These Altmount Kits by Balmar offer the opportunity to install a larger alternator than would otherwise be possible. This one is the 48-NSP-4.6 and is for the Nanni N4.6 marine diesel
Alternator upgrades using a single drive belt are limited by the amount of power that can be transferred by the belt but using a serpentine belt considerably increases the possibilities. The kits include a set of serpentine pulleys to go over the crankshaft and water pump pulley and replace the alternator pulleys and come with a matching serpentine belt. They are sold for specific engine models only. The alternators need to be purchased separately. If you order an alternator with a kit it will come with the appropriate pulley pre-installed. Installation is relatively easy. The kit comes with two belts but purchasing an additional spare belt is recommended. This particular kit uses a 42.3" circumference K6 belt so when selecting an alternator you need one with a K6 pulley.
Read my BLOG POST about installing one of these kits.
My blog post has installation notes for a different serpentine kit.