Calculate Sectional Draft?

Discussion in 'Stability' started by spincircles, Feb 23, 2008.

  1. spincircles
    Joined: Feb 2008
    Posts: 1
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: Troy,Mi

    spincircles New Member

    For an 11 ft Classic Moth Dinghy, I'm trying to figure out how to
    calculate the draft required to provide about 100 sq inches of cross
    sectional area below the waterline midships. Beam is 54". LOA=11'

    Internet searches turn up how to calculate a quarterback's draft
    rating, and some hydrostatics calculations for a crazy looking flying
    foil cruiser but nothing clear enough for me to apply.

    Can anyone recommend a primer website or good book on the required
    calculations? I can accept that I may have a learning curve to
    negotiate here to gain a working knowledge and am willing to do the

    If you can nudge me in towards the path of knowledge, I'll go there.
    I also have a Hydro Dynamics engineer sitting four feet away from me
    at work, but he has been grumpy and busy recently trying to keep road
    vehicles from blowing up ("ARRGH, deflagration, not explosion..."he

    I know that trim is greatly, greatly subject to where I sit in a dinghy, but want ot capture the draft midships and design a hull with a specific bow and stern trim with me sitting at one specific point in the hull. Intent is to design a hull that trims to allow me to sit forward in light air to reduce wetted area and then slide back in heavier air to promote planing.

    Pat Smith
    Troy, Mi
    1 person likes this.
  2. ivor Bittle
    Joined: Feb 2008
    Posts: 51
    Likes: 7, Points: 0, Legacy Rep: 135
    Location: England

    ivor Bittle Junior Member

    calculate sectional draft

    Try reading my website for starters.Do not ingore the section on bulbous bows.

    Ivor Bittle
    Last edited: Feb 23, 2008
    1 person likes this.
  3. jehardiman
    Joined: Aug 2004
    Posts: 2,870
    Likes: 418, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    There is no direct calculation for real hull section shapes.

    The process is...

    1)Select a forebody, midships and afterbody shape along with an estimated rocker profile from an estimated displacement distribution and total weight.

    2) Run up a fair set of lines and bonjean curves and a good weight estimate.

    3) Using the bonjean curves and the weight estimate, work out the actual displacement distribution curve, waterlines, and trim.

    4) Pull the midships area off the bonjeans or displacement distribution curve.

    5) Check your lateral planes

    6) Figure out the corrections needed to your rocker curves, body shapes, and displacement distribution curve.

    7) Start all over at 1).
    Last edited: Feb 23, 2008
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