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

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

 

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.

 

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.

 

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