Pitch damping in small hull

Discussion in 'Boat Design' started by frank smith, May 14, 2010.

  1. frank smith
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    frank smith Senior Member

    I have been think of a small , relatively large disp. sailboat .
    It would therefore be rather full belled . The question is , what aspects of the hull design would contribute to pitch damping .

    Frank
     
  2. daiquiri
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    daiquiri Engineering and Design

    Longitudinal weight distribution, vertical CG position, sailplan shape and area, fwd and aft hull shapes and volumes...
     
  3. apex1

    apex1 Guest

    The opposite way, longer with the same or less displ.
     
  4. u4ea32
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    u4ea32 Senior Member

    A small heavy sailboat will pitch a lot.

    To make it pitch less, concentrate the weight in the middle, and make it lighter with balanced ends. A fat stern and narrow bow pitches much more than balanced ends. A lighter boat with concentrated mass pitches far less because there is buoyancy to stop pitching and little mass to encourage pitching.

    Before you go any further, I suggest strongly you go sailing on each of the following: A Westsail or similar Colin Archer like boat; a Catalina or similar full-in-both-ends boat; and a Soling or Etchells or similar fine-in-both-ends boat.

    You'll find the boat with by far the best motion at sea is the Soling. Really, no comparison. You'll find the boat you felt seasick on was the Colin Archer. You'll find the boat you are so bored on that you'll wonder why anyone bothers to sail will be the Catalina.
     
  5. frank smith
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    frank smith Senior Member

    To be fair I should say that interest is in a small ocean going boat . Not that I have any intention of sailing across an ocean in a small boat. But I find the design limitation an interesting puzzle. If the boat has to be kept within a length limit of say 12' , and will disp. from 1800-2200 Lbs. , then there are some things that jump out at me suddenly , one is fore and aft pitching .I think you can see where I am going with this . Some may say , "oh no ,not again". But I am thinking about it just the same.
     
  6. GTO
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    GTO Senior Member

    I'm having trouble understanding why concentrating weight in the middle of a boat reduces pitching.

    As I envision it, the lighter the ends are, the quicker they will react to the buoyant force exerted by passing waves. To me, that indicates pitching action will be increased, especially with the banana shaped hull of a traditional sailing vessel, where the stern and even the bow end above the water line.

    If weight is more evenly distributed throughout the hull (relative to waterline beam), the inertia is increased, giving a slower pitch response.

    Anyone care to enlighten me?
     
  7. Ike
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    Ike Senior Member

    It's called moment of interia. If you have a tetter totter and put all the weight at each end it tends to want keep moving up or down. The ends have a high moment of inertia. If you concentrate the weight in the middle the ends are very easily stopped. Same with a boat. If the weight is in the ends, when one end goes up and the other down they just want to keep going even when the water resists the movement.(from the buoyancy of the ends) But if the ends are light, the buoyant force easily overcomes the small moment of inertia and stops the motion.
     
  8. GTO
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    GTO Senior Member

    So you end up with a faster initial pitch, but it dampens out more quickly.
    As opposed to a slower initial pitch that keeps going.

    Okay, I think I can see that.

    I'm guessing the magic is to balance it all so that you don't end up with a pitch that can snap your neck or hardly no pitch at all and the boat imitates a submarine.
     

  9. Ike
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    Ike Senior Member

    right. You want a gentle ride, not a neck snapping ride, but you don't want the bow buried or the stern pooped. You want the boat to rise and fall gently.
     
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