I need to replace the belts on our FL120s. The PO used Gates 9470s (13mm x 1200mm) and they have been on for at least 5 years and @600 hours. Although they still look fine, I don't wish to tempt fate. I think I'll go with those, but find it interesting that others recommend using 10mm width belts.
We also have an on-deck hydraulic drum windlass that is powered by the starboard engine. I've had to replace a few of those belts over the years, and just had one snap in two with very little use on it. Those have all been 10mm wide belts, because I put on what the PO had used. But now I'm thinking maybe that should be a 13mm belt also, as it's running on a double pulley (and like I said, the 13mm alternator belt is still fine).
What is the conventional wisdom on using a belt of a particular width?