Average ratings are taken from data compiled by the United States Sailing Association, published annually. PHRF stands for "Performance Handicap Racing Fleet," and its ratings are used to determine the relative speed potential of any given class of yacht.
An initial handicap rating for a new class design is assigned based on the boat's critical dimensions, its similarities to other rated yachts, the designer's speed predictions, and other such theoretical data. Observations of actual racing performance are then used to adjust the design's rating from time to time. What is rated is the boat itself, not the skill of its crew or its gear. Thus, a skilled crew using new sails and gear may be able to sail, say, a J/24 faster than its rating would indicate.
The handicaps are given in seconds per nautical mile around a race course. For example, a J/24 (page 294, average PHRF 174 seconds per mile), racing on a ten-nautical-mile course against a Dufour 24 (page 287, average PHRF 240 seconds per mile), would give the Dufour a handicap of 66 seconds per mile, or 660 seconds for the ten miles. Thus, even if the Dufour finished the race as much as 660 seconds (11 minutes) after the J/24, the Dufour would still tie the race.
For more on the PHRF system, see http://www.ussailing.org/phrf.
Not every sailboat has a PHRF rating; where none has been found, the expression "NA" (for "Not Available") is used in this guide.
Maximum Speed (also known as hull speed)
This is calculated using the formula: Speed = 1.34 x VLWL.
That is, the theoretical maximum speed of a displacement hull (as opposed to a planing hull, which obeys different hydrody-namic rules) is approximately 1V3 times the square root of its waterline length. Above that speed, the waves a displacement hull makes, which are induced by the boat dragging a hole in the water along with it as it plows forward, become so large that the boat expends all additional increments of motive power trying to climb out of its hole. Planing hulls can escape that fate by climbing out of their holes and skimming the water's surface.
Hull shape and wave conditions sometimes alter the 1.34 factor a bit, but basically 1.34 is the applicable factor in most cases.
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