hard steering to port while cruising

The rear of the rudder is quite flat and thick..I´ll look for a picture so you can get and idea.
Yes the packing gland is fine, I had the rope system replaced and now I have double rubber o-ring in the port instead..

I wish to address this issue also this winter...but would like to heard opinions before..
First I have original steering cable, or at least since the boat is mine it was never replaced.
Boat steers out of the water fine and smooth to both sides, the same standing in the water or cruising at idle speed or while docking.
Now when I get to planning speeds it noticeable harder to steer to port than to starboar...also If I don´t hold the wheel it turns alone to starboard...
I blame this behavior to the fact that my rudder is not original and has no fin to compensate the prop wash....
Can the prop wash be the culprit of this hard steer?
I know that I could replace the cable just to the fact that is to damn old, but also I don´t wan´t to spend the $200 or so to have the same problems...
I would like to havee neutral steering!! so I guess some work need to be done in the rudder, but also wonder want kind of work are we talking?