[TakeFive]

...I guess the ultimate test would be to have another boat tow me so there is no propwash and no sail influence on the helm. I'm pretty sure I know what the results would be.

You don't need to get towed. Just motor up to hull speed and put the motor into neutral. Then see if the rudder responds in the same way while coasting.

 

[TakeFive]

Yes, a 1994 MK1 Catalina 42 has no rudder bearings. Newer boats have them.

Since my original post I've spoken to a surveyor who has seen this exact problem before. The solution is to have a new rudder made, but correctly this time - one that is symmetrical on both sides, as it should be.

If the rudder was asymmetrical, why didn't the installer notice it before wasting his time and your lift money on it?