trimaran proportions

Discussion in 'Multihulls' started by raedy07, Feb 23, 2016.

  1. raedy07
    Joined: Dec 2014
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    Location: philippines & spain

    raedy07 New Member

    hello, i wonder if there are established 'total lenght to overall width' proportions for trimarans. reason is i would like to decrease total width as it can be very hard to enter our river, it can be really crowded with other boats. the fact that high-tides are at night doesn't help. i would compensate the decrease in width with increase of flotation in the outriggers so the righting moment would not change. is there a problem with this approach? the boat in question is a plywood deepV philippino-type outrigger motor 'banca'. essentially i tri i guess. cheers, Radek
  2. Tom.151
    Joined: Jul 2009
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    Location: New England, USA

    Tom.151 Best boat so far? Crowther Twiggy (32')

    Would love to see a picture or two - a banca is usually a very pretty boat.

    "Typical" beam/length ratio for much older boats is near 50% which was prox same as catamarans of the day. More recently and for higher performance boats it ranges from 67% to 75%. For racing types it goes to 100%.

    If you add lots of buoyancy to a banca ama , the older small log style anyway, you might find that (for a motor banca) the added weight might make it possible to eliminate one ama - making it a single outrigger and reducing the total beam even more. Just saying.

    Without seeing the boat and getting a feeling for the weights and displacement of the current configuration it's pretty much guess work on my part.

  3. raedy07
    Joined: Dec 2014
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    Location: philippines & spain

    raedy07 New Member


    RAINBOW'S END: LOA 27m, width 16m, draught 1.5m, displacement 8-10tons (?), motor 230hp Mitsubishi 6D22. mainly used for scuba-diving day-trips and also longer excursions. we bought her 2years ago, when prior owner gave up using her as coastal cargo&passenger ship. since then, slowly-slowly, we're converting her to best suit her new destiny :) i next want to turn my attention to the outriggers; i think they're too low (should be just 'kissing' the surface?) , too buoyant (the number of bamboos was increased from 6 to 10 without anybody being able to explain why) and arrangement should b more V-shape - now they're slapping d waves, doesn't feel right. i suppose that's why 1 cross-beam cracked, as the wood was sound & no matchstick (20x25cm)

    Attached Files:

  4. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    Raedy- This is going to be a steep learning curve for you if you are not a naval architect.

    The optimum spacings and displacement of the hulls can be found in the library of Cyberiad. Search Leo Lazauskas, a member of this forum. Also in this forum is a free software Michlet, also by Leo.

    Bamboo outrigers works for the small banca because it is very pliable but when this big banca are made, they use wood which makes it very stiff and prone to cracking.

    Unfortunately, no rule exist in the Philippines regarding the design/strength of the outrigger. MARINA, the governing body doesn't have it. What you can refer to for the design are the international Class Societies rule. DNV and Lloyd's Register has design standards for crossbeam.

    You might also find the study conducted in Japan by a Filipino naval architect Dr. Glen Aguilar regarding the forces on the outriggers.

    This has been extensively discussed in the local forum of Filipino naval architects. PM me so I can show you the leads or how to become a member.

  5. oldsailor7
    Joined: May 2008
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    Location: Sydney Australia

    oldsailor7 Senior Member

    Sounds like you should simply beef up your crossbeams. :idea:
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