[HandsFree]

I have a 1997 3100 open with 454's. Here in the great lakes we troll at very low speeds (i.e. 1.8 to 2.8 mph). In order to do that I run one engine at a time. When I use the port engine my volts stay between 12 and 13 but when I use the starboard engine the volts slowly drop, the last time it was down to a little less then 11. When I raise the RPMS to over 1000 it begins to slowly rise. I am thinking it is the alternator, is there an easy way to check it? Could it be some sort of regulator that is not working properly. Any suggestions?