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I'll take a guess. Perhaps the water pressure was greater on the outer surface of the rudders than the inner due to the engine pushing the water in between the two rudders, perhaps causing a bit of a vacuum. At 12 knots, this may explain the rudders bending, inward, especially since they're plastic. You're lucky they didn't break. I broke three IDA rudders to date, though granted, a custom (huge) size which is no longer being made. All breakages were under sail with motor tilted up. None involved grounding or collision, just pushing water. I now use stock rudders.

Leon

Hardcrab,

I had a simular situation as your "On our boat, it can "push" (port turn) further than it can "pull" (stbd turn).
It seems to be limited on the pull stroke by the engine coupling bolt/pin hitting the fiberglas at the exit hole."
It happens to me when motoring and rudders raised I found if I did not pull the port rudder all the way up ( 3/4 inch less), I had about the same amount of motor swing to port as to starboard.

The rudder mech. has a plate that hits the transom when the rudder is all the way up.

Will