If the rudder
gets loaded due to hydrodynamic properties of the hull
then sail trim will have limited results in unloading. Look into Hiscocks book where they tell the story of their boat built (the one in NZ).
Big barn door rudder
may simply get too loaded. Remember wheel
give arm/leverage and so do NOT press it too hard as other components may give.
Surprising the designer
would not care about giving his boat a light helm