Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That is what I meant about "OP's choice of alternators may not be the best
". That particular alternator has a relatively slow slope up to full power.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.

Off which end? My Yanmar can handle a lot of torque off the back end but
the front crankshaft bearings are not designed to handle a lot of side load.
Double belts will stop the slippage but a double pulley extends the center
of load further from the bearing increasing the leverage of the side load.

There is also the principle that diesel engines are much happier running at
1800 RPM than they are at 800. For many reasons running at idle under light
loads for long periods is hard on diesels. I am not saying don't do it.
Just that there are better ways to tackle the problem.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com