Most dinghies sailors won’t care less about rudder design, as many classes are fairly prescriptive about the shape and size of their rudders – to be ‘in’ class.
If that applies to your sailing boat class then, you just need to keep your rudder blade in perfect condition, fill any nicks and dents and ensure that it is a smooth as it can be.
However, if you sail a a dinghy in a development class, or even some restricted development classes, you may find your class rules allow virtually unlimited choice. The question then is ‘what is best’.
This is where is gets rather technical, so I did a little web research and found the following:
It may be of use…. take a look.
Discussion on rudder profiles – http://www.boatdesign.net/forums/sailboats/rudder-daggerboard-profiles-937.html
Free foil design software (GNU Public Licence) from MIT – http://web.mit.edu/drela/Public/web/xfoil/