“The bow thruster on our 2005 348 Open was acting up last fall as well. It sometimes slowed to a VERY low RPM and sometimes would not spin at all.
The problem turned out to be jammed brushes on the thruster motor. A shop specializing in electric motors fixed the problem in a few minutes. They said the brushes swelled, causing the problem. Root cause was operating the thruster at low RPM due to weakened batteries – I didn’t recall running the thruster with poorly charged batteries, though.