(Includes standard equipment for offshore classes)
The US SAILING Portsmouth Yardstick is a widely used method of rating boats of different classes sailing the same course. It is derived from actual records of classes of boats with thoroughly documented ratings.
The Primary Yardstick for the compilation is the Thistle at 83.0. This class was selected in 1961 because of its class organization, wide geographic distribution of active fleets and activity at clubs conducting
races for other classes as well. The rating of 83.0 was the Portsmouth Number that keyed the early
Dixie Inland Yacht Racing Association (DIYRA) ratings for other boats sailed internationally under the
Royal Yachting Association Yardstick at that time.
The Primary Yardstick turned out to be a happy choice with exchange of data and Portsmouth Numbers between DIYRA and other clubs in North America. Nationwide participation using this common reference ultimately led to North American Yacht Racing Union (NAYRU) assuming in 1973 the responsibility for administration of the DIYRA Portsmouth Yardstick, now known as the North American Portsmouth Yardstick.
Portsmouth Numbers are defined as the length of time boats would take to sail a common but unspecified distance. For example, the distance a boat rated at 80 could cover in 80 minutes should be able to be covered in the same race in 95 minutes by a boat rated at 95. In deriving the numbers, boats have in almost all documented cases sailed on courses including the three basic sailing angles: beating, reaching and running. The numbers provide a direct comparison of the performance of different classes.
Assumptions made in generating Portsmouth Yardsticks (D-PN) include:
- That each boat placing first in each class was sailed to its true potential by a perfect crew according to flawless strategy;
- That all boats sailed the same course, experienced the same wind/water conditions and degree of interference of clear air;
- That all one-design boats conform to class specifications and rules, and use sails specified by the class; and
- That boats with multiple sail inventories (genoas, spinnakers, etc.) utilize the proper sails for the wind conditions and legs of the course.
The first and fourth assumptions obviously are paradoxical. The time of the first boat of a class, however, is used to derive handicaps as a function of wind velocity and, ultimately, Portsmouth Yardsticks (D-PN), thereby approximating the assumption that the boat was sailed close to its potential.
A moderating effect in the determination of a number is that at least two classes are involved in determining a datum for handicapping, the class in question and a reference class, all subject to the four basic assumptions. Whenever this results in a boat yielding a handicap value with a probable error exceeding prescribed standards, this datum is rejected. Such a result usually means that a boat sailed into a hole or was in a favored position regarding a wind shift; conversely, the reference classes can be affected.
A modification to the calculation procedure, i.e., determining an average elapsed time and handicap for a hypothetical reference class from all reference classes participating in a race, has minimized these variances involving the reference classes.
Examples of classes with well established D-PNs in addition to the Thistle are: C Scow, Flying Scot, Laser, Snipe, Sunfish, Star, Hobie 16, Hobie 20, Nacra 6.0na, Tornado, Catalina 22, J/24, San Juan 21 and S2 7.9.
These and other classes are designated Secondary Yardsticks. A Secondary Yardstick is the Portsmouth Number (D-PN) of a one-design class or offshore class boat for which the D-PN for consecutive years is reproducible to better than ±1 and for which Wind Velocity Handicap Factors (HC) for each Beaufort Number (BN) range from 0-6 can be determined. The Portsmouth Number 83.0 for the Thistle is generic to all the Secondary Yardsticks.
Examples of the use of Portsmouth Numbers are:
- With computer or calculator derive corrected time (CT) from elapsed time (ET) for Portsmouth
Numbers (PN) based upon the equation:
CT = ET / PN x F
where the handicap conversion factor is F.
F is 100 for the common usage of PN in the equation.
- A club may compute time correction factors (reciprocals of Portsmouth Numbers) for multiplying the elapsed time to obtain corrected times. The scale can be adjusted arithmetically so that any desired boat becomes a scratch boat.
- Portsmouth Numbers may be used to compute time on time allowances for each boat. The allowances are normally in the form of decimal minutes per 100 minutes of elapsed time.
- Conversion of PHRF, MORC, IOR and PMA ratings to D-PN and Wind Handicap Factors.
Wind Handicap Factors (HC) or wind handicaps, when used in conjunction with the factor F = 100, are an extension of the Yardstick that was conceived and evolved by the DIYRA Portsmouth Numbers Committee. The objective is to provide more realistic comparative performance of boats taking into account wind/wave conditions. The need became especially evident when displacement boats were competing against planing hulls and when catamarans were competing in very light or very heavy conditions. Basically, their HCs will reflect the true performance characteristics of a boat across the wind spectrum (driving force).
The derivation of HCs is based on the Thistle, as the Primary Standard, with its basic D-PN of 83.0 assigned for Beaufort 0 to 9. The exceptional light wind performance of this boat contrasts with the difficulty of keeping her on her best sailing lines in winds above 18 knots. Accordingly, classes equipped with trapezes, with a lower power/weight ratio, classes simply underpowered, and catamarans, will show a more marked improvement in boat speed relative to the Thistle as the wind velocity increases. The important fact to keep in mind is that the entire system of Wind Dependent Handicap Factors (HCs) stemming from Portsmouth Numbers (D-PNs) has been kept internally consistent.
A second extension continuing to be evaluated is modification factors for allowances to D-PN and HCs for class design options, sail plans, multiple sail inventories, equipment, overall weight and number of crew. Some data already have been obtained; however, until sufficient data for calculations of significant values can be accumulated, these factors are currently assigned on a theoretical basis (calculated from other rating systems).
The US SAILING Portsmouth Numbers Committee has been evaluating a base boat concept for offshore classes. D-PNs and HCs are now determined with the assumptions that offshore classes carry full sail inventories, correct sail choices are made and that allowances are made for those with reduced inventories, excess weight, etc. Under a base boat concept, a standard boat would be defined for each class (sail inventory, engine type, propeller) and allowances made for deviations from this base boat. The D-PNs and HCs would then represent the base boat of each class.
Currently, for offshore classes, a standard boat is considered to be equipped as follows:
- Genoa LP between 150% and 155% of "J"
- Spinnaker pole length equal to "J"
- Spinnaker maximum width 180% of "J"
- Spinnaker maximum length 95% of jibstay length
- Unmodified hull and appendages
- Lightweight engine and folding or feathering prop, 2-blade solid prop in aperture, or retractable outboard motor
- Full length battens allowed without penalty
- Boat in racing condition
- All above are subject to exceptions defined by class rules or the manufacturer
It is assumed that all modifications are made to improve boat speed; therefore, the D-PN or HC may need to be modified accordingly.