Reducing pitch of a spar buoy

Discussion in 'Stability' started by floating, Apr 22, 2013.

  1. floating
    Joined: Jun 2009
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    floating Junior Member

    For a spar buoy of about 1m diameter and 10m draft, I am tasked with designing a mooring that will reduce the spar's pitch in moderate waves. The present mooring design is a compliant single-leg mooring with a bridle that attaches to the spar. Would adding other mooring legs (3 legs instead of 1) reduce the pitch? I am not supposed to alter the structure of the spar (e.g. adding a counterweight or buoyancy), just the mooring.
  2. MikeJohns
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    MikeJohns Senior Member

    Usually you look at the forcing function and change the mass of the spar buoy to de-tune the response. But this sounds like a simple stability issue. I think you'd be better off modifying the buoy than the mooring. Greater displacement required by the sound of it.
  3. tom28571
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    tom28571 Senior Member

    While working with an old Navy whaleboat for a race committee boat, I find that plastic cone shaped "flopper stoppers" are great in reducing rolling by reducing vertical motion of each side. Hanging a set of these (properly sized) under the spar should work with no modification to the spar itself.
  4. jehardiman
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    jehardiman Senior Member

    By "pitch" I assume you mean off vertical axis motion. And by 'bridle" do you mean a two point attachment to the buoy as opposed to a single swivel at the bottom center of the spar?

    As Mike and Tom imply, unless you want to go a 6-point rigid mooring (...not advised)the only way to change the response is to change the ratio of mass to drag of the buoy for the frequency of waves to want to minimize. Note this does not fix the response issue, just pushes it to other frequencies where you don't care about the response. Without seeing the spar and mooring design, not much more can be said. There are a whole lot of things that can be tried. Go get a copy of Buoy Engineering by Henri Berteaux of WHOI, John Wiley & sons 1976 or his 1991 re-write.

  5. floating
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    floating Junior Member

    Yes, by pitch I mean the declination from vertical in any direction, and by bridle I mean a 2-point attachment to the spar's side. The bridle goes to a subsurface float, then a line goes from the float to an anchor.
    Thanks everyone - the message is consistent that a mooring is not going to reduce the spar's excessive pitching motions in waves.
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