The Portsmouth Yardstick (PY) or Portsmouth handicap scheme is a system of handicapping used primarily in small-boat yacht racing.
The handicap is applied to the time taken to sail any course, and the corrected time can be used to compare widely different sailboats on even terms. Portsmouth Numbers are continually updated with empirical data from race results worldwide. Numbers are provided for differing wind speeds (when the data are available), plus a composite number that can be used when the wind speed is variable or unknown.
The Portsmouth numbers are administered in the United States by the Portsmouth Numbers Committee, and in the United Kingdom by the Royal Yachting Association, based on annual input from a large number of affiliated sailing clubs. While the two systems are similar, the US version (D-PN) uses handicap numbers scaled to 100, while the RYA version changed to a scaling of 1000 in the 1990s.
Each class of boat is assigned a "Portsmouth Number", with fast boats having low numbers and slow ones high numbers—so, for example, in the case of two dinghies, a 49er might have a RYA-PY of 740 while a Mirror has a RYA-PY of 1385 (these are the actual RYA Portsmouth numbers for 2011, but note that adjustments are made each year).
In a race involving a mixed fleet, finishing times can be adjusted using the formula:
where Scale is 100 for US numbers, and 1000 for UK numbers, and Handicap is the applicable Portsmouth Number for the given class of boat. Each boat's time is adjusted with the formula, and then the adjusted scores are compared to determine the outcome of the race.
For example, a PD Racer (a semi-open homebuilt class, and the slowest listed boat) has a D-PN of 140, and an A-Scow (the fastest listed centreboard boat) has a D-PN of 61.3. If an A Scow takes 1 hour to finish a given course, and a PD Racer takes 2 hours, the handicapped times are:
So the PD racer, although it took twice as long to finish the course, would be declared the winner. Numbers in the UK handicapping system would yield similar results, though some slight variation is to be expected due to variations in the race data used to compile each handicapping chart.
The Portsmouth Yardstick is used for dinghy racing and small keelboats or multihulls. Larger sailboats are more likely to use the Performance Handicap Racing Fleet handicapping system in North America, or the IRC handicapping system in Europe, Australia & New Zealand.
There is a linear correlation between the D-PN and PHRF, allowing the following conversions formulas:2007 D-PN