Glenn Ashmore wrote:

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.

Off the front crankshaft and or a PTO shaft. The engine is an 8 litre
6 cyl natural aspirated 160 Hp engine which weights 1000kg and is a
proper industrial marine engine as used by fishing vessels - not a
bleeding micky mouse Yanmar engine designed for pleasure yachts. I
have done a certain amount of research and I am amazed by the complete
rubbish being spouted about alternators.