Thanks all for your suggestions. Since the problem occurs with the thruster disconnected from the rigging, I don't think the problem is the rigging. I haven't taken on a drop of water through the bow thruster seals on my current cruise so far, which has included motoring into some good sized seas. So I think I the next time I have the boat out of the water I will start with callipers, followed by precise machine-shop measurements and milling, if needed. Perhaps a previous owner of servicer simply over-tightened a band/hose clamp, causing an out-of-round condition. I'll report my findings after the next haul-out.
Jim
SM384 Sirena Azul
Ketchikan