Possibly rudder parallelism is not a good word. When you cross the rudder blades in horizontal plane, rudder blade section chords(red lines on the photo) should be parallel between themselves.
I beleive that the ABA AMRO boats found that 2 degrees toe in on their rudders gave the best results, would this be the small for cats.
![[Linked Image]](http://www.dotan.com/im/rudder-parallel.jpg)
