My IP although a full keel is a spade rudder. It's an unusual spade as it does have a "lobster strap" attached to the bottom of it. Older IP's apparently exhibit rudder shudder just as you describe, you feel it worst in the wheel
but can feel and hear it in the hull
. I believe it's caused more than anything my on older IP's the keel is pretty thick and I think there is turbulent flow off of the keel, exciting the rudder.
Shimming the bottom of the rudder takes some of the play out of the rudder and greatly diminishes the shudder to where it's pretty much un-detectable.
Non laminar or turbulent flow off of your keel I feel can most certainly excite your rudder.
Look up aerodynamic flutter to see what I believe may be going on, yes our speeds are much lower, but water
is much more dense.
I'd look at the keel, it's the only thing that changed, or possibly look to see if you rudder may have excessive play
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