Questions about Small Cruising Catamaran Design and Construction

Discussion in 'Multihulls' started by ahen, Feb 3, 2017.

  1. ahen
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    ahen Junior Member

    Hello.

    I have decided to create a small catamaran (around 12 ft) as a school project and have created a design in solidworks for the hulls. I am a complete novice to boat building (I was surprised I got the anti-spam question while registering correct), but I have some technical knowledge. I am not looking for a perfect design, but I want to put enough thought into the thing so that it actually sails somewhat well. I decided on a catamaran for a few reasons: they look fun to sail, they have a low draft, and I have always thought they were the coolest looking boats. I don't really want anything fancy, so I decided on making a displacement hull simply because it seems easiest to design. Following that mentality I think the tortured plywood method will be the easiest and quickest way to build a hull, so I am designing stations that plywood, and then fiberglass, will be formed around.

    Is this a good approach? I am completely open to criticism/changes that would make the whole process easier. I plan to start by building the hull and work from there. I want to know if there glaring problems with my design, things that will destroy the boat the second it hits water.

    Because I am not planing on using a V-shaped hull, I want some sort of dagger boards/center board for sailing upwind. I would prefer them to be removable so I can land on beaches easily and avoid ledges.
    Also, I was thinking about connecting both hulls with plywood and fiberglass, having a large platform to sit on.

    What other considerations should I have while designing a hull?

    I don't know how to upload my complete file, so I am including some images.
     

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  2. TANSL
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    TANSL Senior Member

    Welcome to the forum and encouragement with your work. Even if you have to change things, do not give up.
    Those shapes do not look apt for plywood. You will have to torture it a lot, maybe even to the rupture, so that the board can be adjusted to that double curvature. Think of multichine method if you want to use plywood board.
     
  3. upchurchmr
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    upchurchmr Senior Member

    That hull shape could be made with strip planking, but never in tortured plywood.

    In tortured plywood the slope of the shape can't have quick changes, it needs to flow from one section to the next. That means vertically within a section also.

    You might read about tortured plywood in this book, its is a good reference for boats - but in particular plywood using glass/ epoxy. http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf

    Looks like a nice shaped hull.
    You are going to need to pay attention to how much the boat + boards + rudders + rig and sailor will weight.
    That's pretty close to a shape I made for another reason, but it only had 100# flotation per hull.
    You will need some freeboard - meaning you need some of the hull above the water for safety. you might keep the waterline where the semicircular section meets the flat sides at a center section.

    Good luck, seems like a great project.
    One suggestion - copy everything you can from an existing boat. At least until you are convinced there is a good reason to change.
     
  4. ahen
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    ahen Junior Member

    Thanks for the quick responses.
    I was planning on stitching about 4 or so pieces of plywood and then bending it over the form, 2 for the sides, and two for the semicircle on the bottom. Does the stern half of the hull have a gradual enough curve? I see that I should make the bow more rectangular to fit with the pieces of ply, but is the back good enough?
     
  5. TANSL
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    TANSL Senior Member

    In my opinion, if you do not change the shapes it is best that you forget to use the plywood board, unless you use strips of plywood with which what you are using is the method of strip plancking with a little suitable material.
     
  6. upchurchmr
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    upchurchmr Senior Member

    Its hard to tell on an un-dimensioned drawing.
    Go to the Gougeon book, it has a table of minimum radius for tortured ply.
    The bow has no chance at all.

    If you look at a Tornado catamaran, that was the first tortured ply boat I ever saw.
    The bow comes to a sharp point at the bottom, and the angle between the sides gradually increases until about 150 degrees at the stern. The Gougeon book has an illustration of a Tornado being built.
    It doesn't seem like you understand torturing. Generally you do not bend over forms.
     
  7. Stumble
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    Stumble Senior Member

    Is designing it required? I would really suggest for a first build that you buy a set of plans than matches your requirements. Richard Woods has a 12' cat I think.

    As designed, besides being difficult to build, I doubt it will have enough capacity at 12' long to carry a human.
     
  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Do you want a sit-on-top catamaran, or a sit-in-hulls catamaran?

    Here are two 14' plywood sit-on-top catamarans by Richard Woods:
    Pixie 14
    Quattro 14

    And a 16' plywood sit-in-hulls catamaran by Ray Aldridge:
    Slider 16
     
  9. TANSL
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    TANSL Senior Member

    Junior, your design should be improved and, perhaps, you should think first the material that you are going to use to change the design according to that material. But that does not mean you have to throw it away and buy another man's design.
    I say this because I am sure that what you are trying to do is design your own boat. So, go on and good luck.
     
  10. Corley
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    Corley epoxy coated

  11. Ilan Voyager
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    Ilan Voyager Senior Member

    First welcome to the forum.
    A cruising catamaran of 12 feet is a very curious project, as 12 feet is truly a minimum for a simple breach catamaran. Cruising means berths and amenities I see as very difficult to fit in a 12 feet catamaran.
    I think you were meaning a beach catamaran. I counsel you a 14 or 15 feet which won't cost more but will be able to take an adult and 2 children for a sail from a beach.

    To be able to design a boat, you have to know a bit about boatbuilding and a lot in naval architecture...Sailing catamarans are pretty tricky to design, you'll discover when you'll go deeper in the subject. Your question raises in me some doubts and interrogations.

    As a school project yo must have something easy to build in a reasonable number of hours, not requiring a full well equipped shop , and dirt cheap. I shall add if the boat is made in the school the building method has to be not smelly, with limited risks of hazard fire and not use toxic products or dangerous tools. Or you will be in trouble with liabilities and insurances.

    How old are the children? And how many children will be involved in the project?
    Because making only one beach sailing catamaran with 20 12-14 years old children is a very frustrating project. Too complex, too expensive, and how the 20 little monsters will sail with one catamaran without bitter disputes? Or just 10 mn tours with one adult after a long waiting time? A catamaran will also need a dedicated trailer and a suitable place for sailing...

    Whatever the age of the alumni, you are in the obligation to propose a viable project, and you have an obligation of good result as other people's work, time and money is involved. In clear words there no place for guessing and learning in the design. The catamaran must be flawless, sailing well, at the good weight, correctly built and reliable...

    Designing yourself your own boat is a different issue. You are losing your own money and time. If it's a piece of junk, that is tour own problem.

    Many years ago, a girl friend of mine, who was teacher of 24 children 10 to 12 years old in a school of Paris (France) had the idea of a project able to federate the very multicultural children in teams, cheap so the parents could afford it and easy to build with hand tools.

    She asked me advice after a good bed gymnastic exercise. I proposed to her flat bottom canoes, 1/4 inch exterior plywood, 1 inch square clear pine pieces, glued with ordinary construction Sikaflex plus some screws and painted with alkyd paint. Material you could find at this time in any material shop.
    The almost 8 feet canoes could be piled like the fishing doris of the old Banks schooners, so the 12 canoes could be taken on an ordinary flat trailer and used at the nearest nautical base of Paris (Les Mureaux) and have access to the installations, security boats and material, etc...
    The canoes could be tied together in catamarans, improving the security and the fun.

    The local materials shop sold the materials and tools at cost and also .gave a lot of things, I and 2 parents made the cuts of the plywood and templates, lone operation needing electrical tools and all the project was great fun for the children. The resulting canoes were not masterpieces but had very artistic and creative decorations.
    Besides even the poorest parents could afford it without pain, as one of the rich ones, involved in catholic charity organizations, took care of the financing.
    The launching day at Le Mureaux was a great one. Nice barbecue, plenty of good food from all the continents, nice weather and the children had great time with the canoes. All slept exhausted in the bus...The canoeing day became an institution.

    That was about the choice of the project.

    If you can afford catamarans as school project, compounded plywood is not a good method. I know it pretty well as I have been involved and built several boats in compounded, from the canoe to the 30 feet trimaran, including several 18t and 20 feet racing catamarans. I almost forgot also a monodromic proa.

    It's not a good method for a school project because of the following reasons;
    1- The plywood needed is a marine 2.5 to 6 mm (depends on the size) of very high quality, first rate soft wood like okoumé with none defects, with 3 to 5 equal plies glued with phenolic resin. There is maybe now 3 plywood makers in the world, all in Europe, able to make these products and having it in their catalogs. The prices will leave you open mouth. if you add the shipping from Europe to for example South America, that becomes stratospheric.
    And there is no alternative as the quality of the plywoods has dropped badly these last 20 years so skin door plywood (all now with urea no waterproof gluing) and exterior grade plywoods (too many defects) are now totally unsuitable.

    2- If the method is fast, it asks for a level of "technicity" in boat building rather high, or you are going to break a lot of very expensive plywood. It's an eyeball and subtle method, so a lot of people are unable to build by eye and obtain two identical hulls. It's mental issue for many people.

    3- Unless going in very delicate cuts, patches and other technical tricks, the method is almost unable to get enough displacement for catamarans under 16 feet.

    Well designed hard chine plywood, stitch and glue method is almost as fast to build, far easier and you can get a good beach catamaran with it. There are several good designs.

    Read the The Gougeon Brothers on Boat Construction which is is now free. Chapter 24 details compounded plywood. Stitch and glue is not detailed, but you'll find information in Internet.
    The link
    http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    Very nice little beast. Perfect for a young sailor. Well detailed plans but not for he total beginner as there no detailed manual. Thanks to Mr Tucker for such nice free plans.
    The cost can be kept very moderate.
     
  13. sailhand
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    sailhand Junior Member

    Hi junior member and good to see someone so young interested in boats/sailing/catamarans in particular. google arafura cadet, these are a simple flat bottom hullform however they are quite quick on the water. the best feature they have from your perspective is a very simple design and build process. off the beach cats are somewhat narrow with beam/length ratios quite high. if you are wanting a more useful multipurpose type of craft then I suggest a beam lenth ratio of around 12 to 1 this will give your project the displacement required for a solid deck and even a camping/cruiser type craft. I have designed a small catamaran tender with a full selfdraining bridgedeck and it is quite useful in that role. the hullshapes are not optimised for sailing but rather motoring with a small efficient motor and rowing. however the deck arrangement may lend itself to your project quite well. yes it will be slower and have more windage than a straight off the beach cat but do you want a camp cruiser or a racer. my design allows you to keep relatively dry and with the full bridgedeck being almost flat the whole length of the dinghy we have many times used it for pitching a small tent and camping in comfort. the small instant pop up tents that you throw in the air and they pop open are great for this purpose just make sure you get the one with fibreglass rod instead of spring steel or it wont last long. one of the latest high performance sailing/foiling cat dinghys, the ufo foiler dinghy by dave clarke and his father steve of c class fame has dead flat keel panels to aid the hydrodynamic lift. you can check out my dinghy design on this forum if you search my posts and there are 2 youtube videos on it, search magic carpet 3.5 powercat catamaran on youtube and you will find it. again my hull design does not provide the best sailing hull that would be for you to design but for the camp cruising aspect in a 3.5 metre vessel I think it works great. a combination of the deck design modified to suit your requirements, perhaps lower for less windage and sailing hullforms may be the best way to go for a camp cruiser in this size. look to the hobie bravo for an easy rig to step and furl to pack away at night for easy camping. the quickest and easiest build method for you is monocoque constructiun in pvc foam, it is more expensive than the ply epoxy route but not significantly and the build is sooooo much quicker and easier to achieve the same strength and rigidity in the structure. you really could get away with just two bulkheads and the transoms if the rig is kept reasonable and the righting moments/ beam conservative also. good luck and feel free to contact me if you would like more information. you might even cosider free standing cat rigged. i have attached a picture of a simple monocoque type build so you can see what I mean, this design would not suit you but the build method is very very quick and easy.
     

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  14. sailhand
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    sailhand Junior Member


  15. ahen
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    ahen Junior Member

    Thanks for all the replies. I don't really want to build a someone else's design, but I am grateful for all the samples I can look at and take ideas from. I decided I am going to switch to chine construction, I don't think the curves are super important.

    If I make a cat with a chine hull with essentially the same shape as my previous design will it sail? I understand that true mastery of the craft requires a lot of knowledge, but that really isn't an option for me. As long as it floats, won't break, and moves when the wind blows I'll count it as a success.

    On a side note I am a student, so liability isn't really a problem; money could be however. I am currently assuming a local lumber company or home depot could supply me with all the wood I need for a chine hull, but I haven't checked. I live in the US so I didn't think finding materials would be a problem. Somewhere I read I could get by with exterior plywood if I can't get my hands on marine grade. Maybe this wont last forever but I thought it could last for a while with a water-tight fiberglass and epoxy seal and paint. I don't plan to sail open seas so it doesn't need to be a fortress.

    Also my school has a very good shop, I am planning on using a CNC to cut out stations for example. All the tools I could imagine I might need are there.
     
    Last edited: Feb 4, 2017
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