I have a 175A 24v alternator on my 54. With twin pulley v belts.
It is the set up with the alternator to port and separate from the 12v alternator and raw water pump  belt. I know some 54’s are set up with the alternator to starboard and run off the same belt as the 12v and pump belt. Not my Amelia.
I am disappointed with the life I am getting from a matched pair, only about 100 hours. I tried to get the green stripe quality in the UK and was sold some blue belt.
The pulley grooves are for 15mm or maybe 16mm,  so that they do not bottom out. That is, so the sides of the belt grip the pulley. Which is what I used.
I have now put a pair of belts that the previous owner bought which are 13mm, but a matched pair (so it is bottoming out) as an experiment, to see how they perform.
Does anyone with my twin belt set up, recommend any particular brand and size/specs?
It would be nice to find a brand that does not shed dust and that lasts for 300 hours or more!

Nick
Amelia Corfu area
AML 54-019