Catamaran hull cross-section

Discussion in 'Multihulls' started by Le_skipper, Oct 22, 2020.

  1. Le_skipper
    Joined: Oct 2020
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    Le_skipper New Member

    I was wondering why some designers include outboard ''bulges'' on their hulls. By eliminating them and moving the hull bottoms outboard (see image), the hull center-to-center beam would be increased, improving the boat's lateral stability. The interior volume would be identical. I understand that the port and starboard hulls would not be identical anymore, but for a custom boat this is not important.
    Any thoughts all you experts?

    Olivier Blanc

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  2. Alik
    Joined: Jul 2003
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    Alik Senior Member

    This is always a compromise of accommodation, structure, performance, stability...
    On the second option I would expect more slamming. Once heeled, the submerged hull shape would be affected.
  3. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    For those who want more room in the hulls, some flare can be good. In your diagram you show that the flared hull allows bench tops each side of the interior walkway. I have a version of this where the flare is much higher - instead of flaring to full width about 400mm above waterline my 38 footer flares about 1.2 m above the waterline on the outboard side. I like it very much. The lower hull shape has more bench space to the inside of the hull but you do not get as wide a bench top and interior storage. The high flare means you get more bench and storage space without some real problems.

    I have designed and built two other boats using the top profile and it is problematic. Slight miscalculations with the build/weight study and the low flare shape will have drastically different properties to that designed. Also when overloaded, the flare will get low to the water on the lee hull when hard pressed. But I like the shape for getting wider bunks into the hull low down. You can use this shape but it requires that the boat never gets heavier over its lifetime and that is a hard ask.
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Welcome to the forum.
    It is not easy to judge the why of the shapes of a hull without being able to analyze more than one cross section.
    Stability on a catamaran is not usually a problem.
    Making the hulls symmetrical can bring the cost of the mold to something like half that of asymmetrical hulls. This is not of great importance in cases of series production but it does, and great, in case of a single hull.
  5. Le_skipper
    Joined: Oct 2020
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    Le_skipper New Member

    When the large inboard chamfer panel starts going below the waterline, the waterplane area increases quickly, isn't that good for load carrying capacity? Is the increase in drag really bad if the hulls are already very slender? Would it slam in some wave conditions?
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    So what is the main problem, an increase in resistance from the rapid increase in demihull beam, with further immersion ?

  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I would say that the bigger hull gonna take more power, pound more unless you start off with plenty of bdeck. One of my favorite designs is the Malcolm Tennant Domino 20.

    gotta ask how it compares 2AC4DD05-CA03-4575-B4FE-1702208B7027.png
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