Let me put my oar in the water a bit re this rudder thing.
A rudder steers a boat by applying a force athwart ships at the stern. This force pushes the stern sideways and causes the boat to turn or stay on coarse as necessary. It generates this force by accelerating the water to one side as it passes through the water distorting the streamline of the water. Two mechanisms are at work here. First, the streamline encounters the rudder on the forward side and is forced to the side by the increased pressure and second, the streamline is distorted at the back of the rudder by a low pressure. Most of this force is generated near the leading edge of the rudder. Adding area at the trailing edge increases drag faster than it increases authority.
The pressure is a function of the mass of the water, the angle of the rudder, and the relative speed of the boat. The low pressure is similar except that it is limited by the head pressure of the water. This head pressure is zero at the surface and increases at a rate of one half pound per square inch per foot of depth. A rudder that extends above the surface will cavitate at any speed to a depth where the stream pressure and the head pressure are equal.
There is also an end effect which reduces the authority of the rudder. Water on the pressure side washes around the bottom and top surfaces to the low pressure side leaving a vortex in the wake. Adding horizontal end plates at the top and bottom of the rudder prevent this wash and increases the apparent height and authority of the rudder. A bonus is that the vortex is significantly decreased so that the end plates don't add any significant drag.
One more thing about rudders. The force generated by the slipstream / rudder is at right angles to the rudder. The force that steers the boat is the vector component that is at right angles to the boat. The other vector component is the induced drag which slows the boat.
A very small amount of weather helm is a good thing. It makes the boat easier to steer, produces very little induced drag, and actually pushes the boat to windward. As the rudder angle increases, however, the induced drag of the rudder increases which decreases boat speed. At a rudder angle of ten degrees, The induced drag is about 20% of the total force generated by the rudder.
This represents some of my thoughts re improving rudder authority on your boats. Hope it helps.
Bill Buebel, CM381 Shadow