Wayne,
My guess is that it is not electrical. I think your flex plate drive is
either loose or worn out. The alternator has bearings only on the outboard
end. The engine rear main bearing acts as the other armature bearing. This
mechanical connection is usually done through a flex plate which is bolted
to the engine flywheel and a splined hub drives the armature or rotor. To
check this, the alternator must be removed from the engine.
Steve

"Wayne.B" wrote in message
...
I have an 8KW Westerbeke diesel generator (about 12 years old) which
has an odd vibration issue at times. This happens most often under
moderate to heavy load and lasts for 15 to 20 seconds at a time. The
vibration is so violent that the guages are impossible to read and it
shakes the entire boat. Vital signs like temperature and oil pressure
look normal, there is no exhaust smoke, and other than the vibration
episodes, it starts and runs very well. There is ample raw water flow
and the electrical specs for voltage and frequency check out OK.
I hate to replace it if there is a simple fix.

Anyone have an idea what the problem could be?