It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.
Beam does enter into it as as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch.
The CR is : Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). Ratios will vary from 5.0 for a light daysailer to the high 60s for a super heavy vessel, such as a Colin Archer ketch. Moderate and successful ocean cruisers, such as the Valiant 40 and Whitby 42, will fall into the low-middle 30s range.
Do consider, though, that a sailing yacht heeled by a good breeze will have a much steadier motion than one bobbing up and down in light airs on left over swells from yesterday’s blow; also that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet. Nor will one human stomach keep down what another stomach will handle with relish, or with mustard and pickles for that matter! It is all relative.
These 30 sailboats have the highest Comfort Ratio of all known sailboats.