They're not the same...The
Portsmouth rating is derived from actual race results over time, and is one standard for all locations. From the US Sailing Portsmouth Yardstick explanation:
The 2008 North American Portsmouth Yardstick for centerboard boats, multihulls, keelboats and offshore classes are posted herein. The Yardstick is a widely used method of rating boats of different classes sailing the same courses and is used by clubs and fleets across the country. It is a time-on-time handicapping system and is derived from actual records of classes of boats with thoroughly documented ratings. Portsmouth Numbers are defined as the length of time boats would take to sail a common but unspecified distance. The formula is easy to use:
CT (Corrected Time) = ET (Elapsed Time) X 100 / HC (Handicap)
The
PHRF ((Performance Handicap Rating Formula") is intended to "level the playing field" when dissimilar boats are racing each other. The PHRF assumes the boat is sailing to its fullest potential, and the differences between the performance of the boats in the race is then decided by the wisdom, skill and perhaps luck, of the skipper.
The rating may vary from location to location, based on the local racing organization’s calculations. A single local organization may have different ratings for different sailing venues. The rating may also change as the committee looks at individual races.
A quick conversion table can be found here:
http://www.ussailing.org/portsmouth/table_v.aspSo the H18 at 71.3 or 4 would be around 97 or 98