How far should your tiller rudder extend into the water for a 20' swing keel sloop?

Answer 1

There's no single right answer that will cover any 20' sloop, the size and shape of the keel the form of the hull and how the sails are ballanced will all affect what size rudder is needed, as well as the shape of it. Also the answer will depend on what handling characteristics are desired.

As for just the depth of the rudder, usually it will go to slightly less depth than the keel when the keel is at it's deepest. Some lifting keel yachts, especially smaller ones, also have a lifting rudder.