1999 S250 ox66 voltage drop

[reelhectic]

Installed new batteries and cleaned the cable terminals. Went for a test drive and started out at 12.4 volts and after about 30 minutes the voltage indicator on my speed gauge would drop, by the time I got back to the dock it was reading 11.2 volts.
Any ideas......I am thinking maybe the rectifier/regulator or the stator but don't know how to check if either is not working.
Would appreciate any help !!!!!
Thanks

[jb123]

i checked my output by running on hose, unhooked batt and used a voltmeter on pos and neg coming from motor.
try unhooking EVERYthing else from boat except motor and see if the voltage still drops