Sailboat directional stability: center of gravity (CG) related to center of lateral resistance (CLR)

Discussion in 'Hydrodynamics and Aerodynamics' started by Federico Ferretti, Dec 27, 2019.

  1. Federico Ferretti
    Joined: Mar 2019
    Posts: 3
    Likes: 0, Points: 1
    Location: Italy

    Federico Ferretti New Member

    Hi,
    according to this document (junkrigassociation.org) in order to have directional stability

    "The Centre of Lateral Resistance, CLR must be positioned aft of the boat’s Centre of Gravity, CG. Then, like an arrow, the boat will want to go in a
    straight line. With the CG aft of the CLR the tiller can not be left for a second or the
    boat will throw itself into a sharp turn. Such a boat is simply directionally unstable.
    "


    I think this is not intuitive, in my opinion the CG must be positioned aft both CLR and CE:
    the weight (CG) should be behind the towing force point (CE) in order to move on a straight line.

    What is the truth and why?

    Thank you in advance
    Federico
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It sounds like nonsense. There are many factors involved in making a boat directionally stable. The simile to an arrow is not accurate at all. Boats operate on the interface between two dissimilar fluids; unlike arrows. For example, boats that exhibit large changes in symmetry when heeled will be directionally unstable in gusty winds.
     
  3. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    I agree with Gonzo in that such a claim is nonsense. The relationship of CB and CLR is off interest but there are way too many other factors to assign that notion to a design concept.
     

  4. Earl Boebert
    Joined: Dec 2005
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    Location: Albuquerque NM USA

    Earl Boebert Senior Member

    Agreed that the statement is nonsense. The question of directional stability was of significant concern to model yacht designers in the free-sailing (pre-radio) era and I've read every article I could obtain on the subject. I consolidated my efforts in a paper for the CSYS some years ago. I never encountered a statement such as that quoted by OP. What does seem to contribute to directional stability is to have the longitudinal center of floatation coincide with the longitudinal center of buoyancy/CG. There is evidence that the (to put it mildly) controversial methods of Admiral Turner are actually an elaborate way of achieving such a result.

    Cheers,

    Earl
     
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