Simple Info!

Discussion in 'Multihulls' started by trackerscout1, Jul 3, 2013.

  1. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I have no doubt you could build a boat, but I do seriously doubt you could design one, that was anything close to successful, unless you count floating as the only goal.

    Boat design is all about volumetics and simple math. It would be nice if we could just "wing it" based on some practical experience, which you seem to have, but it doesn't work this way. If you have difficulty with the volumetric aspects of the process, you'll never figure out how much boat you need, or how it (the volume) should be arrange. This doesn't even address the sail balance portions of the equations, which can delve into the black arts at times (well it seems like this to most) and don't even start me up on foil sections.

    Without a basic understanding of the concepts and principles involved (such as how much a cat's hull should displace and where to distribute volume), you're best advised to by a set of plans, which are cheap, given the full scope of a project like this.

    Yep Richard, they (L Gato) usually are seen with the main raked well aft and tend to plow a bit uphill, but given the age of the design, a reasonable trainer for a novice. It's not my cup of tea and I'm sure there's plenty of better choices, but it's the one I had at first glance, so I posted it.

    Lastly, if your hull(s) displace 372 lbs. (6 cu. ft.) you'll be swimming a lot (couldn't resist).
     
  2. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    I'm not very good with Imperial measurements. But isn't two people at 150lbs each, plus 100lbs for an 8ft catamaran fishing boat near enough 6cuft? Metric is SO much easier

    And to OP. 12ft x 1ft x 6in is just the overall size. Typically you can expect maybe only 2/3rds of that displacement by the time you have "knocked off the corners"

    Richard Woods
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Standards now suggest 170 - 180 lbs. as a human, which I usually add 10 - 20 pounds to for a dayboat, to include beer and Fidel the wonder dog. Getting sufficient volume on a 12' cat shouldn't be difficult, though getting the structure light, strong and stiff enough to perform well, is a different matter. This is an area L Gato falls a little short (it's a bit heavy).

    This is an area CJ should explore, as you've noted previously. There's no reason CJ couldn't get a set of plans, make informed modifications to the scantlings, material and build method choices, not to mention possibly some shape changes, to improve the abilities of the original. Styling is anything you could want CJ, though again an eye on light, stiff and strong structures are key toward cats, if you want any level of performance from them.

    I'm not completely sure, but L Gato is probably over frames. At over 200 pounds, it would seem so. I have an 18' mono design that comes in at less then 220 pounds, so maybe a taped seam conversion would be in order on L Gato.
     
  4. trackerscout1
    Joined: Jul 2013
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    Location: Ogdensburg, N.Y.

    trackerscout1 Junior Member

    PAR,
    Thank you for your input. I do appreciate that having plans and building from them, even with some minor alterations, is probably the best way for a beginner to go. That is not an option for me as I do not have the money to spend on plans, if I am going to acquire the fiberglass and resin, and other materials that are essential for building the boat. Most of what will go into this boat, as far as lumber, is high quality leftovers from other projects, of which I have quite a lot. I am doing this on the super cheap, and if all it does is float and slowly make its way out onto the lake, well than that will be pretty good by me. As I have said this is only my first build, I am by no means expecting a super fast racing cat, or a blue water cruiser. Also, I may not have the best grasp on the mathematics associated with boat design, but I do have time to learn it. Earlier today, I had no idea how to find out how much weight an individual hull could carry, and now I do. In fact I have a very good handle on that aspect at this point, not exact by any stretch, but enough to allow me to breathe a little easier about my hulls.
    Allow me to explain what I mean about having time to learn. If this build hits the water, and then hits the bottom, I will be frustrated, but I will not be deterred. Once I have managed to build a boat of reasonable quality, I will either critique it myself, or find someone who can check it out for me, and make adjustments from their for my next build, which will be a little bigger. After that boat is completed and I have had time to test, I will figure out what needs to be tweaked, and improve upon the design from their. So on and so forth until I have a design that fits my needs beautifully, and if that never happens, oh well. At least I will have had a great time trying, building and trying some more. That is what I do, I build things. Sometime they turn out wonderfully, sometimes they need some work, but for me, the process of actually building is both educational and life affirming.
    I have been researching boat design, looking at every boat I can, and going out on the water in every type of boat I have had access to for years now. Unfortunately the books that I have been able to acquire are not written for multihull enthusiasts, nor have they contained a lot of specific information on the mathematical aspects of boat design. This is why I have asked for help. I want to learn the formulas and be able to accurately deduce the proper size shapes and characteristics of a boat design. But in that area, I will need someone who can explain what to do, and why to do it. I hope I am being clear enough for you to understand what I mean, sometimes I can't find the proper words or turn of phrase.
    Mr. Woods, I assumed that I would loose a little displacement as I rounded off the corners and whatnot, but as the original number for even a single hull is higher than I need by almost six hundred pounds, I think I will be ok, on that aspect at least.
    Now, I have noticed that some of the newer cruising cats hull shapes seam to be almost upside down in comparison with a typical hull design, with the widest part of the hull at the bottom, at least in appearance. My first thought upon seeing this design, was that it seams like it would have to be very stable with a reduced amount of whetted surface area. What are your thoughts on this? By the way, this is an open question, to anyone who is interested in contributing positively to the discussion.

    Thank You
    ~CJ~
     
  5. redreuben
    Joined: Jan 2009
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    Location: Beaconsfield Western Australia

    redreuben redreuben

    TC1
    There are a load of free plans here;
    http://www.tdem.co.nz/links/free-boat-plans

    Scroll down for multihulls. Tornado anyone ?

    Whist I appreciate you are on a budget the small cost of plans will more than pay for themselves in lost materials from errors and huge savings in time. imho.
    RR
     
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  6. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    Slightly confused. A catamaran hull 12ft long will not have a draft of 2ft and a WL beam of 1ft. Even if you mean total freeboard, the 2ft from keel to deck will be excessive.

    6in draft (half the WL beam) would be considered more normal

    Richard Woods
     

  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A $50 set of plans is hard to beat. Free plans often tend to be incomplete and not well thought out, incorporate no longer available materials and use obsolete techniques.

    This format is a very difficult way to absorb the knowledge to self design a boat. This said, a few here have done just this, with questions about every aspect of the process. One is currently under construction and I suspect it'll go well (he's had some professional help on the side). From the concept stage to the actual, cutting the first parts stage, has been about 2 years. This would be normal, because it's a lot to absorb. He's also be pretty "picky" about what he wants (a good thing), so maybe if you're especially talented and pick things up quickly, you can half this time.

    Plans eliminate all the math, shapes, balance and scantling needs a design requires. Like I said, you can still make it a custom job, but the underwater areas and sail plan will work, without having an engineering degree under your belt first.
     
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