DIY Simple Catamaran Sailboat Design

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

  1. peterAustralia
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    peterAustralia Senior Member

    Outrigger canoes, catamarans, trimarans - all are good

    I have sailed an outrigger canoe, and they are pretty stable. My outrigger was 14ft long, deep vee and had added weight for stability. Thus it totalled 30kg. That is massive righting moment.

    It seems you want a catamaran, that is fine. It looks as though this is what you will build, regardless of what any of us say.

    My suggestion is start building now! Start with a frame, add the plywood later. Use the frame for strength and chine logs. 16ft seems a good rough size to start with. Buy some 6mm plywood, get gorilla glue to bond your plywood to the frame. Use cheap galvanised screws for additional strength. We are talking about a boat that has a lifespan of six months then it gets thrown away, so galvanised is fine.

    When you cut an 8ft x 4ft plywood sheet in half, it ends up at 8ft x 2ft. Thus 2ft from keel to gunnel is a good rough estimate.

    You join your two 8 x 2 plywood sheets together lengthwise, so you get a 16ft x 2ft sheet. A couple of these together, join the ends together, add some wood to space out your hull. Guess what, you now have a boat shape. You need two of these boat shapes. Add a solid bridge deck. Use 4x2 or 3x2 for your mast and also your crossbeams., cheap and simple.
  2. bregalad
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    bregalad Senior Member

    Please visit the link Skyak provided. Read everything on that site.
    Consider that Matt Layden, the designer and builder of those small sharpies, and the Bolducs who are later owners have cruised those boats for many years. Little Cruiser is 25 years old and has been sailed from the Bay of Fundy to the Bahamas. It has made something like 10 trips to the Bahamas each lasting a couple months or more.

    The Bolduc's many cruises have all been mostly drama free. The boat has been maintained in coastwise seaworthy condition for decades without great financial outlay.

    They may not be the boat for everyone, but they do serve as a blueprint for small, well found inexpensive cruisers.

    There are two podcasts with the Bolducs totaling 90 min or so
    Part one

    If you want to continue on the small catamaran path you might want to read 'Beach Cruising and Coastal Camping' by Ida Little

    Ida and Michael Walsh cruised the Bahamas for 17 years using a Hobie cat and an old Grumman canoe.

    Webb Chiles has completed 5 circumnavigations, including going most of the way around in an 18' open boat ... a Drascomb Lugger.
    His web site is and a pdf of much of his open boat voyage is available for free on his site ... Link to the book

    Rory McDougall sailed his 21' Wharram catamaran around the world and later to 2nd place in the Jester trans-Atlantic race.
    His blog

    Think through this carefully before you start whittling on wood. Once out of swimming distance from shore things can get very interesting very quickly.
  3. SamSam
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    SamSam Senior Member

    I've never seen birch plywood in anything other than an indoor cabinet type plywood. Birch wood itself will rot in about 15 minutes if left outside. Is this the same guy telling you about tree sap resin? What was the story on that anyways, is that something on the shelf at a store, labeled as marine grade tree sap resin or something?
  4. WilliamPrince
    Joined: Oct 2013
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    Location: Mexico

    WilliamPrince Junior Member

    Skyak - I feel like an idiot for not answering these questions earlier now.. I definitely have some strange specifications for this boat, and of course that will factor into the design/construction. Let me elaborate on my intended voyage then, so all things can be considered.

    I plan to sail from San Jose del Cabo, where I am currently living/building, to mainland Mexico, then along the coast down to Costa Rica. After that, the boat does not matter to me, but I hope to have it last until February of next year.

    As for the "epoxy and glass" suggestion - are you speaking of fiberglass? And fiberglassing the entire hull? The fiberglass may not be expensive, but the Epoxy sure is... Although if it is the only way the boat will last 3 months sitting in the water, then I can consider it.

    Like I mentioned earlier, I am looking to treat the wood with something to waterproof it before the outside coat of epoxy... I've been looking into CEPS and Danish Oil and Polyurethane+thinner... What is the best option here? If it would help, I could upload pictures of the products available near me. It is damn hard to decipher them and figure out exactly which one does what I want, without ******* anything else up.

    I am going to be sleeping aboard, although pulling up on a beach whenever possible, pretty much every night if I am able. Because of that, I am not sure about a V shaped hull, I am thinking more of a tapered flat bottom; essentially a V shape with the pointy part chopped off.. Like an upside down ziggurat... Like the side view of a lampshade... Okay you get the point. That way, I get the best of both worlds, and pulling it up on a beach is no problem.

    As for what boat I am building... I have very little sailing or boating experience, and I can only really make my choice based on what I read online, and hear from people. Based on what I have seen and heard, a cat is the best option for me in many ways. A monohull could be more bang for my buck, a trimaran or an outrigger canoe might be more suited for my journey, I might be wrong about a cat being superior, but I think it will get the job done. It doesn't need to be the most agile, quick, or smooth riding vessel ever; it just needs to be seaworthy.
  5. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Peter, that is some of the most straightforward and helpful building advice I have heard yet.. Remember, I am almost a total noobie when it comes to this, so sometimes simple description fits the bill!

    I just have 2 clarification questions for your explanation...

    Firstly, you keep referring to a "deep vee" shape, are you talking about a single chined hull? And by deep do you mean a large angle or a small angle made by the connection? What do you think of the way I described my hull shape design in the post above?

    Secondly, when you talk about making a 16x2 foot plywood, are you talking about using this rectangle, unmodified and untrimmed, as one side of the hull? This seems too simple to me! But maybe I am just overthinking it... Will a 2 foot deep hull even be deep enough? Besides, a 2 foot deep hull in a single chine V shape is different than a 2 foot deep hull in a flat bottomed double chine hull... What are your thoughts?

    Lastly, are you vouching against a stitch and glue style construction? Or should I just do the stitch and glue over the frame as you say?
    Last edited: Oct 16, 2013
  6. peterAustralia
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    peterAustralia Senior Member

    Because you dont really have much access to epoxy, fiberglass and microspheres, it will be hard to make the filets that are used in stitch and glue. So here is the issue, two peices of plywood come together at a joint, they form an angle, how to you build a strong joint?

    One way is to use a fillet with microspheres, epoxy, and fiberglass inside and out. This is a good method. The old style method is stringer frame, you use a chine log. Say you have a 40mm x 40mm timber. You plane it to the angle you want. Then you can glue and screw the plywood to each angle. You have to have some reinforcing at the joint.

    Fiberglass, microspheres and epoxy make a better joint. but since you do not have these, you need to use a chine log where plywood meets. At the gunnel it has a different name, a sheer clamp, but basically it is the same, a long piece of wood.

    Durability of your boat without epoxy. You might be able to buy a gallon for 100 dollars. Polyester resin is half the price (fiberglass resin). Fiberglass resin is ok if your boat is only going to last a few months. Fiberglass resin is pretty good stuff, your boat wont last years and years, but should last a pretty long time

    If you cant afford either, and you are going to paint your plywood, I would hope that your plywood is of reasonable quality. Poor quality plywood will rot very very quickly. If you have cheap plywood it will last longer if you coat it with epoxy or fiberglass resin.

    I am worried on your budget you can only afford very poor quality plywood. I could easily spend 2000 dollars on a boat like this. Good plywood is 80 dollars a sheet here, maybe there is some plywood in the middle, not too cheap, not too expensive.

    Hull shape, that is up to you.

    Flat bottommed is easiest, like Gary Deirking's Wapa. You will need a leeboard though. So mid section has 3 sections, bottom left and right

    Next is probably deep vee. Roughly 25 degrees each way, to make a 50 degree angle. Basically this means a triangle where the vertical height is the same as the horizontal width. So mid section has 2 sections, left and right

    Next is to add a chine, this is more complex. You can have 2 each side, so midsection has 4 layers of plywood.

    this boat has a chine - see
    better boat, but more complex

    here is a deep vee hull
    simple effective. You can have 1 hull or two, up to you, ourigger canoe or catamaran, either is fine.

    here is a flat bottommed boat
    easy to build

    16ft x 2ft plywood gives a rough shape for your hull, it will need trimming to add rocker, lift the bow and lift the stern. It is a rough start.

    I would see if I could buy a gallon of figerglass resin for $50. Should make your boat last longer.

    Because your budget is so small, not sure if you can build much more than this

    thats why I liked Roonio, it had a solid bridge deck for sleeping on. Also it had more storage in the hulls

    If you want to mess about a little with plywood shapes, get some cardboard and stickytape, 16 inches x 2 inches is your starting point. Make 2. Trim each at the stern and bow, join them at the bottom, you have a boat shape.

    You kinda have to decide what boat you like the best.
    Wharram Melanesia
    Solway Dory tri

    something else

    get stuck into building the frame, add plywood later. Your running out of time
    You can see a boat frame here, post 27

    From now, you have to decide which boat you like the best, which you can afford, what plywood is available where you are, the cost etc etc. 6mm plywood would be a good starting point. The very cheap stuff is useless, hopefully there is something a little better where you are.

    I have come close to the end of what I can do

    I like Roonio and Flaquita personally, some of the other 16ft cats looked good too.
  7. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    now we have a much better idea of your needs. You are making this trip alone or with crew?

    My ideas about the best boat for your trip are leaning toward a double end monohull. The primary reason is that I think you are going to need to carry so much weight in supplies (200+lbs) they will serve as ballast if you pack them right. The little cruiser site I mentioned last has a page of their provisions and equipment, take a look at it. You are going to need an enclosed boat because of the environment. You are going to need to sail at night sometimes (lights, battery, solar, VHF). You are going to need to stay put on anchor for days in a storm waiting for better weather -comfort. You are going to need the ability to pass through breaking surf with confidence and row when the wind dies. Surf and heavy loading are why I think it should be double end. A pointed tail does not add drag when it sinks and it divides following waves rather than smacking the transom and stuffing the bow. Transoms weaken the hull in racking anyway and you don't need to hang a motor.

    For a sail I think a junk rig would be best. Power low. Easy reefing from inside the cockpit. Balanced easy control. I would build a box mast and fill it with foam flotation.

    We can consider an outrigger but I don't think you should plan a skinny unstable main hull that leaves you high and exposed.

    About epoxy/glass. I will say it again, for 'waterproofing' protecting the plywood, adding strength, I don't know of anything better than epoxy. I think you should have epoxy and a layer of glass BELOW THE WATERLINE -the portion of the boat that is under water continuously. The majority of the boat can be house paint. Epoxy does not seem overly expensive to me -we should check price. What is your budget for this expedition? I have one suggestion for the budget -don't make the boat 'disposable' -do it right and it can be sold at the end. For a plywood boat that is to be dragged up a beach when it is not sitting in seawater there is no better investment than epoxy and glass on the bottom.

    I am not a fan of 'gorilla glue' -it needs large matching surfaces, it doesn't fill, and can not be repaired -nothing sticks to it. Above the waterline I would use "PL premium construction glue". There is a glue section on this board you can check.
  8. bpw
    Joined: May 2012
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    bpw Senior Member

    An old fiberglass beach cat will be far cheaper the anything you an build, and much better.

    Give up thoughts of going to Costa Rica, sure if you had lots of experience and skill you might pull it off, but you are not there yet. Between you and Costa Rica is one of the nastier pieces of water in the world, the gulf of teahunapec. Remember, if you stop paying attention for a moment and flip you cat somewhere a bit offshore you are as good as dead.

    For bumping around beach to beach inside Baja a beach cat will be fine if you are careful but don't take it outside. It takes lots of skill to run breaking surf in a beach cat, and even experts destroy boats trying.

    Honestly, a kayak sounds like a better boat for the type of travel you want to do. Cheap, simple, easy to build, easy to drag up the beach and can get through a bit of surf on the beach.
  9. Manfred.pech
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    Manfred.pech Senior Member

  10. bpw
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    bpw Senior Member

    And I second skyak on his idea that a monohull will be better in almost every way than a cat for what you want to do.
  11. Skyak
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    Skyak Senior Member

    micro-balloons are for making a light weak sand-able composite. They are expensive and I don't think he needs them at all. I always use "wood flour" (fine sawdust) and fine chopped fiberglass scraps for filleting. There will be no shortage of these materials when you begin building.

    My thought is he doesn't need cheaper crappier materials and to think less do more -he needs build a more efficient smaller boat. Less time, less material.

    The outrigger canoes I see are cool craft I would have recommended before I knew his travel plans. How do you reef sail? Where do you sleep? Where do you stay in bad weather? Where do you cook? Crap?

    I know voyages have been made on such open craft but look closely -did their foul weather clothing cost more than your boat?
  12. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Okay, based on what you have told me, I am open to looking into building a monohull for this. Less time, less materials all sound like good things, and although I am ready to start building now, a few more days to plan won't be the worst thing. I am unsure about changing my plans so far in, but in fact a monohull is what I learned to sail on, so perhaps it will be best in other ways. So let me give this idea a chance.

    If I was to build a monohull, what design would you reccomend? I now have to factor in a daggerboard, which could be complicated to incorporate, and what about pulling it up onto the beach? Would I still use a flat bottom 2 chined design? Should I extend the length? Or is a 16 foot monohull as capable as a 16 foot cat? I will start looking into monohull design plans, if that is really what you think I should pursue, I trust your experience over my intuition.

    As for the waterproofing... I have attached a picture of what I have so far. This is stuff I have collected cheaply by people who have been interested in my idea, the epoxy was practically given to me by a guy who owns a yacht repair dock. I have the tube of 5200 as glue, I've been highly reccomended that for below the waterline. I also have access to reasonably cheap fiberglass, so fiberglassing below the waterline might not be out of the question.

    Construction will start in the next few days - I thought I had a place to build all this time so I had been focusing on other things, but I just found out that it will not work out with that place, so I am running around to find another place as soon as possible. If I do decide to switch my design to a monohull, I will need slightly different lumber, so I need to make this decision soon.
  13. bpw
    Joined: May 2012
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    bpw Senior Member

    Why are you so set on building? you would save a lot of time and money buying an old boat. And could chance you could sell it in the end for what you pay.

    If the goal is to build a boat, great, but you may as we'll build something good.

    If the goal is to go somewhere on a boat, don't waste time and money building.

    If you do build, buy some plans, will save lots of time and money.
    1 person likes this.
  14. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    Peter, I have been messing around with a few interesting hull shapes, I don't really fully understand what I am going for here, so I am just kind of going by looks. Anyways, what do you think of this monohull idea? This honestly wouldn't change the construction method a huge amount, just a deeper larger (longer?) hull...

    As for the joints, I actually can afford at least 2 gallons of epoxy, I have one already and could get another one if it is required. I wonder though, what additives do I need for the different parts of this construction? I want to use something for three main functions, like I mentioned earlier in this thread.

    I want to treat the plywood before anything with some sort of penetrating sealer (CEPS? Danish Oil? Is there an additive/thinner to Epoxy that I can use?).

    I want to strengthen my joints, regardless of what kind of construction I use (Epoxy mixed with some additive.. 204 or 206? Or 409?? I'm not sure)

    I want to treat the outside of my boat with some final protective layer, possibly including fiberglass. If I use fiberglass, it would make sense to use fiberglass resin? But without fiberglass, should I just paint pure epoxy resin on?

    Attached is the pic I forgot to attach in my last post.

    Attached Files:

  15. El_Guero

    El_Guero Previous Member

    You are getting there ....
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