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PHRF Interpretation
In theory the numbers represent the seconds per mile that your boat is slower than a 12 meter (rating 0)sailing on an olymipic triangle course. In theory a boat that rates 90 is 30 seconds a mile faster than a boat that rates 120.
The actual ratings are not derived from measurements but from regional comparative performance results of the fleet of similar boats relative other known boats. In other words, if you buy a production boat, that has been raced under PHRF, it originally had a rating assigned and over time that rating may have been adjusted to reflect how well that class of boat has done relative to its rating.
There is no useful information about seaworthiness in a PHRF rating certificate. On the other hand, and IMS rating certificate contains a lot of useful information about the seaworthiness of a particular design, such as its angle of positive stability and an approximation of its vertical center of gravity and buoyancy.
Jeff
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