Building a Low Cost 12’ trimaran

Discussion in 'Multihulls' started by joe1947, Apr 12, 2013.

  1. joe1947
    Joined: Mar 2013
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    Location: Bali Indonesia

    joe1947 Junior Member

    Hi all

    I am living now in North Bali, 5 minutes from the sea. The see here is mirror like with light winds except January and February when the waves can be up to 2’ with strong winds. Amazingly, all the fishing boats are trimaran but nobody is sailing for fun. All have also sails like a reversed lateen rig for slow sailing while fishing (in addition to the engine, not outboard, just with simple long shaft), but nobody sails for fun.
    I am intending to build a 12’ trimaran to sail it for fun. Low cost and getting it ready fast as I don’t own now a boat are the main issues. I have made the design based on many articles I have read and hopefully it will sale fine. The reason to go for a trimaran is that is safe under sails. I mean if the AMA begins to go too deep into the water, you can release sails so the chance of capsizing is very small.
    The AMA’s connection to the VAKA, the mast and the booms will be from bamboo that is available here and is very cheap.
    The AMA’s and VAKA construction from 6mm plywood covered by a layer of fiberglass with polyester.

    Any opinions regarding the design will be very much welcome.
    Attached are the drawings of the AMA, VAKA, mounted boat and Hydrostatic reports. Will be happy to provide any additional information.
    Length: 3.6 meter
    Beam: 3.0 meter. VAKA beam 62 cm. AMA beam 31 cm.
    VAKA Displacement at 18 cm water line: 168 Kg. Free board: 41cm
    VAKA & AMA’s Displacement at 24 cm water line: 253 Kg. VAKA Free board 35 cm
    AMA Displacement almost submerged: 128 Kg, at angle of heel 15º
    Joe
     

    Attached Files:

  2. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    I am not a NA, but I like the concept. You will need a few bulkheads to hold the shape, and I would have the gunwales come together at the stern to reduce the drag of the hull.

    You also do not show any dagger boards or rudders, which you will need of course. The rule of thumb is the dagger board should be about 5 percent of the sail area and the rudder about half of that.

    You should also give some consideration on how you are going to attached the beams at both the outriggers and the main hull, the forces are quite large. I have seen some simple attachments where they were lashed on, you should more them a bit closer together to give you more structure to lash to on the hulls.

    Is there a reason to make a tri? A small catamaran is usually just as fast, and only has two hulls to build, rather than 3. And you get a larger trampoline deck to lay on for the same overall size boat.
     
  3. joe1947
    Joined: Mar 2013
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    Location: Bali Indonesia

    joe1947 Junior Member

    Hi Petros

    Thank you for the input.
    I hope that the V shape of the AMA’s will eliminate the need of a dagger board or lee boards, for simplicity in building.
    The bulkheads I am planning at the transom and in front of the mast rises 8 cm above the deck (VAKA & AMA) and will be reinforced to hold the beams with two bolts at each place. I am intending to use for the beams 6 to 7 cm diameter bamboo reinforced in the lace of the bolts.
    The reason to make a tri is because I am intending to teach sailing my son (14) and my daughter (just 3&1/2 now) and as I am 65, can’t move fast enough to prevent capsizing. I have sailed Hobbycat and capsized more than ones not feeling when is coming.
    With a tri, if the AMA going too deep, I can release sails easy and avoid capsizing (as I understand from reading articles comparing cat’s and tri’s).

    Joe
     
  4. AMW marine
    Joined: Apr 2013
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    Location: United Kingdom

    AMW marine boatbuilder

    looks like a lovely design go for it i say

    rgards ash
     
  5. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    the hull is not going to give you much lateral resistance, better plan on at least a lee board or you not make a lot of headway. Otherwise, it will not point into the wind at all and even going down wind the rudder will not be very effective unless you have a dagger board. A simple dagger board is not too complicated. Or you can put a low aspect ratio fixed keel that will still allow you to drag it up on a beach.

    bolting through bamboo does not seem like a good idea, lashing them in place spreads the load out better and is easy to field adjust, replace. no bolts to drop and loose either.
     
  6. joe1947
    Joined: Mar 2013
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    Location: Bali Indonesia

    joe1947 Junior Member

    Petros

    I was thinking that because of the AMA’s V shape it will give me enough lateral resistance if pointing to the wind or sailing side wind if it will be enough submerged.
    The local fishing boats have very thin AMA’s from bamboo and no dagger board, but they are not sailing upwind. They are using the engine.
    I will add reinforcements to be able to use lee bords on the VAKA. What will be the best logitudinal location in your opinion?

    Joe
     
  7. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    best approximation for lee board location is to determine where the centroid of the total sail area will be, and put the centroid of the board about 2-3 percent aft of that location. should be close enough, you can trim the sails or mast rake to eliminate any heavy loads on the tiller.
     

  8. joe1947
    Joined: Mar 2013
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    Location: Bali Indonesia

    joe1947 Junior Member

    Petros

    Thanks for the suggestions

    Joe
     
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