Mary, I wondered the same thing. Wouter conveniently included the rule number so a visit to the F16 website revealed these requirements. Not exactly "grandfathering" in the traditional sense but the result is the same.
6.1 Grandfathering of boats into the Open Formula 16 class
6.1 The head of a local Formula 16 class organisation may give dispensation (called grandfathering) to a particular design or class and allow them to become part of the Open Formula 16 class under the following conditions.
6.1.1 The design may not have hulls longer than 5,30 mtr.(17ft 4 inch.)
6.1.2 The design must have a rated jib sail area which is less than 4,25 sq.mtr. and more than 2,75 sq. mtr. (this rule does not apply to single handed setups)
6.1.3 Both the SCHRS (ISAF) and Texel rating of the design must be equal to or "slower" than the rating assigned to the Formula 16 class.
6.1.4 When grandfathered, the designs are allowed to race against fully compliant Formula 16 designs in declared "Open Formula 16" races.
