I'm new, I want to build a mini trimaran

Discussion in 'Boat Design' started by Willzilla, Dec 3, 2011.

  1. Willzilla
    Joined: Dec 2011
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    Location: New Zealand

    Willzilla Junior Member

    Hi this is my first post here, so here's an introduction.

    I'm 16 and I live in New zealand and I like to make stuff (mainly out of wood). I've been interested in boats for a while and now I'm on my summer holidays I would love to build one. I was going to build a PDRacer and I still could as a starting boat, but that would require all my money and more. I still think its a good idea though but right now I'm looking at other options. I have a bit of sailing experience, I've sailed optimists and helped sail a 30' yacht and a cutter probably about 18-20' (I normally use metric so I'm not that good at estimating feet). I know how sailing works and I know I could quite happily throw together a crude sail boat if my life depended on it! I will also be getting more sailing experience soon too.

    As the title says, I want to build a mini trimaran and to suit my needs I'm thinking I would need to design it myself. This isn't going to be a great boat or anything, just something fun and cheap that I can play with and use as a design/ build project to fill in my need for creation!

    This is what I'm thinking so far;
    I picked up a kayak sail for $2 (online auction nobody else was interested in) and its pretty good. Its a dacron sail (1800mm x 1200mm triangle) and has a wooden mast and boom that's mean't to sit in a hole loosely to spin. The plan was to make a really small boat that I could fit in and sail around on a nice day and maybe so some fishing, but it wouldnt be mean't for it.

    The other option is my friend is going to give me a free old optimist sail which I thought I would use instead because it would have a much bigger sail area.

    So far I'm planning on buying a full sheet of 7mm ply and another sheet of the same that is only 2 3rd's as long. The big sheet would make the whole hull and the smaller sheet would make the 2 ama's (I would just use the full length of the sheets). I've sketched a layout of how the parts will fit and its looking good. This seems like a good size for the boat I want and would easily fit on a roof rack. I'm not completely sure on what wood to use for the aka's and chines but that's not a real problem.

    The aim of this boat is to just be some cheap fun that I can get a bit of experience with, as well as having my own boat to sail. I'm planning to build it the same way a PDR is built, which is simple wood glue and chines from what I've seen. I don't think it would be work glassing this boat because its meant to be a sort of toss around boat.

    Anyway, I came here asking for help on a few design aspects. I reckon if it looks like working trimarans it should work fine so I'm looking at images and videos and drawings if I can find them. The main hull is going to have 2 sides and a flat bottom and I'm wondering if I should put a curve to the bottom or just leave it flat and what effect this may have. Also The ama's are going to be something like an equilateral triangle prism with pointy ends (looking pretty much like most ama's). From what ive seen you want the ama's to be parallel with the water when one is leaning into the water, so I need to make them angled slightly outward from the hull when building? If I use the kayak sail or optimist sail will that affect mast placement? Also does the mast placement matter a whole lot, say I put it close to the bow how would that affect it? I will still try make my boat look as much like other ones as I can with general proportions and placements.

    That's all I can think of for now, any feedback on what Ive said is appreciated! But I'm not likely to turn to building plans that would overall cost quite a bit more in materials etc. This trimaran will probably cost under $100 and that's what I'm aiming for! I dont have a lot of money to be spending on boats! I'm expecting people to tell me to build the PDR though and someone might be able to convince me. It will just cost about 3 times as much (I've already looking into it) which is still cheap as compared to most boats, but most boats arnt even an option! One last question, Would I be able to use 3mm ply and a bit more framing to not spend as much and maybe save some weight? This boat will be pretty similar to a kayak I imagine.

    I can do some drawings if anyone wants to see more detail and analyze them. I would draw them anyway if I was going to put money into this project.

    Thanks for reading!
    1 person likes this.
  2. tunnels

    tunnels Previous Member

    where are you in new Zealand ?? Auckland ??i have a friend is boat designer and boat builder that could help you a lot .
  3. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I would bet you can get a proven design that wont cost all that much more than you plan to spend.

    If so, you will end up with a boat that you know will be safe, and will be able to sail well.

    It would also be easier to build with detailed instructions.

    You may like to consider a catamaran, as in small sizes, trimarans are a lot more work for less accomodation.

    If you do a search of these forums, I am sure you will find something that would appeal. If you are lucky, someone may suggest something suitable.

    Good luck with it all.
  4. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    As designers always say "if in doubt use your eyes and plagiarize"

    So looking at what others have done is always a good start. Have you seen the 3m trimaran class? Might give you some ideas?

    I'd use 4mm ply as from experience 3mm is a bit thin when beaching

    I would recommend drawing your boat before you build it!!

    Maybe my Strike 16 and 18 trimarans will also give you some ideas. The pdf drawing is to scale

    Good luck with your project

    Richard Woods of Woods Designs

  5. Doug Lord
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    Doug Lord Flight Ready

  6. redreuben
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    redreuben redreuben

  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    Some thoughts:

    Unless you have boat-building skills and materials so you make a boat that will not fall apart if left in the water, it is best if the boat is removed from the water after each use. That means a light boat is better than a heavy one. Using 7 mm ply will make an OK boat for a short haul up the beach but perhaps a bit heavy for car-topping. Consider how far you have to move the boat between the water and where it will be stored. The down side to using lighter materials is the construction gets more complicated in order to get enough strength and stiffness for a useful boat.

    If you are limited to a small amount of material a multi-hull will have much less buoyancy than a cat or trimaran. A proa is an efficient compromise. A multi-hull can carry more sail safely than a mono-hull. A boat with the small kayak sail will need to be paddled or rowed upwind, hard to do that with a multi-hull. The Optimist sail is too big to be safely used on a canoe or kayak.

    Designing a boat is not as simple as making something look like a boat. I went that route and my first boat was nice-looking but useless; the next 2 were not as pretty and almost as useless; took me 4 tries to get a useful boat. That didn’t bother me as I was trying to learn design and doing it works better for me than reading a book. But if you want a successful boat with your first build it is wide to use an established design; if cost is a factor there are lots of free designs on the web: avoid the older designs that are taken from old magazines though, newer designs take advantage of more modern materials and will give you more boat for less cost and effort.
  8. Willzilla
    Joined: Dec 2011
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    Location: New Zealand

    Willzilla Junior Member

    I live in Dunedin, down the other end of the country. But thanks anyway!

    To reply to everyone else, my first sketches of "my" trimaran looked a lot like the scarab and I thought I was on the wrong track! But putting aside trimarans for a bit...

    As a simple cat what do you think about using PVC pipes like this? http://rebelcat.com/index.html

    Im not sure how much they would cost and I hope its not much! But I'm mainly looking at the top left picture. I know I could build that but would an optimist sail be suitable, if I build the boat around the sail? My kayak sail would work, it just doesnt have much room under it for doing that. I would have to build upper framing to hold the sail in so I have room to sit! But It might not have to be high at all depending on how I get the sail from side to side. The boom pivots up and I could just grab it and do that. But anyway if I made something like this, Im not sure how well it would work but having a hammock as the seat would be perfect on a nice day! It would make the boat a bit heavy and complicated if I didnt make it bigger though. Otherwise I would just have a few cross bit of wood to sit on and maybe a net. It would be a blast to go fishing with on a nice day at a lake and would easily fit on a roof rack.

    Regarding boat storage, My first plan was to build a bot that could fit in the back seat of a car but that would be too small. We have a trailer I could put it on if I had to transport it. Also I could put it in a boat shed with my friend's boats which doesnt cost much and the smaller it is the more likely there will be room for it. I'm not planning on building anything any time soon that has to stay in the water.

    I like the 3m tri and the scarab but they do look more complicated then what I was planning (with the hull shape etc). I want to build something really basic.

    If I had a boat rigged with the optimist sail, would adding a jib be a possibility? How does this affect keel placement? Its it better to have a keel that you can move forward for the jib and and back for just the main sail?

  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    I don't have construction details but I imagine PC pipes would be heavy. Those pipes look around 12" diameter; regular 12" diameter PVC pipe weights about 10 lb/ft so 2 hulls 14' long would be at least 280 lb before you add fittings, deck and rig. For comparison, a 4 mm ply canoe 12' long weights about 25 lb with seat ready to go, but without sail rig.

    On the other hand, the cat would be far more stable and allow diving and fishing, the canoe would not.

    The PVC would cost more than several sheets of marine ply and lumber needed to build two hulls for a small cat IMHO, at least here in Canada.

    - idle doodlings for a small plywood cat:

    To start with I’d guess a small cat could be no more than 100 lb, and allowing 200 lb for a solo crew and equipment brings the launch weight to 300 lb. Each hull needs at least 4x that in buoyancy, or 1200 lb, which is 19 cu. ft. of water. For a length of 14 ft and allowing an extra 30% for buoyancy lost at the stems gives a cross-sectional area of 1.8 sq. ft., and assuming a roughly equilateral triangular cross-section gives a triangle with 2 ft on each side. That will require 14 x 2 x 3 x 2 x0.8* sq ft for 2 hulls or 4.2 sheets, and since 4 mm weighs 12 lb/sheet the boat skin will weigh 50 lb. So my guess of 100 lb boat weight is about right, with leeboard and rudder, lumber, fittings, glue, paint and rig. You’ll need more than 5 to build it BTW; the 0.8 factor is waste lost to the cutoffs from the curved edges of the ply planks.

    This design basis would yield a boat that could be safely sailed on a single hull and carry more sail than you have. It would also carry more than one person with severely reduced performance. For more modest sailing you could reduce hull buoyancy 50% (probably the bare minimum) which would reduce ply required to 4 sheets and reduce boat weight by 10 lb or so. It would need a smaller rig, I’d guess the optimist rig - around 35 sq ft - would be OK and you could add a jib or genoa later to get more drive from it.

    Obviously you could use less material for the same buoyancy if the hull cross section was square instead of triangular, but you would get a slower boat with inferior stability characteristics.

    I can’t provide a design for you as I am not a boat designer (I do design the occasional canoe but that’s my limit) but these numbers should be in the ballpark.The weights are on the low side, I assumed light duty and small sail area; most cat designs would be higher in both departments.
  10. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member


    Good luck on your project.

    The simplest tri I have seen is here:http://www.duckworksbbs.com/plans/gumprecht/drifter12/index.htm

    I know the plans are for sale, but it might give you enough ideas to avoid the first time problems. There is a high powered catamaran that was originally made in 4MM ply, that should be the biggest thickness you need to go (the original Tornado cat is 20'). But plain plywood gets damaged quickly when it is pulled over the beach. Just a little fiberglass on the bottom would make the boat last a lot longer, if you can afford it.

    Another way to go is to use a canoe hull, if you can find one free or cheap. See this:http://smalltrimarans.com/blog/?p=2063 Of course that is a lot bigger, but made of things you might be able to find.

    Have fun.

  11. rwatson
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    rwatson Senior Member

    THATS the one I was trying to remember .......

    It would be the best solution.

    For a beginner, trying to spend only pennies because you dont want to throw away a lot of money on an amateur design, is a big mistake.

    It will end up costing a lot more than you ever expect as you encounter unexpected problems and have to 'fix' them.

    Getting a proven design, with a proper material list will save you all the mistakes, and in the end, work out cheaper. You will end up with a product that you can re-sell when you want to change boats, as apposed to having to pay tip fees to get rid of it.

    Trying to build a craft for under $100 is like trying to build a car for under $500. Seems like a great idea, but not possible.

    This is from first hand experience. :(
  12. Willzilla
    Joined: Dec 2011
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    Willzilla Junior Member

    I found 4mm ply sheets for $35 each which I could use. I forgot to check if they were exterior grade though. But anyway I see how 4 sheets would be used to make hulls for a cat and if there is any way possible it would be awesome if I could use 1 sheet for one hull, maybe with a few extra bits from another sheet. 3 sheets would cost $105 which I can afford, I don't mind spending a bit more money, as long as its not all at once and I can get the boat finished in a reasonable time.

    I thought PVC tubes with a large diameter wouldnt cost to much but I was wrong so I'm forgetting that! Unless I could find some old ones or something.

    I will get back to Tri's but right now I'm just looking at cats some more (that's what the internet's for, right? :D ) I just noticed most of my paragraphs start with I.

    That boat sounds good, but It still seems like its too much. If I build one that would use a bigger sail rig then what i have then I could need to buy stuff to make a sail (probably from polytarp or tyvek) But it adds more cost and work to it. Do you think it would be possible to build one hull for a cat with 1.5 sheets of ply? Performance is cool and I guess I want a boat to sail it mainly but if I don't have optimal performance I'm not too worried. Just something I can take out on a nice day and cruise around and maybe do some fishing, and also something that could fit in a roof rack. My dad has a caravan and a roof rack which would mean we could go on holidays and go sailing in lakes.
    I'm not demanding you to work out stuff and give me information but if you want to help thats awesome ;) same goes for anyone.

    I'm looking at cats a bit more because I havent really had a good look at them yet and it might be better than trying to build a tri. The reason I chose a tri was because I wanted to make a really mini boat for my kayak sail and it keep its stable building a tri would help a lot. Then i got offered this optimist sail and its kind of been going up in size so now a bigger tri would still be cool but I didnt choose it for what it is really, I just wanted a tiny boat that wouldnt capsize! But now I'm going to make a slightly bigger one.. MAYBE because I dont know yet.

    That one looks very good! It is just getting up to be a bit more than what i planned though. I may or may not because to get funding to make something cool and if I do I will be looking at boats like this! Is that a proa in the background? For what I want, the proa looks much more simple and requires less materials which would suit me better. How do you sail a proa? They could be another option. I was looking at some a while ago and they reminded me of motorbikes with sidecars, would it be hard to make one a bit like that for 2 people? I know it would need to be a bit bigger.


    I just remembered: I know what glassing involves, but what amount of resin and tape would I need to just go around the corners on a small hull and would it be pricey? And how much more would be required to glass the whole outside of a hull? If I was to build a toss around tri or cat I think it would be worth glassing the bottom or just the bottom corners for dragging around and beaching.
  13. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    It comes back to what I was saying about getting more boat for your money with a monohull. With a cat both hulls need to be capable of supporting the entire weight of boat plus crew, and for a small cat it is advisable that the hulls be fully enclosed so you need extra material or the decks. The reason for fully enclosed hulls is to prevent them flooding, so you can get the boat back upright again if it flips. The idea of an old canoe with outrigger floats or amas is a good one that achieves the same end with less work and material if you can get your hands on a canoe, which makes a near ideal center hull for a trimaran.

    For a modest performance canoe-based trimaran the amas do not have to be able to support the entire weight of the boat and crew, just the downward force of the wind on the sail, which is typically around 1.5 lb/sq ft, say 50 lb from the Optimist sail. That’s equivalent to Beaufort 5, you shouldn’t be out in anything stronger than that.

    In the links above - especially the Duckworks one - you can see from the thin crossbeams that the amas do not have a lot of buoyancy. Amas with buoyancy of around 100 lb would give you 2x safety factor and you can easily get 2 amas, 8 ft long with that kind of buoyancy. You can see that the ama hulls tend to have a near-triangular cross-section. If you imagine cutting a ply sheet into 6 strips each 8" wide, you can see how you would get two hulls with a triangular cross-section from that. Now add a keel along the bottom, 3" would be about right, so it looks like a truncated triangle, and you have a fairly typical ama cross section. The triangular shape makes the ama cut the water and waves far better than a square one, and would give the 100 lb buoyancy we are looking for.

    The sketch shows what I mean. The curved deck, shown dotted, is stronger than a flat one but harder to do. The joints between the ply sheets are reinforced with lumber; the inwales can be glued to the side planks (sheer planks) while they are flat to ensure a good joint. The bottom or keel is a solid plank of lumber. I haven't shown frames, 3 or 4 ply or built-up lumber would be about right, and 2 of them should reinforce the attachment of the beams.

    A good waterproof glue should be used, not sure what you can get in NZ, epoxy is the gold standard but there are water-based glues that will do the job if the hull is given about 4 coats of exterior paint or 6 coats of exterior varnish. I see no reason why you would need to apply glass fiber either tape of sheet, if the hulls are handled with reasonable care and the solid lumber keel can take some punishment from rocks.

    The beams need to be strong enough to handle the buoyancy force with some to spare. Note the way the amas are barely kissing the waterline in the Duckworths link: you can mount them a few inches clear of the water, then sail the tri so the amas are out of the water for best performance. With this type of tri ideally you balance the sail force with your weight and the amas are just there to prevent a flip if the wind gusts.

    Attached Files:

  14. Willzilla
    Joined: Dec 2011
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    Willzilla Junior Member

    I like it! I can't find any old canoes online in my area but I know that my cousin is getting or now has an old one! I bet he would like to sail it. Otherwise I would be up for building a nice canoe even if it took a long time, It would be fun for adventures not only sailing and going fishing etc. Now I have to look at canoe plans XD Does anyone know of anything? Preferable small and free plans is always good. The guy I got the kayak sail off showed me a few boats he's built and I think he might have some so I will email him. I will ask around to see if anyone ask and old canoes or anything. typically, how well do these set ups sail?

  15. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    One of the best implementations I've seen is the little wing sailing kit by Ted Warren. It's basically a set of ama's and crossbeams that you drop onto a Kayak. It solves the problem of leeway control by having fixed mini keels on the ama's. Provides a very simple lightweight solution. You could do the same with a kayak or canoe with less sophisticated materials and still have good results.

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