38 foot sail catamaran design questions

Discussion in 'Multihulls' started by Steveso, Jan 17, 2017.

  1. Steveso
    Joined: Jul 2015
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    Steveso Junior Member

    Hi,

    I have been designing a 11.6m cruising catamaran, and there are some things I have been wondering about the underwater hull shape.

    I read in Shuttleworth's article about seaworthiness, where he said that the center of buoyancy should move forward in the immersing hull, and backwards in the lifting hull, but how much it should move forward? And how about the total center of buoyancy? Should it stay in the same position, or would it be beneficial if it moves slightly forward to compensate the rigs lift, which try to pitch the boat forward?

    Also the position of Longitudinal center of effort in relation to LCB and LCF is unclear to me. What is the correct position, and how it affects the stability?

    Here are some specs of the design:

    Sail area 68,4sqm (Bermuda rig)
    LOA 11,6
    LWL 10,9m
    BWL 1m
    Displacement 4600kg
    LCB 55,7% (Lwl from the bow)
    LCF 54,8% (Lwl from the bow)
    LCE 51,4% (Lwl from the bow)
    Beam between hull centers 5,1m
    Beam to draft ratio 2,3
    Mid section coefficient 0,78
    Wetted surface area(include rudders and lar keels) 29,8sqm
    Cp 0,61
    Waterplane area 16.8sqm
    Keel centers are directly below the LCE
    Total keel area 2,14sqm foil Naca-0006
    Total rudder area 0,79sqm foil Naca-0012

    The boat will be flat panel foam construction. I chose to use keels instead of daggerboards because it's simpler to build and I can use keels as a gray water tank. I'm an amateur so all tips and comments are welcome, but don't tell me to buy a boat.;)
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    That is part of the calculations for comfort at sea. If the changes are too large, the movement is very fast which usually induces seasickness. Search for published standards. The increase or decrease of buoyancy are not the only parameters that affect the movement. For example, a heavier rig will slow down the pitching period, which in turn could allow for a hull with more flare or more rocker. All these parameters are inter-related. Ultimately, you can change values and see what the results are, or write an optimization formula, that will be much harder to do.
     
  3. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member


    how about some Fore/Aft Flume tanks for cats?, which seem to suffer from Hobby Horsing more than monos, and also have their own quirky sea-sickness factor.

    If not Flume Tanks, how about a Fore/Aft equivalent of anti-roll fins? Put a couple little lifting wings fore and aft so that when that they are in the water they lift, and when out of the water they don't. If going fast over calm water they would both act as lifting foils, which should be OK, too.
     
  4. Steveso
    Joined: Jul 2015
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    Steveso Junior Member

    Hi Gonzo,
    Do you know where I could find detailed specs of similarly sized catamarans(or any size catamarans)? Or a book recommendations about multihull design? I have Principles of yacht design by Lars Larsson and Rolf Eliasson, many of the principles are the same, but it would be nice to get hands on some book that concentrates on multihull design and is comprehensive.
     
  5. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Unless you are designing an open deck cat I would increase the displacement to say 6T

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. Steveso
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    Steveso Junior Member

    It's a bridgedeck catamaran. I will increase the displacement, and sail area also.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If you are using flat panel construction, is this a Wharram style design?
     

  8. Steveso
    Joined: Jul 2015
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    Steveso Junior Member

    It's a multichine hull.
     
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