lcb cp midship

Discussion in 'Multihulls' started by metin_mehel, Mar 18, 2010.

  1. metin_mehel
    Joined: Jul 2009
    Posts: 158
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: turkiye

    metin_mehel mech.eng.

    Hello,
    I have some questions for small sailing catamarans:
    1-What are the optimised values of cp and lcb? In what conditions the lcb should be located behind midship (to aft side)? In some resources say
    lcb position 6% lwl.. What is the reference and the direction? Is it to aft?
    2-The default value of midship is Loa/2 in delftship program. What will be the best location of midship?
    3-Is it possible modelling a catamaran by free edition (delftship)?
    4-Is it possible to place at least three control point on a vector (delftship)?
    Thank you very much.
     
    Last edited: Mar 18, 2010
  2. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    This may not be very helpful because by "small catamaran" I take it you mean a beach cat. But in "Sailing Yacht Design - Practice" (edited by Claughton, Wellicome & Shenoi) Alexander Simonis presents a case study for the design of a cruising cat. He presents a table of the design parameters for 17 of his charter cat designs. The LCB for his designs varied from 53.1% to 56.43%, and the center of flotation varied from 53.53% to 58.42% of the waterline length. In contrast to Shuttleworth, he places the LCF a little behind the LCB. In the paper he discusses the evolution of these parameters, and their relation to the rig center of effort, as his designs evolved over time. He says of his latest design "It is considered that the fine entry combined with the LCF positioned as far back as possible would reduce the pitching motion as long as the LCB moved rapidly foward when the bows went down. This is also partly why the bows have a slight angle forward instead of being plumb as seen on earlier cruising designs." For this design (the Moorings 4500), the LCB is at 55.23%, the LCF at 56.36%, the CE at 55.95%, and the center of lateral resistance (CLR) at 56.03%.

    I think it depends on what behavior you want from the boat in different conditions. You may want to have everything farther back on a beach cat, so the bows have more leverage with regard to avoiding pitchpoling.
     
  3. metin_mehel
    Joined: Jul 2009
    Posts: 158
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: turkiye

    metin_mehel mech.eng.

    Are these percentages of 'lwl' starting from 'FP' ore 'AP' ?
    In other words the referance point is 'FP' .
    And could you please tell me what will be appropriate cp value for each condition?
    Thank you very much for your assistance
     
  4. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    FP.

    He presents a table of 17 different designs. Cp ranged from 0.534 to 0.598.

    Again, these numbers are for large charter cats, not beach cats.
     
  5. metin_mehel
    Joined: Jul 2009
    Posts: 158
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: turkiye

    metin_mehel mech.eng.

    How about the hull speed (or froude number) while the cp and lcb values are between the range?
    Thank you very much
     
  6. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    "It is considered that the fine entry combined with the LCF positioned as far back as possible would reduce the pitching motion as long as the LCB moved rapidly foward when the bows went down. This is also partly why the bows have a slight angle forward instead of being plumb as seen on earlier cruising designs."

    Tom, this interests me, because I've never been enthusiastic about plumb bows. Both of my small cat designs have angled bows, because it seems like cheap insurance against pitchpoling, though you get less waterline length in a given overall length. How do you feel about plumb bows?
     
  7. terhohalme
    Joined: Jun 2003
    Posts: 512
    Likes: 40, Points: 28, Legacy Rep: 506
    Location: Kotka, Finland

    terhohalme BEng Boat Technology

    Plumb Bows

    I have found plumb bows more much comfortable than angled bows. In immersion, plumb bows has buoyancy much earlier than angeled bows and they don't sink as deep. Also plumb bow boats have bigger longitudinal second moment of inertia, which reduse their piching. Here is the example:
     

    Attached Files:

  8. Questioner
    Joined: Sep 2007
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Planet Earth

    Questioner Junior Member

    Only have a few numbers for A-Class

    ASG3
    LOA 5486.4mm
    LCB 2946.78, 0, -66.13mm (53.71%)
    LCF 3010.02, 0, 0mm (54.86%)
    COV 3043.49, 0, 83.79mm (55.47%)

    Marstrom
    LOA 5486.4mm
    LCB 2996.80, 0, -65.41mm (54.62%)
    LCF 3060.73, 0, 0mm (55.78%)
    COV 2761.02, , 97.73mm (50.32%)

    COV is Center of Volume for the entire hull

    IMO sailed well, it appears that the Marstrom is the better boat of the 2 right now
     
    Last edited: Apr 12, 2010

  9. metin_mehel
    Joined: Jul 2009
    Posts: 158
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: turkiye

    metin_mehel mech.eng.

    thank you very much for these all good information. pitching prevention is so exciting.
     
Loading...
Similar Threads
  1. manon
    Replies:
    7
    Views:
    4,769
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.