The biggest reason that voltage climbs so high on an unregulated system is
that the battery is too small a capacity, and/or is a sealed or "maintenance
free" version. You want a "flooded cell" battery, the largest you can fit
and afford, to run your boat's electrics. Get a group 27 heavy duty battery
and you won't have any problems. A cheaper way of regulating the charging
system is just to use an additional battery in parallel to absorb the extra
charging.

Hey, good to see your still in the group Bill!
I have tried several batteries... I'm currently running a group 27 1000cca. I
have tried 3 BIG non maintenance free batteries. Everyone of them did the
same thing. The factory service manual calls for a minimun of 360 CCA. It
seems like Im not running them down enough to give the charging system
something to do. I just bought a 420 CCA to try that because a few of the guys
in the electricial engineering group said the bigger battery has MORE
resistance to charging once its full and is adding to the problem.
I gave up on electrical "experts", they cant agree on this at all. I'll trust
your experience on this. Money is not the problem. If you know of a specific
brand and model of battery that will work I will try one.
Now for a good laugh... I just happened to see another motor like mine Saturday
at the dock and talked with the owner... Now get this.. this guy didnt have a
clue about his old boat but he let me look at it. The thing is running a
perfect 14.5 volts and he's running a 520 CCA MAINTENANCE FREE battery.
ARRRRRRG.