DIY Simple Catamaran Sailboat Design

Discussion in 'Boat Design' started by WilliamPrince, Oct 10, 2013.

  1. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Hey folks, I am planning on building a small catamaran sailboat, in order to sail around the coast of Baja California. There is only light winds, and no strong currents, but of course I want to make the strongest boat possible. I am also on a pretty low budget, so I'm thinking that I will mostly be building out of 2x4s and plywood (laminated plywood for the hull, and treated with some extra waterproofing.) The basic design for this boat I have drawn and attached to this post, but I have some further questions.

    Firstly concerning my pontoons. My pontoons will be the decidig factor in the success of my boat. They must be bouyant enough to hold me and maybe one other person, as well as the rest of the craft, and sturdy enough to withstand some reasonably stormy seas. Of course I won't be taking them out in a massive storm, but they still need to be seaworthy. Originally I was going to build them with 2x4s and plywood, similarly to this instructable, slightly thinner and much deeper, but the same idea of building. The problem is, I don't have a whole lot of woodworking experience, and making 2 seaworthy pontoons would be a stretch even if I did. So I have a few questions concerning these pontoons:

    How large will they have to be to support myself, some gear, and the boat?
    What is the best design for this particular pair of pontoons? Is that duckboat adequate?
    Would I be able to build a good pair? Given the right tools and the right lumber?
    If I don't build them, what is a good thing I can look for to use as a substitute? A pair of canoes seems kinda silly...

    The pontoons are my main problem, and most important. But I do have one more thing that concerns me and that is the mast. The mast will be either wood or aluminum or steel, and will also support the boom of course. There will be a LOT of pressure on these parts during full sail, so I need to make sure they will not fail me at a critical moment.

    What is the best material for the mast?
    How can I attach it to the deck in a firm and sturdy way?
    Will the frame of my weak little thing be enough to provide the support from the guylines and shrouds?

    I know this is a lot of questions, but really any critiques of my design, or any help is GREATLY appreciated. Thank you very much!

    Attached Files:

  2. eyschulman
    Joined: Jul 2011
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    eyschulman Senior Member

    large used aluminum canoes not silly.
  3. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    Here is a good free catamaran design for two. It uses a similar technique to the duck boat you referenced.

    Aluminum tube would be best for the mast, as long as there is no lightning.
  4. gordanm
    Joined: Mar 2009
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    gordanm Junior Member

    Perhaps this would fill the bill ?: Slidercat
  5. Manfred.pech
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    Manfred.pech Senior Member

  6. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Thank you for all the designs, they are all interesting and I will definitely take them all into account... I have a few further questions. As for construction, would the stitch and glue method be strong enough to build a pair of seaworthy 17 foot catamaran hulls? Obviously I will put some ribs in, but the method itself just seems iffy to me...

    Also, about cold molding. Somebody today told me that I was crazy to even try to cold mold my own boat, especially a 20 footer. Is this possible? I'm on a budget as well... What sealant would be best to use? Polyester resin is ******* expensive, about 50$ a gallon, and I don't really have access to tree sap resin which I would rather use. Regardless, am I an idiot to even attempt to cold mold this thing as an untrained amateur? He talked about it as a mastercraft trade, and highly warned me against it... What are your thoughts?
  7. El_Guero

    El_Guero Previous Member

    Quit listening to bathroom lawyers, even real lawyers can get you in trouble.

    First, you need a real design.

    Second, you need to up your budget - cut back on beer for a month or two.

    Third, poly resin is ok, but you would be better off spending the money on epoxy. Get ready for sticker shock.

    Use MARINE GRADE PLYWOOD. Any other ply disintegrates quickly when wet.

    $500 might be possible, but it would be a very tight budget and would need some 'used' material.
  8. Skyak
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    Skyak Senior Member

    Stitch and glue with epoxy has been used for many years to build seaworthy hulls more demanding than what you plan. Epoxy is not cheap but is worthwhile for the critical function it provides. Marine oakum plywood is a good match with epoxy cost and quality.

    Cold mold is not a budget beginner method.

    The free design I pointed out can be used with good plywood and 'PL premium construction adhesive' because it has chine logs to glue against. With high quality house paint protecting it this can work for years. This is the best you can do if you can't afford stitch & glue with epoxy.
  9. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Okay, talking to a bunch of people, and the drive to build a successful seafaring craft had prompted me to raise my budget somewhat. I am still looking to spend as little as possible, but I am willing to spend money on something that will hold my vessel together. I have heard a lot from different people, and opinions differ; here are the main conflicts I am still unsure about.

    Mode of Construction
    I could build it as shown in my original "duckboat" instructional, not sure what to call that method of construction. Starting from the frame, and attaching the plywood to the chines and clamps around the frame.

    Or I could try the stitch and glue method with epoxy. I'm not sure exactly how to do the stitch and glue method, nothing a few hours on the internet couldn't fix I'm sure. Also I do have doubts as to the strength of this method, but I guess it has been tried and tested. I would put at least 3 ribs in each hull anyways, even if I was using stitch and glue.

    What Wood to Use
    I had not even heard of marine plywood until a few days ago, and my original plan was going to be to use regular plywood (maybe some type of laminated plywood), and treat it with some lacquer or varnish to waterproof it. Would this work? What would I treat it with?

    My other option is to use marine plywood, which I still don't entirely understand the benefits of. I will still need to treat the outside with some finish to waterproof it, so why is it worth the extra money?

    Epoxy, Tree Sap, 5200, Oh My!
    The way I see it, I need some type of goop for 3 main uses. The first use of goop (I use goop here to refer to all these different products) will be to adhere 2 pieces of wood together. I will need to glue frame pieces to other frame pieces, and plywood to the frame. Which goop is the best for this?

    Secondly, I need to fill any gaps or stress zones in the construction with some sort of filler, like caulk, or synthetic rubber or something. What is the best material to do this?

    Lastly I want to lay down some protective layer over the outside of my boat. This can be whatever, I am very open to suggestions and exploring new alternatives. I have heard about tree sap resin, epoxy resin, synthetic resin, fiberglass + epoxy... How can I decipher all this and choose the best and cheapest one for my boat?

    Thanks again everybody you have been very helpful!
  10. El_Guero

    El_Guero Previous Member

    Marine grade plywood is a world better than non marine grade.

    BS1088 and above plywoods use water proof glues and the wood they are using is amongst the best for water resistance if the surface gets pierced.

    They have solid surfaces, in B grade or A grade. They have no voids in the cores - a huge difference.

    The best plywood you can find underneath the quality of marine grades are exterior grade construction plywoods, these do not have solid (veneer) surfaces. The glues are not necessarily WBG glue - water proof. The woods used quickly degrade if exposed to water for periods of time. And they have internal voids.

    You can over come some of the deficiencies, but you increase your maintenance costs and lower your resale value greatly.

    And I almost forgot, marine grade plywoods, okoume is about the top grade, have consistent weight control per sheet. And usually okoume is about the lightest weight for the strength plywood out there.
  11. El_Guero

    El_Guero Previous Member

    Slightly easier would be SOF framed hulls. You cold deck the hulls with okoume plywood, or even cherry and teak for beautiful looks.

    Using polyester skin would reduce the weight and the cost of the hulls.
  12. philSweet
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    philSweet Senior Member

    Building a monohull would be easier and quite a bit cheaper for a given level of performance. That's for the performance you'll get for 500 - 1000 bucks. If you check a few boatyards, you can probably find a 25' sailboat that you can sail away in for $500. Bring a bucket, some rags, and a gallon of bleach. Are you planning to sail the Pacific coast of Baja, or the coast of the Gulf of California?
  13. SamSam
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    SamSam Senior Member

    I want to hear more about tree sap resin.
  14. Skyak
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    Skyak Senior Member

    I am not sure if all your questions were answered but I am going to try short and sweet.

    Mode of construction, the duck boat you referenced has lumber in all the corners -I call these chine logs. Their primary purpose is to provide a large glue and fastening area to every edge. Some glues need this. This construction is not easier or lighter than stitch and glue.

    Stitch and glue epoxy on marine ply is the standard. Easiest to learn and quickest. The result has resale value. There is a world of experience to guide you.

    Which goop -epoxy. If you know what to add it can do it all. I have a race kayak that is ply, epoxy, and glass -nothing else. The west systems build/repair guide is the epoxy bible -it's free. The only place I would use something else is for paint for better UV protection and the ability to be stripped. 5200 has instructions to tell you what it's for.

    Tree sap? Hundreds and thousands of years ago sap was used as a sealant -cooked and applied to seams -'pitch'. The only reason I know about it is the EU banned it and a bunch of historical vessels were threatened.

    As I said before, good house paint has been used with enough success I can recommend it for wood boats -big savings. Most stitch & Glue covers the whole exterior with fiberglass and epoxy -and some also do the interior. Some only fiberglass 2 inches to either side of the joint, inside and out. Every failure I have seen has been on an edge, or where there was a hole for something to mount to. If you build in ply protect your edges!

  15. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Okay thank you VERY much for pointing me to the West Systems Build/Repair manual... I finally think I understand epoxy, as much as I ever will at least. The only thing slightly off-putting is that all these additives are pricy... How much of this stuff am I gunna use? Let's say I use it to glue joints with the stitch and glue method, and epoxy fiberglass over the entire thing. Is that one gallon of epoxy resin? 20? That price tag could go way up real quick. What about polyester resin? That stuff is like 10x cheaper here, could I substitute that for any part without seriously endangering the integrity of the ship? Regardless, you just answered a LOT of my questions, thank you kindly.

    I am diving into the dimension-specific design of my hull, and the more I learn, the more I realize that I am in over my head. The more I hear about the importance of a point of balance in the hulls, and planing vs. displacement hulls, and every little nuance that I never imagined existed, the more discouraged I become. But I am determined to build this thing and I want to do it right. Of course, designs for a 20 foot cat are something that people shell out money for, so I wont ask for you guys to point me to the free ones (although if there are ones, I also won't say no...) I would however like to learn whatever information you have to offer me on the matter - what are the main concerns I should be aware of, is a 2.5 foot beam good for a 20 foot hull? How deep should they be? I'm thinking of tapering to a point in the front, and slowly becoming a flat bottom in the back (similar to the slider design mentioned earlier in the thread). I mean, maybe I am overthinking it, but I feel certain that assuming that any ol' shape will be successful is underthinking it. Answers or insights to any of these questions, is, as always, greatly appreciated.

    Thank you Skyak and El_Guero for your helpful insights, very much appreciated. :)
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