Hi Chris,

I meant rounding the tip edge of the rudder, not the planform. The planform is what you see from your rudder, if you lay it flat on the ground. The wing tip edge is best seen if you look from the leading to the trailing edge (e.g. front view). If you make it round, you help the tip vortex, which is always there, to be stronger and hence more dragy. If you make it sharp, you hamper the vortex a bit. Hence rountig or fairing the tip edge is not good. A simple cut is better, just have a look at modern aerobatic aircraft (which have a symmetrical foil as our rudders).

In terms of planform you have some degrees of freedom from an aerodynamic point of view, many shapes are possible. Some people "believe" in elliptical shapes, however a simple taper ratio of 0.4 is as "good" (depends largly on your figure of merit, which is not only drag) as an elliptical shape, but much simpler to fabricate. There is a small "improvement" possible if you have rounded leading edge on the last 5% of span, as you can see on many airliners when not equipped with wingtip devices. Again second order effects, most important parameters are area, span and the section shape(s).
It is difficult to judge these rudders unless you know what are the most limiting factors (stalling at low speeds, cavitating at high speeds, structural strength), what are the design targets (optimising lift/drag) and what is design philosophy (rotating or sliding kinematics, designed to be loaded or unloaded when sailing straight) , etc.

When we broke the rudder of my friends T, we decided to keep the same area and span, but went for staright leading and trailing edge and designed a new section shape, which should have less cavitation, slightly less drag and better structural stability than a conventional NACA 4 digit section.

Cheers,

Klaus