CG controlled stability is fairly straightforward. A heavy boat, with a low center of gravity, will normally exhibit high stability. CB controlled stability (often called form stability) is determined by the center of buoyancy and its relationship to heel angle. In general, the wider the boat, the more form stability, but at extreme heel angles, freeboard, deck camber, and cabin dimensions all effect form stability.

        Dynamic stability controls how much a boat heels in response to a wind gust or impact of a strong wave. A stable cruising boat will resist these dynamic forces long enough for them to pass safely by. Heavy displacement helps dynamic stability, but the most important factor is the boat’s roll moment of inertia. The roll moment of inertia is calculated by multiplying the weight of each piece of the boat by the square of its distance from the center of gravity. The "squared" term makes the calculation very sensitive to how far heavy objects are from the center of gravity. For example, a dingy with two people sitting fore and aft on the centerline has a smaller roll moment of inertia than the same dingy with the people sitting side by side. Both boats weight the same, have the same center of gravity, and the same center of buoyancy (exactly the same static stability), but moving the people off the centerline greatly increases the roll moment of inertia. If two identical boats are hit by a gust, the one with the largest roll moment of inertia will roll the least. Deep draft, heavy displacement hulls, with long heavy masts will have the largest roll moment of inertia.

        Boat designers rarely provide enough details for us to quantify stability. Prospective buyers typically depend on advice from various "experts", magazine reviews, or antidotal evidence from other sailors. While these sources should not be ignored, its possible to "reverse engineer" existing designs using only basic dimensions, like length overall (loa), beam, displacement, draft, and ballast, and determine their relative stability, compared to a standard. The key to this process is to generate a standard, or "template", which is based on the characteristics of designs known to have excellent stability. If our test boat fits inside this template, we can assume its basic design is reasonable and similar to the boats making up the template.

        The credibility of the template must be extremely high for this process to work. My approach was to base the template on the actual designs of respected and proven cruising boat designers. Few will challenge the quality of the designs by Carl Alberg, John Alden, Bill Crealock, Ted Brewer, Lyle Hess, Ted Hood, Al Mason, Chuck Paine, and Robert Perry. I collected data on 118 boats from these nine designers, and analyzed them with respect to the following six basic non dimensional parameters: