Read the Shark class rules. The rudder must measure in based on the formula. There is a minimum weight and a number of measurement points it needs to adhere to. I'm guessing PHRF-LO is aware of these requirements.
a) The Rudder shall be constructed of solid hardwood or alternately of fibre reinforced plastic over an optional core. The core must be suitable for the applied loads. b) The minimum weight including pintles or pins but less tiller and extension shall be 7.7 kg. (17 lbs.). A maximum of 0.9 kg of corrector weights may be added. c) The blade thickness shall be 40 mm 5 mm (1-9/16” 3/16”). This thickness is to comply for a minimum distance of 711 mm (2’4”) below the transom. d) The depth of rudder below the lowest point of the transom shall be a minimum of 813 mm (2’8”). e) The rudder width between the bottom of the transom and 711.mm (2’4”) below the transom, shall not be more than 432 mm (17”) and not less than 178 mm (7”). Also it must at some point be 279 mm (11”) wide. The width below the 711 mm (2’4”) point is unrestricted. f) Rudder blades shall be fixed relative to the pintles. Neither rudder depth nor rudder profile shall be adjustable. The line of the gudgeons may be set at the vertical. g) The rudder or transom shall be fitted with a suitable means of preventing the rudder from becoming detached from the hull. h) No other foils are allowed above or below the waterline, i.e. endplates, winglets.