DIY Simple Catamaran Sailboat Design

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

  1. peterAustralia
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    peterAustralia Senior Member

    hi william

    I see that your on a budget, been there, done that.

    Simple hull shapes are best. A catamaran will go fast, be stable, is beachable and unsinkable, simply because it is a catamaran. So your catamaran doesw not have to be the sleekest smoothest boat to still work well. The 18ft Hawk catamaran shown in the above link looks really good.

    Now if your on a budget, what you can do is this. Build one Hawk hull, for the other hull build something really simple, but much much lighter. This will save you weight and money.

    For a mast, wood is cheapest, use some polyester ropes as stays, to keep the mast from falling down. Not the best, but cheapest.

    look at this one, built in 2 days
    http://www.tacking-outrigger.com/red_tri.html

    Forget about resale. You can save money by forgetting about epoxy, just use good quality plywood and paint. Your boat will last a few years before rotting away. However in a few years you probably wont want to go sailing again in the same boat.

    What sort of layout am I talking about
    http://www.mit.edu/people/robot/alaska/alaska.html
    http://www.flaquita.net/

    If I was you, and your on a tight budget, I would build Flaquita, forget about epoxy, just use plywood, wood, screws and nails. I might go for a simple cheaper rig though. Plans for Flaquita are only 100 dollars.
     
  2. Skyak
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    Skyak Senior Member

    You are in over your head if you think you are going to build a 20ft catamaran on a tight budget. From your first post I directed you to a basic design about 16ft with instructions. My reasoning was you could add up the time and cost for a reference point -did you do that and decide 'I want to spend more on something bigger and more expensive!'??

    My advised path if you want to sail a 20ft cat
    1 sailing lessons
    2 consider what owning a 20ft cat takes Still in?
    3 look at used 20 ft cats and compare to what it would cost you to build and the result
    4 if you still want to build buy plans with support

    Beach cats have been raced for more than 30 years. Outdated designs are available that are better than you can build by a greater margin than you can comprehend without years of experience.

    20ft cats have no reason to be other than speed. monos and tris have better accommodations and simpler use.

    My advice is start from a decked canoe light enough for you to carry, cartop, paddle, but big enough for two plus supplies 14 to 18 ft. Add sail and appendages. Want more? bigger sail and ama, maybe two
     
  3. WilliamPrince
    Joined: Oct 2013
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    WilliamPrince Junior Member

    My comment above about a 20 foot cat was misplaced: I am planning on making a 16 foot craft and I will be buying lumber within the next few days, today I am going to look at the prices of Marine plywood, almost everything is damn more expensive here so I hope this isn't a dealbreaker.

    In fact, the length of my boat will probably be somewhat determined by the size of the plywood, which will probably be 8'x4', in which case my boat may end up being under 16 feet. Is there any real danger associated with having a boat that small? Of course it limits where I can take it, but 15' is fully capable of shore cruising, yes?

    Also, I am thinking of doing building the hulls like this: http://m.youtube.com/watch?v=Wuoy1dGOxFI mainly because it is the most in depth tutorial I can find. I think I will need about 10 pieces of plywood for the two hulls, maybe more. If marine plywood is too expensive, I will probably just buy normal plywood.

    How long will normal plywood last in the water? Lets say I paint it with house paint, or whatever other measures will make it more seaworthy, will it last for a month? Or it will it disentagrate in a few days?

    That being said, I'm coming down to the final stages here. Thank you guys so much for all the repeated answers to my nooby questions, without this thread I think my chances of success would have been much lower.'
     
  4. Skyak
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    Skyak Senior Member

    Your one scarf 15-16ft cat should take 4 sheets for the hulls, and there isn't much use for ply in the beams or rig just the boards and rudders. The waterline length to beam ratio on performance multihulls tends to be 11 to 1 or more. This 16ft cat will have a capacity of about 400lbs and sail 10KPH without too much difficulty in the same wind speed on a beam reach. It is not anyone's idea of the most seaworthy cruising boat and you would be wise to stay within 3-5 miles from shore and watch the weather with paranoid obsession.

    A 16ft ply canoe might take 3 sheets, with a fourth to build accommodations flotation lock boxes, hiking seats. It might sail 10KPH with skill and difficulty and have a capacity of 600lbs plus. The kicker is that a canoe can be paddled or rowed at 4 knots easy by 2 crew.

    That video is the stitch and glue method.

    What is your plan for storage and launching? This is a very important consideration for how useful your boat is and how long it lasts.

    About plywood -there is no 'normal'. What you were told before about positive attributes of marine ply are all true...but marine ply is not the only ply that has these. It is registered, monitored and insured to have these attributes.

    The 'good' plywood I have seen most is birch -often from the Baltic or russia.
    -no knots
    -no repairs or cuts anywhere in the plys
    -even number of equal sized plys
    -even straight grain -no wavy patterns
    -absolutely no voids
    -generally hardwood -not conifer
    -almost all plywood is made with resorcinol glue -you can check a small scrap with the boil test

    Plywood like above, built right with epoxy stitch and glue seams, painted well inside and out with quality house paint, pulled out of the water and inspected after each use and any scrapes through the paint rinsed with fresh water, dried and painted promptly should last more than 5 years. Some have gone more than 10. The plywood is dry protected.

    If you don't promptly repair scrapes, if you leave it in the water day and night, if you leave water in it between use, you can lose a year or more of life for each week.
     
  5. WilliamPrince
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    WilliamPrince Junior Member

    Okay, this may sound strange, but basically, I am going to launch it once it is finished, and cruise with it as far as it wi possibly take me. I guess I can haul it out of the water every week or so and do what I can to repair it, with what tools I have, but it will be in the water for it's entire life. I am looking for it to last 1 month at least, the longer the better.

    At this point I think it is clear that I will not be buying marine plywood, it is significantly outside of my budget, at >100$ a sheet. I think four sheets is a conservative estimate.. I think it will take more like 10 sheets to be honest, although maybe it will be somewhere in between. So, what I will be doing, is looking for the best plywood available, based on the specs you mentioned above, and treating it with some epoxy or polyester resin to waterproof it, then applying a coat of house paint over it. I may even put down a layer of fiberglass, but I'm not sure how expensive that is, or difficult. I will need some kind of fiberglass of course, for the joints, so if I am doing it anyways maybe the extra cost to put a layer over the shell wont be incredibly much more.

    Anyways, I have yet to draw exact designs, because I am an absolute novice and all I have is a pencil and some graph paper and a ruler, and I don't even know where to start... What are my chances of succeeding in this thing do you think? I mean, whatever happens I consider it a success, but what about the actual boat.. Will it even float? Jesus.. I need to complete the project, it is very very important. What are some things I can do to simplify matters? Maybe I will go with your canoe idea...

    Anyways Sky, thanks for all your help man, I dedicate this boat to you if it doesn't sink in 20 minutes.
     
  6. WilliamPrince
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    WilliamPrince Junior Member

    This just in: my available options for plywood are cedar and birch, honestly both looked pretty quality, the guy recommended the birch for marine use, but he gave me the impression he didn't know what he was talking about... Any comments? Would 1/4 plywood be alright? Or should I go for 1/2 or even 3/4?
     
  7. peterAustralia
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    peterAustralia Senior Member

    hi william

    here is a list of mulithull plans from Duckworks
    Buy a set of plans - seriously - even if they are 50 dollar plans
    for a 16ft catamaran, 6mm plywood would be fine (1/4 of an inch), even 4mm would work with fiberglass, without fiberglass i woukd go with 6mm.

    http://www.duckworksmagazine.com/r/plansindex/multihullsail.htm

    from memory, some small cats like Roonio make sense, cheap rig, a platform that you can sleep on, and storage in the hulls

    http://www.duckworksbbs.com/plans/jg/roonio/index.htm

    you can also buy an old second hand beach catamaran, here is a beach cat that did a 500 mile trip
    http://www.tacking-outrigger.com/cairns_to_thursday_island.html

    spending cheap, using your boat for awhile, and then let it rot away is OK. Cheapest sail is a tarpaulin or polytarp, wood is cheapest for mast, polyester rope from supermarket is cheapest for lines. Yes it wont last long (a few months), but it might suit your needs
     
  8. WilliamPrince
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    WilliamPrince Junior Member

    Hi peter, all those plans look great, I especially like the "Hot Chili" by Jeff Gilbert and the "La Chatte" by Glen-L.. Those both look very applicable to what I am looking for. Here's my problem: my most limiting budget in this project is time, I need to build this to completion in less than 1 month and a half. Also, I am all the way out here in Mexico, so sometimes shipping itself can take a month to reach me! I have contacted a few places to see if they could possibly offer their plans in a digital form, but I'm not holding my breath.

    Heres what I think I will do: basing my ideas of what I can see from pictures and rough sketches of similar boats (the Slider cat, the 18' Hawk, La Chatte, etc.) I can make a hull which closely mimics their shapes and sizes. I can't do much more than that, and I don't have the knowledge to see what hull shapes would be good or bad, so my best bet is to plagiarize from those who know what they are doing.

    Skyak, I looked more into your outrigger canoe suggestion today, and I honestly think that may have been a good idea, and possibly might still be. It just seems to me, based off my little knowledge of my study of boats in the past week, that it is more in my favor as a cruiser to have a catamaran, given the stability and unsinkability (somebody mentioned that...?) Anyways, I think I will go with the cat for now, if I am able to create 2 nicely shaped and balanced hulls I am prepared to be filled with such an immense feeling of pride and relief and satisfaction than I have ever felt.

    This boat project and the learning and challenge that goes along with it has become a very big part of my life - I will be spending the next month and a half building and perfecting this boat every waking hour, all of your words are greatly appreciated

    As one final thing - does anybody know how I can treat plywood (non-marine grade) for waterproofing? I've been pointed to teak oil and "danish" oil, to saturate the wood with those, but anything oil based will make it impossible to adhere, no? As always, all help much appreciated!
     
  9. peterAustralia
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    peterAustralia Senior Member

    when i have ordered plans from overseas, they usually arrive within a few days, dont see why it would be harder in Mexico. Hey, your not planning to take this boat over to the USA, without a visa are you?

    For your budget, I dont think you can afford hot chilli,,, its too heavy, too much weight

    Build a 16ft main hull, vee bottom, straight sides, that your main hull. add some 4 x 2 timbers, they are your crossbeams (akas). Mast is a square section of wood supported by polyester rope. Smaller hull, just a simple deep vee shape.

    If you need help with offsets, i may have something laying around, please note, these are just offsets. Building in 6 weeks is a pretty tall order, especially for someone that has not built a boat before.

    A flat deck like Roonio is cheaper, easier, sleep under canvas.

    Why the 6 week time limit?

    Your main hull does not have to be that complicated, vee bottom, 16ft x 2.3ft wide, 45 degree angles below the chines, vertical above the chine, Very simple, do daggerboard, add a leeboard if you need one.

    Wapa by Gary Deirking is about the cheapest you can build, flat bottom, flat sides. A 16ft version is available

    http://outriggersailingcanoes.blogspot.com.au/2008/06/waapa-on-launch-pad.html

    http://homepages.paradise.net.nz/garyd/wa_apa.html
     
  10. peterAustralia
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    peterAustralia Senior Member

  11. Skyak
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    Skyak Senior Member

    William,
    I do my best to answer all your questions but you have not answered any of my questions. How will you store, transport, and launch this boat? Does my description of the care needed for a painted plywood boat seem reasonable to you or do you need less care? Without these answers I can not answer you with any certainty and you can not appreciate the value of my answers.

    I think you gravitate toward a catamaran because you are insecure in your sailing capabilities. Catamarans offer some ability to set the sails and not pay attention. A small monohull is actively sailed.

    "hot chilli" is a POS -the plans don't tell you anything you couldn't figure out. "La Chat" is a fine ply beach cat -I don't think you could build it without the plans because of the bent ply construction.

    This will give you an idea of the direction I would recommend.
    http://www.solwaydory.co.uk/products/sailing-canoes/osprey-outrigger-sailing-canoe/
    The traditional Polynesian outriggers have some merit but they also carry some outdated style and limitations.

    I gave you my suggestion for waterproofing -quality house paint. There is nothing under this but wood, epoxy and glass.
     
  12. Skyak
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    Skyak Senior Member

    Birch will bend better and has more strength -best if you will not cover with fiberglass.

    Cedar is lighter, softer, and more rot resistant. If you plan full coverage with fiberglass and epoxy it might be a better choice. Be sure to inspect for knots or any breaks in the plys. If you see any in any sheet in the pile avoid it.

    None of the designs I would consider need more than 1/4" ply.
     
  13. WilliamPrince
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    WilliamPrince Junior Member

    Sorry Sky, I was taking your questions as sort of rhetorical "remember to think about these points" sort of questions, but I will try to answer them here.

    As for storage and launching, there will only really be one launch, when it is completed. I've had a small problem in the master plan, as the place I was going to build did not end up working out, so I am searchig for a new place to build as we speak. I hope to find one by the end of the day, something sheltered, near a beach or marina, and hopefuly near a concregation of boat knowledgable people. To launch... I will probably just drag it to the beach on top of something to protect the bottom, or put it on some dollies or somethig if the beach is far enough. I will figure out launching when the time comes, I think it won't be too difficult.

    Your description of the care needed for painted plywood is something I have to consider, but I don't think I will be able to care for it much. The paint was a FANTASTIC tip, it will be much much cheaper and easier than fiberglass and epoxy, and although I won't really be in a position to drag it out of the water and fix it up (I could find time possibly, if I carry some paint and a brush with me), I think it will last for 2 or 3 months before my plywood begins to soak, at which point my boat will probably be scrap, which I am okay with. Am I wrong in assuming a paint coat would last that long without care? Maybe there is some finish I can apply to the outside of the paint to add another barrier to slow down the decay?

    As for the choice between the catamaran and the outrigger canoe, you are right about me doubting my sailing capabilities. I think my journey will be much easier with a cat, although the construction may be slightly more complicated. However, I am somewhat torn, as I see positives to both sides. Also, the link which peter supplied about the duo that sailed around the northeast cape of australia in a 16 foot cat, they said that an outrigger canoe would have suited them better. That being said, I think this is a slighly different journey, and certainly through different waters.
     
  14. WilliamPrince
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    WilliamPrince Junior Member

    Peter, no I am not planning to sail illegaly into the US, actually I will be sailing south, hopefuly to Costa Rica if all goes according to plan. The six week time limit does sound tough, but I think I can do it. I can spend about 16 hours a day working on the thing, which, especially in succession, is a lot of time. The deadline is to be in Costa Rica by january, and since I am not sure how fast I will be able to sail, I want to give myself some good leeway.

    Those are great plans - I can definitely base my hulls off them. Again, I am really fcking torn between the outrigger and the cat, and even between this post ad the one I wrote 10 minutes ago, I have flip flopped a few times. When you say that an outrigger canoe is more difficult to sail.. How difficult are we talking here? I have basic sailing experience, and I will of course learn on the go... I am considering more an more the outrigger canoe idea.
     

  15. Skyak
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    Skyak Senior Member

    William,
    my launch and care questions are key to recommendations and your answers are much different than I expected. You intend to coastal cruise open ocean and the boat is all but disposable in 3 months? BIG DIFFERENCE!

    If the boat stays in the water continuously you want to cover the bottom in epoxy and glass -everything below the waterline. We need to think about paint below the waterline too -maybe ablative.

    My favoring boats you can carry was wrong. You want to sleep aboard -how many? You might still want something you can drag up the beach.

    I have been reading the blog of a couple that cruises a simple 14ft sharpie. Not my recommendation, but something to consider. Lots of good advice and experience.

    http://www.microcruising.com/lc1.htm

    I need to think a bit.
     
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