Is there a well-defined relationship between Cg and motion?

Discussion in 'Boat Design' started by maritimeman, Apr 21, 2023.

  1. maritimeman
    Joined: Mar 2023
    Posts: 5
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    Location: New York

    maritimeman Junior Member

    Hi all,

    I'm curious to understand if there's a good way to predict the motion of a vessel (say a barge) in the ocean given a Cg. It's my understanding that, for a given lower Cg, the motion in most axes improves - sort of how ocean wind turbines use a very deep ballast to keep upright.

    Taking aside that swells will vary and produce an impact, is there a good way to understand how Cg may improve or change Pitch, Roll, and Heave motion? Put another way, I suspect if you had a wind turbine with a 3000ft deep ballast, it would not pitch/roll very much at all.

    Less concerned about Sway, Surge, and Yaw, also because intuitively I don't think the Cg has a big impact on those.
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yes and No. There is a broad correlation between a vessels GMT (Transverse Metacentric Height) and its natural Roll period. Since KG (distance above keel of the Cg) is a factor in GMT, it affects the roll period (T), but is not the only factor. There is a similar effect for GML (Longitudinal Metacentric Height) on natural Pitch period. KG has no effect on natural Heave period (not exactly true, but let's not confuse the simple case). Surge, Sway, and Yaw are generally not periodic except when forced.
    Note the emphasis on natural. Most motions in operations are not natural, but are forced by the seaway. That is a whole other chapter in the textbooks.:eek:
    Last edited: Apr 21, 2023
    DogCavalry and bajansailor like this.
  3. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    To follow up on JEHs post.

    The natural periods of motion are determined by the attributes associated with a restoring force. By that I mean, Roll, Pitch and Heave.
    These are the 3 motions that are influenced by the hull shape/size.

    Rolling - GMT
    Pitching - GML
    Heave - Aw

    The hull shape, and mostly the ratio between under water volume and the waterplane area, influences these 3 natural periods of motion, in association with the location of the KG.

    And then some.
    Seakeeping is terribly complex.
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