Mark,
My Sly Cat Racing rudder design has the flat surface near the end of the rudder for a couple of reasons. First, if the taper goes all the way to the end of the rudder, the cross-section where the front vertical water transfer channel is becomes too thin, weakening the rudder. Second, I have larger diameter vertical transfer channels than my competition (to increase water flow), making the need for the incremental material at the back of the rudder. Third, machining the rudder for flat sizes from tip to trailing edge makes the leading edge angle smaller and more susceptible to damage from contact with floating twigs, etc. Fourth, water comes off the sharp 90* edge on the back of the rudder just as well as the "standard" rudder.
In a nutshell, this design allows for increased water flow, increased strength (they are also 7075 aluminum, not 6061), less prop walk, require less rudder input for cornering (increased cornering speed), and can be shortened more than my competitor's rudder with the same heading holding power, resulting in less straight line drag.
By most performance metrics, my rudder designs prove to be best.
