GBM wrote:

We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is
charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to
be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been
slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess
in the
50-60 deg C range (120-140F), so I would keep my hand in contact
with it.

Couple of things.

Yes, your alternator getting hot is quite normal.

You want to idle the engine around 1,400 RPM to prevent "wet stacking"
which happens when the engine is not hot enough provide combustion of
all the fuel in the chamber and that leads to VERY expensive repair
bills.

The sheave ratio between engine an alternator is about 2:1, so at
1,400 engine idle, alternator is about 2,800 RPM and probably putting
out maybe 25-30 amps, if you are lucky.

The 1,400 RPM will also help minimize belt slippage.

My guess is the 55 amp output will require an alternator RPM of
between 5,000-6,000.

It's just the nature of the beast.

Have fun.

Lew