Originally Posted by sailingdeacon
Another clue that may be key: even with the strong pull to port, the rudder is aligned straight foreward and aft. To me this implies a right hand paddle wheel effect rather than an imbalanced rudder under power.
Is your prop-shaft angled downwards or do you have a sail drive with a nearly horizontal shaft? The more the prop-shaft is angled downwards, the more torque component gets into the vertical. Mine's at 12 degrees down, so sin(12deg)=0.2 or 20% of the torque applied to the prop is trying to yaw the boat instead of trying to roll it. However, this effect is not rudder dependent, except that the rudder must counter it. As you see a reversal of the force on the rudder, I think it you have and overbalanced rudder. If that's the case, extending the trailing edge of the rudder might help.
However, if you have a downward sloping prop-shaft, and a rudder that changes shape with depth, then the spiral of the prop-wash will apply differential lift and un-balance a balanced rudder.
Worth 0.2 cents?