[paulk]

SA/Disp. ratios are a little like horsepower ratings for a car. They relate how much power (sail area) is available to move the boat (displacement). If you compare cars, you can find a big one with 300 hp and a small one with 300 hp. The smaller one will tend to be "sportier". Boats are harder to tell apart than cars, as far as displacement goes. Two boats could be tthe same size, but have very different displacements. This makes the SA/Disp. ratio very useful in doing what you''re doing, comparing boats to one another.

There are other criteria that can also affect performance, even if two boats have similar size, displacement, and sail areas. Hull type and shape can make a big difference -think about full-keel, fin keel, and centerboard hulls. Because of this, if you want to measure performance, it makes sense to use a tool that tends to take all these factors into account. PHRF (or also IRC) does this. On a triangular course, the Catalina 36 would be expected to sail 9 seconds per mile slower than the C&C 36. You may not sail triangular courses. On one point of sail or another, or in certain conditions, the Catalina may actually be faster than the C&C. (In light air with a bit of chop or powerboat slop, the Catalina''s heavier displacement may tend to keep them going, while the C&C gets stopped more easily by the waves because of its lighter displacement.) Overall, however, the rating will give better insight into the performance of a boat than any single ratio could, because that is what it is designed to do.