Regular alternators of the type fitted to cars (and most boats). are
regulated to about 14 volts.
This level is chosen to avoid gassing at the battery. They will charge at a
high current if the battery is low but as the battery gets charged up the
current will fall off drastically. They are basically intended for cars
where the battery will not very often be used when the engine is not
running. They top up the battery following the use of the starter motor and
feed the car loads whilst the engine is running. They are not intended to
recharge a flat battery.

Most boats seem to use this type of alternator, probably because they are
readily available from the automotive market.

They can do better on a boat if fitted with an alternative regulator. These
are sometimes available from the original alternator manufacturer but also
as an aftermarket add on.

You need to find out what type of regulator is fitted on your existing
alternator as it may be ok with an alternative.

Alec





"Jeff" wrote in message
...
Gm1234 wrote:
...

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.



I have a 110 Amp Balmar on a Yanmar 2GM20FC (18 HP). When I'm powering
at 3000 RPM the engine runs significantly hotter when charging. I
normally
limit the charging current to 30 AMPs when traveling if I know the
batteries are discharged. Also, if my wife wants to use the microwave
while we're powering, I'll have to throttle back a tad.

On the other hand, when charging at anchor and running at 2000 rpm or
less, the engine stays so cool that the hot water heater doesn't get up to
full temperature.