Stability in Island Packets


(reprinted from the IP Owner's Newsletter Spring 2012

              STABILITY . . .    by BILL BOLIN, VP, Sales and Marketing

We here at Island Packet Yachts have all become very familiar with stability and seakeeping issues, starting with Bob Johnson's decade-long effort in the 1990s as the sole U.S. sailing industry representative participating in the creation of new international standards for sailing yacht stability.  I have taught a number of seminars on the subject at boat shows over the past two years and thought I’d share my overview of general stability issues and how they relate to your Island Packet.

Stability is defined, in boat designer parlance, as the ability of a yacht to resist capsize and once capsized, the ability of that yacht to recover to an upright position.  The first part of the equation, resistance to capsize, comes mainly from two different sources: the yacht's form stability and the yacht's displacement.  Picture a Jon boat (flat bottomed, beamy) and a canoe (round bottom, narrow).  The Jon boat has more form stability than the canoe and is less likely to capsize.  One can also understand that a heavier displacement yacht will be less likely to capsize than a lighter one.  Imagine a large ocean-going container ship and a small fishing runabout.  It's going to be much harder (take much more energy) for the heavier ship to capsize.

So how does one gauge the actual differences in stability between various yachts?  Most of you have probably seen a stability curve in sailing magazines or product brochures.  It illustrates the overall stability characteristics of a yacht, its sail-carrying power (stiffness) and how able it is to resist or recover from a capsize.  Let's see how these curves are generated and what they mean.

At rest. a yacht has two equal and opposing forces acting on it: gravity, holding the yacht down in the water, and buoyancy, keeping the yacht afloat.  A designer locates the geometric center of these forces (with the center of gravity labeled as "G" and the center of buoyancy as "Z') and will picture them as shown in Figure 1..

When a yacht heels, the center of gravity remains fixed, but the center of buoyancy moves as the shape of the submerged hull changes (Figure 2).  
The horizontal distance between the moving center of buoyancy and the fixed center of gravity, referred to as distance “G-Z”, is called the “righting arm” and can be plotted on a graph for various angles of heel (Figure 3, below).  As the yacht heels at increasing angles, at some point the two centers are again directly above one another and a heel angle beyond this point will result in the yacht going to a fully inverted position and remaining there until some outside action allows it to recover (waves, wind).  This point where the G-Z distance is again at zero is the yacht’s “limit of positive stability” (LPS).  LPS occurs around 120 degrees of heel for a typical fin-keeled boat, and on average about 140 degrees for an Island Packet model.

A stability curve can also supply you with the “righting moment” of a yacht, or the force (torque) trying to push the yacht back into an upright and static position.  In Figure 4, I have taken the displacement of a typical 46-foot fin-keeled yacht (Brand XYZ) and an Island Packet 460 and multiplied these numbers by their respective righting arms (in feet) to get “foot pounds of righting moment (torque)” at various angles of heel.  

The resulting curves show the amount of torque each yacht exerts throughout the stability range.  Note that at just 15 degrees of heel (a typical sailing angle) the IP460 is exerting over 20,000 “foot pounds” of torque that balances the heeling force of the sails.  Let the sheets go and the yacht comes back upright quickly!  And the fin-keeled boat, at this same angle of heel, has about half the righting moment, due to both a shorter righting arm and lower displacement, which translates to about half the sail carrying ability (so reefing early might be a good idea!).

One more very important observation to glean from the stability curve: the shaded areas “under” the curve above and below the horizontal axis relate to the amount of energy needed to heel the yacht IPY design up to the LPS and then for it to recover from capsize beyond that point.  Note in Figure 4 how much more energy is needed for an Island Packet to get to its LPS vs. the fin-keeled yacht, and likewise how much less energy is needed to right the Island Packet after a capsize compared to a typical fin-keeled boat. Which yacht would you want to be on at sea?

Our owners tell us that one of the things they like the most about our yachts is their high level of safety and seakeeping.  With a combination of high righting moment, high LPS and low level of energy needed to recover from a capsize, it’s easy to understand why, in this regard, Island Packets are truly in a class by themselves.