All that is needed is the measurement of the old rudder shaft, from the wear line of the T-piece to the start of the actual rudder. 

Single Ski Rudder

Good for all conditions.

Double Ski Rudder

Surf Rudder

Eliptica Rudder

Amazing in down-wind conditions.

Vlei Rudder

Perfect for using on flat water. Guarenteed not to pick up any weed!