gudmundur writes:

So how the hell is it
'Regulating'? I seem to find there is NO regulation at all, and often
find my battery voltage above 15 volts. This seems like a poor design,
and hard on batteries also.

You must think in terms of current, not voltage, when it comes to such
issues. Did you measure the current?

You trust the voltmeter? Has it been calibrated?

The *battery* regulates the voltage from any current-limited source, and
small outboard alternators are typically quite limited in current
delivery (5 or 10 amps, less at higher voltages). Unlikely that the
alternator could deliver enough current at an elevated voltage to be
"hard" on the battery.

Fully charging a 12 volt lead-acid battery requires around 16 volts.

Hard on your battery? Easy enough to test with an ammeter. Get the
battery fully charged, run the engine for a while, and measure the
charging current after it levels off. A big battery can take quite a
few amps in, on top of a full charge, and not be harmed.

"Hard on a battery" is a 100 amp automotive alternator sticking on the
high voltage. That will cook a battery.