hello Terry
I have tried sailing with the rudder blade up, (a stone was caught in the cheek, after running aground) and the rudder was not very efficient, I was having to use much more rudder effort to control and the rudder was almost on full lock to keep a course to windward.
I had to give up and start the motor to assist the steering.
I believe the deeper the rudder is in the water the more "bite" it has in the water. Also adding area to the rounded top trailing edge would move the centre of effort aft and would affect the balance of rudder and ultimately the whole boat trim.
I wouldn't recommend doing it.
There have been some boats modified with a transom hung rudder similar to the Seal 850, these were reported to be a better arrangement with more balance and less lee helm.
But this is quite a serious modification to consider doing, and I would advise trying to obtain copies of any drawings done by John Baker.
Mike Edwards
Seal 28 "Aztec"
Mike Edwards
Seal 28 "Aztec"