Trapezoidal hull ..... ?.??

Discussion in 'Multihulls' started by Rick29, Apr 2, 2015.

  1. Rick29
    Joined: Apr 2015
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    Rick29 New Member

    Hi guys,
    I'm trying to choose a hull cross section for a 3.6m home built, plywood cat.
    Never built before, brief thoughts or recommended reading on trapezoidal hulls.

    I have read "hull cross sections" at MultiHulls Inc but no mention of the above,
    Or does that answer the question, no one does it?

    Cheers
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    First, you need to define what the boat will do and what materials will be used. The cross section is not usually the first design criteria to focus on. However, if this is for example a slow speed houseboat, a rectangular section will be fine.
     
  3. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Even for a fast displacement boat the cross-section will be of minor importance, if it is not intended for competitive sailing. The displacement and the slenderness ratio (LWL/Displ^0.3333) will be two major factors in choosing the principal dimensions in this case.
     
  4. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Can you give us a little sketch, there have been lots of things called trapezoidal.
     
  5. Rick29
    Joined: Apr 2015
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    Rick29 New Member

    Trapezoidal hull,

    Thanks guys,
    I'm new at this, more info from me. An open deck day sailer Cat for closed waters on my local bay. Max two people.

    My guidelines are:
    300kg of load, boat excluded
    Simple to construct from plywood,
    Trailerable, 3.6x2.4m
    Single handed sail,
    Beaching and shallow draft 300mm/1ft or less.

    Similar sail layout as a Hobie 16, prob recycle a 2nd hand,
    I have resonable wood skills, but still want to keep construction to its most simple and cost effectiveness, hence choosing a chine/flat surface type hull (less rounded surface) and less waste by using dimensions that are multiples of materials on supply.

    The beam and Loa also suite an existing trailer I own and my road laws (no permit required).
    I also wanted to build a rigid design, not assemble at boat ramp.
    Speed is not a main priority.
    No daggers or keels, thus the thinking of asymmetrical or trapezoidal hull section.

    Don't know how to download drawings yet,sorry you'll have to read this.
    Cross section (front view) is an 800 deep x 400 wide hull but the inboard bottom of each hull has a section missing, full length, being from the midline of the bottom of hull, 45 degrees up to inboard wall of hull.
    I have chosen 45 deg as a starting point, less aggressive would reduce crossmember stress in down pitch, yes? Crossmembers planned are rectangle alloy tube, with a 2.4 x 2.4m open deck plan. Real basic!

    Dimensions of hull cross section from front, in a clockwise direction from top left (outboard and deck corner) are:
    400 across deck,
    600 down vertical inboard wall,
    283 down 45 deg chine to bot of hull,
    200 across bot of hull: and
    800 up the vertical outboard wall.

    The bow and stern shapes.....later.

    My first build to learn mistakes and practice the basics of design and construction. My library has some older books, 70's and 80's.
    Better reading?

    Cheers
     
  6. rattus
    Joined: Feb 2008
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    rattus SeƱor Member

  7. Rick29
    Joined: Apr 2015
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    Rick29 New Member

    Rattus,
    Thanks, this looks very helpful. More attention to disp/load/sail area.
    The joinery goes on.

    Cheers
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    300kg load in a 12 foot sailing cat, should kill any tendency to runaway speeds.
     

  9. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I agree, 300 kg of useful load is quite high for a single-handed boat and shoal-draft cat. At this point, you might as well think about a monohull.
     
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