The problems began when I used traditional logic statements to sort my database. Computer database programs are great at sorting out data within a discrete range, such as Disp/LWL ratios between 250 and 300, but these CRISP logical terms totally exclude all boats outside the selected range. This simple technique proved to be too crude and did not produce a useful list of candidate boats. In the real world, a value moderately less or greater than our crisp limits might be good enough for at least some consideration. Even the boats that pass the CRISP filters are not easily compared since they are all ranked the same. Again, in the real world, values closer to the midpoint of a range are often preferred by designers, at least as a starting point, and should be scored higher then those at the edges. It quickly became clear to me that traditional crisp logic was just too rigid a tool for evaluating small changes in cruising boat parameters.. What I needed was a process that would organize the boats in a manner sensitive enough to discriminate the best features for my Ideal Cruising Boat template.

        I was introduced to a possible way around this crisp logic dilemma by Jim Manzari, a gentleman I met on the Internet News Group, rec.boats.cruising. Jim shares my interest in evaluating cruising sailboats and was using a novel approach based on Fuzzy Logic. This is a relatively new field of mathematics, invented by Prof. Lotfi Zadeh at the University of California at Berkeley. Fuzzy logic replaces the familiar crisp logical statements such as "greater than 250 and less than 300" with linguistic statements such as CLOSE or VERY CLOSE to 275. Without rigid crisp logic boundaries, these "fuzzy logic variables" can be used to blur the edges of a logical set and allow each member in the set to be ranked individually.

        For example, instead of only showing Disp/LWL ratios between 250 and 300, a Fuzzy logic approach would assign a score (between 0 and 1) based on how close each boat was to the ideal Disp/LWL. A score of "0" indicates no compatibility with our concept and a score of "1" indicates perfect compatibility. For our purposes this score is called the Degree of Compatibility (DOC). In this example, a minimum of 230 and maximum of 370 might be specified, along with a region between the two that is close to the ideal value, lets say 280 - 320. Boats in the ideal region would score a perfect DOC of 1, while those on either side would have progressively lower scores. Boats outside the maximum or minimum would receive no score.  Instead of a large cluster of boats all logically the same, the Fuzzy logic process returns a list of Disp/LWL scores that can be sorted from 0 to 1. Just what we need for comparing various designs.

        The real power with this technique lies in the ability most of us have to approximate values based on our collective experience. Defining a single best value is very difficult compared with identifying a range that bounds the best value. For example, it should be no problem getting a group of experts to agree that good sailing performance is desirable for an offshore cruising boat. Likewise, they should all agree that the SailArea/Disp ratio is one important factor in sailing performance, and, that it should be high enough for good performance but not so high that seaworthiness (crew fatigue, rig strength, and weight) is compromised.

        A group of experts would probably never agree what the perfect SailArea/Disp ratio is, but they would probably consider starting with a Fuzzy set for an offshore cruiser having a minimum SailArea/Disp ratio of around 14, an optimum range CLOSE to 15 through 17, and a maximum of 19. This would filter out the under powered motorsailors and the red hot racers, all in one stroke. Usually several iterations are needed before the experts become familiar with how the process works and form a consensus. This typically involves reviewing the data and identifying what factors made some good boats score low and vice versa. After lots of heated debate, a consensus will be reached, the DOC variables can be adjusted, and the real fun of evaluating the boats can begin.