Starting out in Boat Design

Discussion in 'Multihulls' started by AstroTux, Jul 19, 2006.

  1. AstroTux
    Joined: Jul 2006
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    AstroTux Junior Member

    Hi,

    I'm interested in designing and ultimately building my own powered catamaran, and constructing it primarily from wood.

    I've got a basic idea for a design, but would like advice on several points (and more if you know of anything I haven't mentioned). I'm not expecting to have this built by next week, and do appreciate some study will be involved.

    * Can you recommend any design packages that can do various calcs based on the design (such as waterline levels etc)? I've already been looking at the open source FREE!ship program ( http://www.freeship.org/ ) which seems to do quite a bit, but I appreciate that it may not be comprehensive. Is it suitable for basing a design on? Thoughts on the app in general?

    * I need some help with the terminology; just reading a few posts in this forum has been educational! :)

    My basic idea is for a catamaran that is 15-20ft long, with a deck that is about 8ft wide. My big problem is determining the draft etc.. and general hull design after this point, hence why I'm here. :eek:

    Using FREEship, I've tried to create a hull that generally appears to be about right, that I can then work on further, but even my most basic attempts fall apart (it is either too wide or too tall). :( It just seems ... wrong.

    Any help would be much appreciated! :)

    Best regards,
    Robin.
     
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    FreeShip is actually a pretty good tool once you learn it. No, it's not comprehensive, no CAD program ever is. FreeShip is a hull modeller, and a very good one at that.
    Is this the same cat you mention in your "ocean-going cat" thread http://www.boatdesign.net/forums/showthread.php?t=12803 ? Leaping into boat design with something this complicated might not be such a hot idea. What you describe is very small, definitely not ocean-going. I'd suggest two things: 1) Stock up on good books (and read them :) ) about cats, offshore cruising, boat construction, etc, others here will recommend their favourites. 2) When you're ready to design the boat, find a naval architect with ocean cat experience to help you. After some reading, thinking and budgeting, you'll have a pretty clear idea what you want. The NA will help you turn your ideas into something safe and seaworthy. Given a list of requirements, some sketches, some discussion as to design philosophy and direction, an NA can either recommend some stock designs (of which there are plenty, take a look around) or come up with a hull and structure to suit you.
     
  3. AstroTux
    Joined: Jul 2006
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    AstroTux Junior Member

    Hi,

    Thanks for the info! I wasn't suggesting in the other thread that a 20ft boat would be ocean-going. It was just something I was wondering so I thought I'd ask.

    Are you referring to a Catamaran being complex, or an ocean-going varient?

    Basically what I'd like to do is build a boat that is sufficiently large enough to live on comfortably, be able to handle something like the North Atlantic sea (the area between Scotland and say Scandanavia, which I understand can be very wild), but not so big that it would be unsuitable for use on the wider of the inland waterways. Finally, the boat would be designed/built by me, for me, so I'd like something that is flexible and wouldn't have me looking around for another boat when I find a limitation in my own boat (i.e. unsuitable for going across to the continent). I doubt I'd be wanting to sail across the Atlantic to the USA etc.., but I hope you have a better idea of what I would like.

    If I'm asking for too much, please let me know. :(

    If it would be preferrable to get on a naval design course, I'm certainly prepared to do that. For me, this is a very long term project.

    Best regards,
    Robin.
     
  4. nero
    Joined: Aug 2003
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    nero Senior Member

    very long term project. Keep that in mind and you should be able to design and make something.

    Multihulls are complicated to design. Much more than a monohull. There are no books on designing a catamaran.

    It took me two years to find the time and tidbits of info and my style. I am in the second build season. Things are not going quickly.

    And to keep in mind that all your efforts and money may end up as something that floats ... but not much more than that.

    You should learn a alot along the way. Things like hard chined plywood boats do not look so bad ... after you spend all the time strip planking to make something smooth and sexy. smile
     
  5. AstroTux
    Joined: Jul 2006
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    AstroTux Junior Member

    Hi,

    OK. At this point, what I'd really like to understand are the more technical aspects of the hull. One thing that became very apparent very quickly from looking at FREEship, were terms like LCF, KM etc... - where can I find out more about this? I'm going to the library tommorow to look for any books on hull theory, but was wondering if you knew of any good resources that explain the theory of hull design etc..?

    What I'd thought of doing was to design a hull that is good both in terms of weight capacity and speed, then build two of these symmetrically. From looking at existing Catmarans they appear to be symmetrical in design.

    Thanks for the advice so far! :)

    Best regards,
    Robin.
     
  6. nero
    Joined: Aug 2003
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    nero Senior Member

    Most sailing cats have symetrical hulls ... below the wl (water line). With power cats many are not. Google boat design and catamarans and power catamarans etc. Surf a lot and you will be able to compare ideas and designs.

    The fun is when you start to draw your dream boat. Everything is a comprimise. for example; Bridgedeck to wl clearance limits the height of you bridgedeck cabin and the height of your bunks and the headroom above them. It spirals around a bit and eventually you accept to live in a different space than your dream.

    there was a page on the net with boat terms listed. let me know if your googleing doesnt find it.
     

  7. rayaldridge
    Joined: Jun 2006
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    rayaldridge Senior Member

    I'd have to say that I disagree that a multihull is more difficult to design successfully than a monohull. After all, in some thirdworld countries, a couple of balsa logs and a coconut mat sail will go like the dickens, so how hard can it be?

    I think that the aspect that makes monohull design more difficult is that the boat heels and therefore has a different underwater shape under every wind condition and point of sail. That seems more difficult, at least if you're attempting to design from first principles. On the other hand, there are a lot of monohulls already designed that work well, so a designer's job may be easier if originality is not a principle design goal.

    There are several good books that deal with catamaran design. I like Thomas Firth Jones, who's published several books that deal with his various designs, some of which are cats. There's Chris White's book, and you can find a number of articles in various magazines and research groups like AYRS. John Shuttleworth has a series of articles on cat design online, and they are excellent.

    Anyway, I think that if you're not too proud to do your design within the commonly accepted parameters, you can do a fairly good job with a cat or proa. I don't mean to make it sound as though I think it's easy-- I'm drawing a little cat myself, and I find that the deeper you go into the process, the more decisions have to be made, with an enormous complex balancing act of tradeoffs to be performed. But I think an amateur can do a decent job of cat design, especially a small cat, which you can chop into and modify without too much angst if it doesn't work quite right at first.

    I'd have to say that the hardest aspect of cat design is scantlings. You want a strong boat, but it has to be lightweight, and that kind of design is an art as well as a science. You have to be pretty ruthless about letting go of your most fondly held ideas, if they don't fit into that strong but light conceptual framework.

    Ray
     
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