Prismatic coefficient for deep vee planing hull

Discussion in 'Boat Design' started by Mamaboo, Aug 8, 2008.

  1. Mamaboo
    Joined: Aug 2008
    Posts: 5
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    Location: USA

    Mamaboo Junior Member

    I am an amateur who fiddles with boat design for fun. So far, I've learned enough to be dangerous, but I'm having fun and am hungry for more knowledge. Lately, I am working on a 37' deep-vee Express-style sportfish to be powered by triple four stroke outboards on an integrated bracket. The hull looks pretty good to my eye, and the displacement is right where I thought it would be at about 19,000-20,000 lbs loaded, but prismatic coefficient is in the .84 range when using midship section to calculate it. From what I've read, this seems a tad high, but I'm not sure if that is the case or even how important it is in a planning hull that should top out around 45 knots. Also, I'm confused on what section area I should be using to calculate the PC. Should I use the midship section even if it is not the largest section (which happens to be the aftermost section)? I'd love for someone to educate me on this subject. Attaching lines drawing in case anyone wants to critique it.
     

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  2. ruysg
    Joined: Feb 2008
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    Location: Sao Paulo, Brazil

    ruysg Junior Member

    You shuold use your largest area section to calculate your Cp, wich isn't necessary at midship. You should get a more reasonable Cp then.

    Hope it helps. Good luck with your boat
    Ruy
     
  3. Nomad
    Joined: Feb 2002
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    Location: Perry, Florida

    Nomad Mold Trader/Boat Builder

    I can't help you with your question but I have a few questions for you. What is the beam of the boat? Is it 37' LOA? Where did you come up with the weight estimate? What is the deadrise?
     
  4. Mamaboo
    Joined: Aug 2008
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    Location: USA

    Mamaboo Junior Member

    Beam is 12'6". I need to clarify LOA, which is actually 39' 8" once I decided to add the integrated bracket. I actually started the design with the idea that it would use a bolt on bracket, in which case the molded hull was 37'. Deadrise at transom is around 22 degrees. At the dwl shown in the drawings, displacement is calculated at 18,900 lbs as calculated by Freeship. I am aiming for a fully loaded displacement with gear, crew, fuel, etc. of no more than 22,000 pounds, which from my research is pretty much in line with real weight of similar express boats on the market such as those from Pursuit, Contender, or Grady.
     

  5. Nomad
    Joined: Feb 2002
    Posts: 462
    Likes: 2, Points: 0, Legacy Rep: 12
    Location: Perry, Florida

    Nomad Mold Trader/Boat Builder

    Is this a project that you are just playing with or one that you intend to sell? If it's the latter of the two send me an email or PM. I have a few pointers that I could offer you if you are interested.
     
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