"Capt John" wrote

Depending on the boat, and what you have in it, 30 amps might be just
fine. Your alternator was reading 13.2 volts because the batteries were
fully charged, it didn't need to put out any more. On a small engine
like yours, you may not be able to install a really large alternator,
it may load the engine down too much. If you need lots of power, a
seperate generator may be the best way to go.



Good points!

On my boat, I replaced the LR-135-31 with a Hitachi LR155 that came off a
4-cyl later model Yanmar. Only change needed, was to drill out the mounting
bracket for a larger hinge bolt (or add a sleeve for the alternator) and
make a new plug for the "L" connector.

This alternator puts out about 25A at idle speed just after starting (using
clamp-on meter). This is more than enough for normal weekend use, but for
long term live-aboard with no shore power, a supplement will be needed if
the boat has refrigeration and other large draws . I am thinking about
adding an 80W solar panel.

My engine is a 15HP 2-cyl Yanmar and max recommended alternator is 80A. At
normal RPM with this alternator actual output at prop would be less than
10HP. I don't have power curve for the 1-cyl 12HP but I used to own one on a
27ft boat, but I have seen them on 32ft boats and then you need every HP you
can get. In that case, a means of disabling the alternator may be needed
when you NEED maximum output from the engine - there is a way to do this,
but I don't have the details.