Stability Driven Design - Part 1

Designer Tim Kernan gets down with the issue of stability, one we know little about - in any sense of the word! Enjoy.

Of all the parameters governing a sailing yacht's speed potential, its stability picture is arguably the most critical. I say "picture" because stability can be developed in several ways and must be considered within a highly dynamic context. Optimization of various stability-producing factors has lead to significant speed gains in recent years, pushing the state-of-the art in design forward. In this series of articles we'll take a look at stability in recent design trends and how it has been manipulated to produce ever faster yachts, beginning with an overview of the basic factors governing stability.

First, let's define "stability" as the yacht's tendency to remain upright in reaction to inclining forces from the sail plan, waves, etc. Inclining represents two of the six degrees of freedom governing a vessel: pitching and rolling (heeling), but we are mainly concerned with heeling as it relates to the heeling/driving force from the sail plan, and this is generally quantified in terms of RM or Righting Moment.

We like RM. RM enables the yacht to resist the heeling force from the sail plan, and since heeling force comes hand in hand with driving force, the greater the RM, the greater the potential driving force. Greater driving force = greater good. How is RM developed? It is developed through a combination of hull form parameters "form stability", and weight distribution "pendulum stability". Simply put, when a yacht is heeled its center of buoyancy (B) moves to leeward (to B1) while its center of gravity (G) remains stationary. This creates a force couple between the two called the righting arm, or GZ. B & G must align vertically in the equilibrium state due to the force of gravity. (Fig 1) RM is the product of GZ and Displacement, so RM can be increased by 1.) Increasing Displacement or 2.) Increasing GZ. Let's assume we don't want to increase displacement-for pure top end speed potential we want the maximum RM for the minimum displacement. So we need to focus on GZ, and there are several ways of increasing GZ. For a fixed keel yacht with non-moveable ballast there are two options: 1.) lower the center of gravity (G), or 2.) cause the center of buoyancy (B) to move further to leeward. That's it, in a nutshell.

It can become expensive to drive CGs ever lower and lower, and it becomes impractical beyond a certain point given draft restrictions and the cost of engineering hydrodynamically efficient appendages with extreme spans, as well as super-light above water line components. And while the struggle for ever lower CGs continues, designers have started exploiting other methods of increasing RM by increasing GZ. Increasing waterline beam is one way of doing this, ie; moving the center of buoyancy further to leeward. Design trends in the Open 60 and mini-Transat classes are extreme examples of this. When these wide-beamed hulls heel the CB moves to leeward very quickly, developing large initial GZ values, but this is at the expense of ultimate stability. In this regard these boats are much like multihulls and can have low angles of ultimate stability-ie; the angle at which GZ becomes zero and then negative. At this point the yacht has no tendency to right itself, and as was the case in some early Open 60 designs, the yacht can be quite comfortable floating upside down. The form stability is such that even with the CG well above the inverted waterline, GZ remains positive in the inverted state creating a positive inverted Righting Moment, or what I refer to as "Wronging Moment". There are other disadvantages to large waterline beam in proportion to length, namely that both form drag and friction drag are significantly increased at low heel angles, creating a vessel that for maximum performance needs to be sailed "on the edge, heeled" at all times, requiring huge expanses of sail in light air to generate the optimum heel angle (akin to doing "the wild thing", as performed by those crazy multihull sailors). Additionally, largely asymmetric waterlines when heeled (created by fat sterns and pointy bows) create fore and aft trim issues in addition to significant changes in the hull's center of effort, affecting helm and balance.

So short of any impending breakthroughs in hull form design the quest for stability turns to the mechanical twist on the other end of the equation, i.e.; moving the fixed CG, or unfixing the CG. Canting keels and moveable ballast are all the rage. This really changes the game, not only because of the ability to generate more righting moment, but to develop more righting moment independent of the yacht's angle of heel. This changes the entire stability picture, and in the next installment we'll take a more detailed look at moveable ballast, how development in this area is changing hull forms, and why form stability may soon be obsolete.

09.08/06