Member

    I admit that my prior successes designing, modifying and repairing - sometimeS large - composite structures, have made me somewhat 'arrogant" I have not yet designed the main connecting beams between the hulls for exactly the reasons you have all mentioned. I am actively looking for advice on the magnitude of the loads I should design to and am actively looking for someone to run a finite element analysis on the proposed structures. Has anyone looked at my web site? Thanks for all the great input PLEASE KEEP IT GOING!
     
  2. mydauphin
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    mydauphin Senior Member

    Glad you have the intelligence to go beyond your ego. That is the hardest part of the whole process. Many of us are very successful in some other field and think that building a boat is easy, and at the 20' level - yes it is easy. Beyond that it starts to get complicated. As I mentioned, I have built many boats, but when I got into 70' footer all my cost and time estimates were wrong by at least 100%. There are many great people on this site - Start a thread need help in reviewing structural aspects of a design. I think you will be pleasantly surprised the help you get on here. Others continue wanting to build their dreams with no clue what they are doing. I mean really no clue.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, I have looked at your website.

    Unfortunately, you're doing things back to front.

    You really need to stop.....draw stumps, and ask the questions you should have asked before you put "pen to paper". To continue, on a vessel of this magnitude, the way you are (as seen on your website), is dangerous!


    You should start by:

    1) A proper working GA
    2) A full spec (SOR)

    Can't do anything without these.
     
  4. fredscat
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    fredscat Junior Member

    Please expand on your reply. I have built the hulls and am very confident that they can handle all the loads. The connecting members, as I pointed out, i will need help with as they are beyond my engineering capability. I have no FEA (finite element analysis) experience and must admit, I am not even sure what loads I should be designing to. I have - on a preliminary basis - assumed the STB Bow and the PORT Stern to be supported by steep, high wave crests then apllied 5 g at the CG (center of gravity) Are these assumptions anything like realistic?? I am also not familiar with the acronym "GA"
    and "SOR" please expand
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Well, the fact you're not familair with GA & SOR is a sad reflection.

    GA = General Arrangement.
    SOR = Statement of Requirements.

    Without these how on earth do you know what you are designing too?

    But, if you're confident, why are you asking how to deisgn the connecting (beam) members? Surely you have taken these into account when designing the hull?...if not, QED!
     
  6. Richard Woods
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    Richard Woods Woods Designs

    I agree with Ad Hoc, and the others

    How can you build the hulls without knowing how you will join them together?

    Actually the important thing is not "how" but "how heavy" will the cross structure be? Regardless of how you do it, it is always a large % of the all up weight. Without knowing it's weight and C of G I don't see how you know how much displacement there needs to be in the hulls??

    What else have you not calculated??

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  7. fredscat
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    fredscat Junior Member

    All, your critiques are taken with lots of respect. Here are some brief answers that may help explain. I am a mechanical engineer with quite bit of aeronautical experience gained afterwards. I am considered by many an expert in composites, fabrication design and repairs. HOWEVER, I know very little about the every day "stuff" in the marine environment and that is why I am asking all these "dumb" questions.

    On th design sequence - hulls and connections - I completed some preliminary, I think conservative estimates on the loads and designed the connecting beams based on that. I want someone else to double check in order to possibly save some weight / cost. The beam to hull connections are TO THE INSIDE SKIN of the sandwich hulls and will beefed up as needed after design loads have been verified. My initial weight estimates for these structures have been very conservative and likely high. I have designed and built man carrying small aircraft and feel confident that with input from others I can complete a save vessel I am not building a racing boat, down to the last layer of glass and last ounce of epoxy.
     
  8. mydauphin
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    mydauphin Senior Member

    You don't get it. The main forces are not the weight of the hulls, it is the crushing forces of all the other forces. I am not a naval anything, but neither you, I, or anyone on this planet knows how your complete boat is going to sit on the water. How much water it displace, its center of gravity etc... Especially on a cat. Yes, it is important to save weight, but that is secondary to the whole design. It is like designing a race car by making it as light as possible and not knowing its aerodynamics, or even what engine your going to put in it. How do you what size sail to put and how do you balance the whole thing out. Trial and Error?
     
  9. gonzo
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    gonzo Senior Member

    If you need to "beef up as needed after the design loads have been verified" , you don' have a clue. I don't believe you are an engineer of any type. Building something without structural analysis or enough experience for rule of thumb is really stupid.
     
  10. WestVanHan
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    WestVanHan Not a Senior Member

    Wow that's a huge project.....

    A few years ago, for some odd reason I got it in my head to build a 50' by 20' power cat,which is simpler than a sailing cat.

    But after spending $1500 for advice from a catamaran designer, I very quickly decided the 7,500 hour build time was wayyyy to much time and money.

    A 97' x 42' sailcat..how many hours will it take to a basic finish-20,000 or so?

    Good luck
     
  11. Richard Woods
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    Richard Woods Woods Designs

    There have been several previous threads from posters asking for help with a partially complete design.

    The major problem you will have when approaching any professional is one of liability. Basically who is ultimately responsible for the design??

    Suppose you asked me to design the structure and I said "do A" but you say "No, I've already committed to B by building the hulls, or it doesn't meet my interior requirements, or I don't want a rig like that, or....". Then what?? Do you ignore what I say, in which case you've wasted your money and my time. Or do you agree, in which case you've wasted what you have done already??

    Having said that, it's good to know you are an expert in building lightweight composite structures. But surely even a passing knowledge in multihull design would help??

    Put it another way, my skills are the opposite of yours. If I was to start building a man powered plane before designing it wouldn't you make the same comments to me as I make to you??

    I agree most home designed boats float, (unlike most home designed planes which never - fortunately - get off the ground). But that doesn't mean they are good boats. Seems a waste of USD1,000,000 or so not to get a proper design completed first.

    And before you ask I am not personally interested in drawing a 90ftish catamaran - why so big anyway??

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  12. keysdisease
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    keysdisease Senior Member

    INSIDE SKIN of the sandwich hulls

    I hope thats just a mistake of semantics, "inside skin." Hopefully you are talking about a bulkhead. :eek:

    Steve

     
  13. Alik
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    Alik Senior Member

    Building 90-footer without complete design is gambling. The cost of mistakes would exceed times of naval architect's fee. But let them ahead if they like it... :)

    I always let this type of customer hit the wall before explaning the basics to him...
     
  14. War Whoop
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    War Whoop Senior Member

    Fred rethink your connection, into a properly engineered "hard Spot"
     

  15. mydauphin
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    mydauphin Senior Member

    May sound crazy to you experts, but when I looked at a large cat. I was think of using epoxy/carbon fiber hulls with an aluminum frame and separate carbon fiber center area. Simply easier for me to build strong aluminum box truss like a F-14. But again I am not a carbon fiber expert either.
     
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