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.
The Portsmouth Yardstick was developed by Stanley Milledge, who was in charge of handicapping racing at the Langstone Sailing Club in 1947 using the Island One design as the scratch boat (having a value 100). In 1950 he received support from the Portsmouth Harbour Racing & Sailing Association to produce the first edition of the Langstone tables for club use when they would be known as Portsmouth numbers. In 1960 he handed over the administration to the RYA and in 1976 a new YR2 format was used, with the Langstone tables being removed in 1986. The Portsmouth Yardstick was extended to multihulls in 1973 and from 1977 four forms were used, for dinghy, multihull, keel and cruiser. Due to the increasing performance of boats, particularly multihulls, the numbers were increased by an agreed factor of 9.462 in 1995 and expressed as three or four digit numbers.
In the United States, the Thistle was chosen as primary yardstick for compilation in 1961 with a value of 83.0, which corresponded to its RYA PN rating at the time. Other boats were compared using their DIYRA (Dixie Inland Yacht Racing Association) rating to produce the D-PN number. This proved successful and in 1973 the responsibility was passed from the DIYRA to the North American Yacht Racing Union.
Wind Handicap Factors (HC) are an extension conceived by the DIYRA Portsmouth Numbers Committee to take a more realistic account of wind and wave conditions for different classes. This produces a factor based on F=100 for each point of the Beaufort Scale from 0 to 9. Further extensions are being evaluated for offshore classes to take account of sail inventories, excess weight, etc.
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 2012, but note that adjustments are made each year).
In a race involving a mixed fleet, finishing times can be adjusted using the formula:
- Corrected Time = Elapsed Time × Scale / Handicap
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:
- A Scow: 1 hour × 100 / 61.3 = 1.63 hours
- PD Racer: 2 hours × 100 / 140 = 1.43 hours
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.
Examples of boats and their Portsmouth Numbers
There are hundreds of boats that have a Portsmouth Number, or D-PN, or both; the table below gives some notable examples. An official table of D-PNs is published on the US Sailing website. The classes included below are those used at the 2012 Olympics, the 2012 Paralympic Games, and the 2012 ISAF Youth Worlds.
* Used primarily for match racing hence not enough fleet races to calculate PY.
Other handicap systems
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 conversion formulae:2007 D-PN
- D-PN = ( PHRF / 6 ) + 55
- PHRF = ( D-PN − 55 ) × 6
- "History of the PY" (PDF). Royal Yachting Association. Retrieved 23 June 2013.
- US Sailing. "Section I". Retrieved 23 June 2013.
- US Sailing
- "RYA Portsmouth Number List 2013" (PDF). Royal Yachting Association. Retrieved 27 March 2013.
- "Centerboard Classes". US Sailing. Retrieved 31 July 2012.
- "Multihull Classes". US Sailing. Retrieved 30 July 2012.
- "The RYA Portsmouth Yardstick Number List for 2004" (PDF). Royal Yachting Association. Retrieved 17 September 2012.
- "Keelboat Classes". US Sailing. Retrieved 31 July 2012.
- "RYA Portsmouth Yardstick List 2010" (PDF). Royal Yachting Association. Retrieved 21 August 2012.