I don’t know whether this could be it, but if the foil was not rebuilt absolutely symmetrical to the other original side the one with a larger curve will generate more “lift” than the flatter side and may cause the blade to want to rotate, especially if the centre of rotation isn’t in the same place as the centre of lift of the hydrofoil. I’m not sure how sensitive a rudder would be to such a phenomenon, given the speeds we tend to travel, but in theory at least, this could happen…
I hope for your sake I’m wrong and there is a simpler solution.
Dave
The membership association for owners of Dufour Yachts