Beginner Builder, designing a sailing catamaran for Blue Water

Discussion in 'Boat Design' started by Joe M, Mar 30, 2018.

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  1. Joe M

    Joe M Previous Member

    So I want to learn to how to determine how much in material I would need to build a 30-40 ft Cat hull out of the following materials. Lets say if I decide on a boat length of 38' and Beam of 18'. With 1/2" foam core. I will mention more attributes because I am digging for tools in case I want to change the size. Like for example do I really need a 38' boat for my self to sail blue water?(yes I realize that is just a fancy term)

    3/8" or 1/2" Foam core. (I want foam core due to its ability to take certain shapes, not marine ply)
    Fiber glass with carbon fiber stringers and re informant of decks and mounted items(probably at least one or two sheets depending on witch sheet size I end up using on the inside of the hull(foam)).

    Now my understanding is to use high density foam on rub plates, chain plates and another other mouthed surfaces. Plus using high density foam for edges of decking, the front and keels of the cat hull.

    My problem is I cannot find how much in fiber glass I need to use from both 3/8" and 1/2" foam cores. I realize that the strength is more about thickness that just adding as much fiberglass and carbon fiber as you can and that the hulls can be around 10mm-16mm thick for Blue Water sailing vessels.

    Has anyone ever seen the construction or a broken hull from a blue water cat ranging between 30'-40'?

    I don't want to hear "strength is relevant" blah blah blah. need details please not your personal theory on strength of composite materials. I have read many comments about theory and relative nonsense that doesn't tell how to determine how much material you should use.


    This is what I was thinking for layering, I am sure this is not sufficient but that is why I am here.
    Lets say if I decide on a boat length of 38' and Beam of 18'. Should I nice thick layer of matt on the outside for blistering repair flexibility? Should I add more layers on the inside or out side? maybe both? Should I change my cloth types or add more layers of different cloth types? Like bi axial vs triaxial?

    Inside
    ________
    carbon fiber 1-2 layers (large weave for rigidity.)
    --------
    12oz fiberglass bi axial or triaxial 1-2 layers
    --------
    3/8"-1/2"foam(with fiber glass tape to run along certain lines of the hull from stern to bow , holding the foam planks together until the layers of fiberglass and carbon fiber is added. Planks vertical and tape horizontal)
    --------
    12 oz fiberglass bi axial 1-2 layers
    --------
    Matt fiberglass depends on how thick it needs to be for blister repairs in the future.
    --------
    Gell coat
    _______
    outside

    Maybe in the end ill might find a salvageable cat hull and reinforce it with a sanded mechanical bonded fiberglass and carbon fiber.(that would be a cool discussion).
     
    Last edited: Mar 31, 2018
  2. coopscraft
    Joined: Dec 2013
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    coopscraft Junior Member

    Start with professional plans. If you want to design your own, start smaller. You could, for example build a scale model and test it. To do the scale model process correctly you should have some understanding of dimensional analysis, engineering properties of materials etc.

    Regardless of what size you design, study existing designs in that size and construction method and do similar. Don’t try to think outside the box until you really understand what’s in the box. “Don’t try to run before learning to walk.”
    Some math that might be useful to know:

    D=fE(l^2)/(w•h^2)

    Learning something like marine architecture by trial and error may seem entertaining, but it could also be deadly and in a boat that size it is for sure crazy expensive. Stand on the shoulders of giants.

    The sea ALWAYS finds out what you did wrong.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    I'd have to agree, your scantlings seem way too light and not particularly appropriate for a vessel of this size. Working up the scantlings and laminate schedule is simply a set of load based equations and not especially difficult, though you do need to do the math. Finding suitable plans isn't difficult nor costly. You can change the interior to your desires and exterior aestedics as well, within reason. The math is done, it'll float with the decks facing up, come launch day and you can feel safe farther from shore than you can swim back to, when the crap hits the fan.
     
  4. coopscraft
    Joined: Dec 2013
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    coopscraft Junior Member

    Joe, now that I’ve had my nap, I think my above reply comes across a bit mean. That’s not my intent. I do think you can build a great boat, and maybe even design one. Your title post says beginner and you admit that you’ve seen a lot of technical mumbo-jumbo that doesn’t lead you to an answer. That means you’ve been looking in the wrong place or lack the background to know what you are looking at. The question of how to know how thick your structure needs to be is not something that can be tutored in a single thread for a project with the complexity you describe.

    As a starting point, you could get the textbooks for sophomore engineering and study them. This would give you a sense of the math. Then you would have some intuition when comparing designs. Study other designs. Start with smaller boats as beginner design projects and survive the sea trials. There’s nothing like some hands on practice to get a feel for what works.

    The engineering subjects you’ll want are, Engineering Statics, Engineering dynamics, Strength of Materials. The university library in Bozeman probably has corresponding textbooks in these subjects. Most university students take these in three quarters consecutively and take vector calculus in parallel. You don’t have to be an engineer to design a practical boat but the subjects covered above will give you the science background that will really help to understand why things are done a certain way.

    Smaller simpler boats are better for getting your feet wet. I built bolger’s elegant punt strictly to the plans for my first project. At 7.5 ft long, it was cheap and still a good learning experience. After that and after reading hundreds of pages on classic dory design and glued-lap construction, I modified a gardner designed dory skiff to my own purposes. I built it successfully and still use it after 20 years. I wouldn’t dream of taking on a project like yours without an expert looking over my shoulder—and I’ve taken those classes.
     
  5. Ike
    Joined: Apr 2006
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    Ike Senior Member

    Actually this is probably a good way to go. It would be a good learning experience and teach you about using various materials. Sure you can get a book that will teach you this but there is nothing quite like hands on experience. It teaches you what works and what doesn't. The advice given here is good. Don't try to reinvent the wheel if you don't know anything about wheels. Start with a wheel and modify it to do what you want.

    PS: My daughter went to MSU at Bozeman. Great school, nice town, good people. Her husband is from there.
     
  6. Joe M

    Joe M Previous Member

    I have had so many people tell me I am wrong about something. I usually prove them wrong. That doesn't mean I am going to go out and just buy materials and start building. So really was the comments necessary? I just feel like you jumped into the chat to have your ego stroked... I am not here for your intellectual reinforcement.


     
  7. Joe M

    Joe M Previous Member

    Yes I agree, I was hoping to get more information on how the hulls thickness is determined but it seems some people just want to jab me with their ego. (I don't mean you)

    That is really cool MSU Bozeman is a beautiful place, people from MT are usually very good. I have considered going there for college but I have too much going on medically. So i might never be able to go to school for engineering. It sucks it has always been a dream.


     
  8. Joe M

    Joe M Previous Member

    Yes that is what I am trying to discover how people determine the load bearing and static tolerances. The problem I have found with some plans is that they just use a lot of extra material to get the strength they need instead of determining what they actually need. That increases costs a lot. I don't have the luxury of building a $100,000 boat. I am how ever convinced the catamaran can be built for under $30k. I just won't be fancy and will have recycled parts.

     
  9. Joe M

    Joe M Previous Member

    This post is mostly a toss in the dark to see if anyone has information on the engineering analysis and calculations of hull strength and static tolerances.

    I will how ever being incorporate stringers in the design This will significantly improve strength without the massive cost increase.

    I found something just now.

    The Elements of Boat Strength: For Builders, Designers, and Owners https://books.google.com/books?id=yAZeHJ3f_4AC&printsec=frontcover&dq=Design+a+composite+boat&hl=en&sa=X&ved=0ahUKEwiIr5ugqJbaAhUB32MKHdcvAOQQ6AEINDAC#v=onepage&q=Design%20a%20composite%20boat&f=false
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Dave Gerr's book is a good start, though the scantlings tend to be heavier than necessary (likely an intentional thing on his part). In the end, i'll still be much cheaper to refurbish a previously well loved cat, than to build new. This has been proven time and time again.
     
  11. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Joe M the respondents have tried to be helpful. There are many members here who are professionals with impressive credentials. The base their commentary on proven principles not relative nonsense. If you continue to rebuff them with "Blah blah blah", you can be sure that additional responses will become rare or non existent.
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If this is what you're looking for, the answer is yep, we have information on the engineering analysis, calculations, etc. We get them the way most do through an engineer degree. I have three: chem E, Structural and an NA compliance CT.

    There's no short cut to the engineering disciplines you must adsorb. Now if you're a bright boy, maybe with some engineering in your background, this will be a relatively easy thing and engineers do this daily; research. You can pick up much of what you need through several books, though given the size you're looking to start with, not so much. You start small, which is much less costly if you've screwed the pooch somewhere, without knowing it until launch day. I've been lucky and each and every design has floated on it's LWL (mostly) at launch, but this isn't always the case. I did design a racer that was so tender, we needed two more crew to hold her upright on launch day, the issue was an easy, if costly fix. It was a racer, so intentionally fidgety underway, but she did need help right away.

    Start with say the dinghy to your mothership, using the same building process as the mothership, so you can cut your teeth on a much smaller project and get a feel for how this is all going to work. You might just hate it and you've saved a lot of money and effort. Then again, you might find you love it, but maybe the build method needs to change or some other import consideration.

    Let me guess, you're young, have little practical experience, little if any engineering understanding and not much 40' catamaran sailing under your belt, let alone building large, heavy eventually sailing objects, but you're full of vigor and ready to get started. I mean how hard could it be, right? It's not though you do need a good grasp on collage prep level, 12 grade math and learn all the subtleties of the process, including the black magic stuff, that will take years to pry from those that actually know.

    If you gave me $30,000, I could find a $10,000 cat, say 35', in need of most everything. I'd drop maybe 5k on new sails, another 5k or rigging and some equipment upgrades and another 10k or repairs, more upgrades, engine repairs, etc. You'd have your $30k catamaran, but this wouldn't be my first rodeo, so your costs will likely differ. As has been mentioned previously, you're not going to build in this scale for cheaper then a re-freshening of a once loved, but well used cat.

    Lastly, no one in their right mind, "use a lot of extra material to get the strength they need instead of determining what they actually need" . . . This isn't the way things are done and hasn't been the way things were done since the DC-3 prototype and every engineer understands the reality of this, as the design, spec'd like this would never get built. Materials and labor are way too precious to waste in this fashion, so get a grip, do some research and when you actually get a clue, stop back in with a reasonable path to your huge 35' catamaran project, that you may envision.
     

  13. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    Going to close this thread now so that it does not become more negative. Reported posts which seemed to insult another member who replied have been deleted.

    Definitely do not want anyone to feel belittled or disrespected; when you ask for advice and input for free, you will not always like every single reply; you are free to take advantage of replies which you find useful and ignore those which are not useful to you. The site definitely wants everyone to have an opportunity to enjoy learning and sharing knowledge, but at the same time, please respect members who spend their own time to share their experience. Please don't let a quick remark cause so much irritation. Sorry it didn't work out. <thread closed>
     
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