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Thread: 18' Foldable Catamaran Sailboat Design From Scratch

  1. #1
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    Default 18' Foldable Catamaran Sailboat Design From Scratch

    This is the rundown. I've been designing a wooden structure, foldable catamaran sailboat with 55gallon drums that compose each hull from scratch. I have a good grasp of the construction of these hulls and how to best construct them, but I am lacking in many other areas. The boat would be foldable along the ridge of one hull with large hinges that would allow it to swing upward. It would also have strong barrel bolts along the bottom to be fastened after unfolding and before use. Here are some of my problems, I was hoping someone would have some suggestions for.

    A) What to build the Mast out of as it must be fairly large for the displacement is fairly large and it will not be very efficient through the water?
    B) Is the current design structurally strong enough to take the pressures of sailing on it?
    C) Will Barrel bolts be strong enough to maintain the connection when on the water?
    D) Netting must be used for the floor to keep any additional weight off, what is the cheapest yet still safe way to go about this?

    And oh yeah, this would be used on small to mid-size lakes nothing crazy, and I would not expect it to set any type of speed records.

    Below are three sketches of the hinge idea, the bolt lock mechanism, and the top down view of the structure in which you can see the additional support around the area where the mast would be located and the sections in which each 55-gallon drum would be placed on each edge.

    Thank You for any tips or advice,

    Sail With the Wind

    20181105_173858.jpg20181105_173341.jpg20181105_165421.jpg

  2. #2
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    Dear SWTW;

    I applaud your efforts to come up with a new idea to get out on the water inexpensively. Unfortunately, I'm not going to be an enthusiastic supporter of this particular idea. To answer your itemized questions;

    A.) Buy a copy of Skene's Elements of Yacht Design - there is a clear explanation of mast structures, how to calculate the stresses on them, and a good introduction to sail area vs. displacement to determine how much sail you need.
    B.) No.
    C.) No.
    D.) There are commercially-available catamaran nettings, but they are expensive. You might look for commercial fish netting, but it might be difficult to find in Chicago. You will need to teach yourself how to mend netting, and how to sew in a bolt-rope.

    Ideas are good things, but to get them to sail successfully you need to apply sound engineering, too. It is not out of your reach; it just requires effort and study. The information you need is out there, you need to find it and apply it.
    Hope for the best, but plan for the worst.

  3. #3
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    I think any folding structure on a multihull would need a considerable overlap in the folding parts, or a considerable separation between the hinging parts -- in other words, a "leverage base" -- forgive me, I don't know the engineering terms.

    You can get a sense of what I mean by looking at various folding/rotating cross-beam structures used on some modern single-outrigger sailing canoes and double-outriggers/trimarans. For example, look a Jim Brown's system on his SeaClipper 20 trimaran -- he uses simple Douglas Fir planks to ease construction for the amateur. These ama cross-beams rotate backward on each side to let this trimaran have an 8.5 foot trailerable beam. The leverage base seems to be between 18 and 24 inches (overlapped beams, once secured in sailing mode). I don't think this is enough for a catamaran (see below).

    Gary Dierking also devised a stainless steel hinge to allow the cross-beams on an outrigger canoe to fold over. These hinges have an approximately 6 inch separation between the hinge part and the pin part, and are made of a pair of 3/16 inch (I think) stainless steel plate with the hinge and pins at the ends of the widest end of the plate. photos here at his blog:

    http://outriggersailingcanoes.blogsp.../it-folds.html


    There is another trimaran builder who makes an ~ 20 trimaran who makes his own fiberglass hinges. I forget his name, but he is well-known.

    Frank Smoot, an amateur who has built many outrigger/tri experiments, uses yet a third folding mechanism that works more or less like the SeaClipper 20's.

    The thing is, hinging outrigger canoe beams is one thing, but hinging catamaran beams is quite another because the compressive stress of the rig is taken by the hull of an outrigger/tri, whereas catamaran beams/platform takes the whole stress. I think if you hinged a catamaran platform, the boat would still have to be nearly as tall (after being folded) as it was wide in the first place because of the overlap between the members needed to deal with the rig stress.

    I suppose one might "pantograph" a relatively short catamaran, but then it would be very long -- but perhaps this is not an impossible situation -- look at the very long Hawaiian outrigger sailing canoes (fiberglas), very light but 45 feet long. A car could tow such a thing, horsepower-wise, easily, but the 'ergonomics' of the trailer might be a challenge. But the catamaran would only have to be pantographed enough to be legally trailerable in width. Say, a 20 foot LOA/15 beam cat would pantograph to 8 feet wide and 30 feet long, and etc.

    The engineering is an interesting problem to think about, in any event. -- Wade
    Last edited by wtarzia; 11-06-2018 at 04:36 PM.

  4. #4
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    I would not expect it to set any type of speed records.
    Well, here's an understatement if ever I saw one.

    As advised already, read up on yacht design. In particular, read up on catamaran design.

    You might not think that your design would tip over very easily. But have you plotted out the stability curve of this proposed boat? Catamarans under 20' (and many larger ones) get flipped over quite easily. At 9' wide, I wouldn't be surprised if yours would turn turtle with a good gust of wind and the crew sitting on the wrong side of the boat. I suspect this because the drums will put a lot of the weight high up. Add enough sail to make this craft move, and the wind will get a grip on it well above the waterline. Once it heels beyond a certain point, what's called reserve stability will drop way off.

    The closest existing craft to your proposal, as you probably realize, is the standard aluminum party boat. They have better shaped hulls with smooth sides and points on the bows, but even so, they don't move all that easily. They take more than a few horsepower to move. And no one I've seen has tried putting a sail rig on one.

    Those party boats, I believe, are generally 8 feet wide. Going to 9 feet as you plan is better. But if you learn how to crunch the numbers, I think you'll find that it's still not enough for safety or any real sail carrying power.

    You can have some fun giving this a shot, but you'll save a lot of headaches and be a lot safer if you hit the books before you start welding.
    -Dave

  5. #5
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    [ATT
    ACH=CONFIG]26241[/ATTACH] Is this what you want? Boat was halfway unfolded when craneman's beepr went of. Boat is Wizard from Richard Woods.
    Attached Images Attached Images
    Last edited by FF; 11-13-2018 at 05:22 AM.

  6. #6
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    This is a horrifying disaster. Which admittedly is somewhat in the spirit of some early catamaran designs. You can build real cat hulls in 2 weeks of part time labour, I have done it on several occasions, and for probably no more than what you are going to spend there, though you may have all that stuff lying around.

    Here is my favourite professionally designed hot mess:

    "Want the challenge of building your own boat, but not TOO MUCH challenge right now"

    That was just one of 14 reasons to build with the Maverick System: The World's safest boat is also the simplest to build!
    Attached Images Attached Images

  7. #7
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    The great advantage of your sensibly chosen form factor of 16-18 feet is that you don't have to fold the thing to achieve trailerability or appropriate behaviour on the water. An 8x16 foot catamaran is just fine. Indeed with the use of some easily managed water ballast the Gougeons managed 32 foot cruising cats on that beam. Last time I emailed with him, Jan was planing a 24 foot version of a similar type.

    I wouldn't worry about stability calcs for two reason: Not all multihull designers seem to have bothered, indeed multis have a lot of self-managing tendencies. But mostly because if I read your plan correctly there are 12 barrels there, and they will weigh nearly 500 pounds alone. So a little head math. An 18 foot cat whose hulls are 12-1 will have a block of 16 inches, or 1.25x 1.25 x18 x.6 (x.76 if you are using fancy curves)x 63= Just over 1k pounds displacement, of which your barrels eat up half. But the deal is cat weight needs to be half displacement, that is typical, so there goes your whole budget. Of course your cans will displace a lot more as they are wider and fuller, but all that is bad. You are in fact building a folding dock. With that weight there would be no reason to build over 8 feet for the form factor as your weight will be so high.

    I wouldn't worry about Skeene. Few people build a boat of this form factor without looking longingly at a hobie rig. Of course that would be not unlike attaching an outboard to a dock then running it flat out as it remains tethered to the shore. But that is another issue.

  8. #8
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    I’m predicting 5+ pages for this one.
    \"A little too tall, coulda used a few pounds...\"

  9. #9
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    I'm predicting going absolutely nowhere.

    A "boat" with barrels won't interest many since it really is just a raft - with none of the good features of a boat.

    Telling someone to ignore Skeene's or other books with naval architecture is almost criminal.

  10. #10
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    Why? I've never heard a small beach cat designer mentioning Skeene or any similar formal naval architect. The problems that small sailing cats face aren't really the sort of thing covered in such a conventional book, are they?

  11. #11
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    Skene's is like a hammer; merely a tool - it's usefulness is dependant on how you use it, and upon what. I just mentioned it as an accessible source of information for mast scantlings and sail area. And yes, the problems that a small cat face in regards to its rig & sails are just the sort of thing covered in conventional naval architecture texts, Chris. The only thing that I don't see covered well is the transverse strength requirements of the deck beams, but there are books out there on cat design that cover this.

    I apologize for butting in on a conversation where I am not wanted - carry on...
    Hope for the best, but plan for the worst.

  12. #12
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    Default Re: 18' Foldable Catamaran Sailboat Design From Scratch

    Oar being stuck in here Back in the 60s, the old man tried a few ideas out on an 18ft cat. Tortured ply with a relatively flat bottom to each hull, but with some rocker built in, but more a bend in the middle of each hull than constant. (This sounds odd and it was) Rig was heavily raked, 25? with a fully battened main with a big roach.
    Point being, it was too wide to trail, so the two main cross beams had hinges in the middle. Bit like the spar attatchments on a wooden a/c wing. They hinged about the lower section, with a bolt put in the upper when unfolded, plus the rig tension held it together.
    Not sure if that helps here... Starting with oil drums is a bit novel for a sailing cat.
    A2

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