Primary and Secondary Standards For Portsmouth Yardstick
All measurements have a reference. In the development of this empirical comparison of sailboats, the primary reference is the performance of the Thistle (83). In England the basis was the National Firefly (100).
When the Portsmouth Yardstick was first being developed in the DIYRA, the Thistle was chosen because in DIYRA nearly every club had a fleet supported by aggressive skippers. The Portsmouth Number of the Thistle at that time was 83.0 and remained frozen at that value during the further development of the DIYRA (and now North American) Portsmouth Yardstick.
For the derivation of the change in performance of one boat with respect to another in a spectrum of wind/wave conditions, the Thistle has been an adequate, but not perfect, reference for the reasons given in Section I. These disadvantages, however, in no way affect the validity of the comparisons within the Beaufort Scale. Catamarans show a particularly marked increase in relative speed; planing boats, moderate. The Thistle, although a planing boat, becomes overpowered near Beaufort 5-6.
Consequently, planing boats of similar character show less variation over the wind spectrum, e.g., Kestrel, Y-Flyer, while boats capable of using increased force, which are underpowered in light air, e.g., Cal 20, Day Sailer, Lightning, Fireball, improve their speed relative to a Thistle.
It was these changes in relative performance of different types of boats that convinced the PN Committee of DIYRA that a "supplement" to Portsmouth Numbers was required; i.e., Wind Velocity Handicap Factors (HC), necessitating additional requirements for reference classes (see Section I).
Current Yardstick classes are grouped by Centerboard, Sailboard, Multihull, Keelboat and Offshore (cruising and retractable keel) classes in Table VII.
There is a strong tendency to interpret the changes in HCs as a trend in the performance of that boat. This is NOT necessarily true. Remember that there are really seven separate "Yardsticks" in the BN range 0-6. Relative performance between boats reads vertically and is for specified conditions.
The Primary and Secondary Yardstick boats are the preferred boats for determining Portsmouth Numbers. Fortunately, almost every set of race results that cooperating clubs have sent us has included one or more of these well-characterized boats.
When these Yardstick boats are not available locally, club handicappers may select a class, as a Local Yardstick, whose performance is consistent in all wind conditions.