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Most engines these days use a V Belt to transfer mechanical energy into electrical energy via the alternator. There are many downsides to V belts. That's why over 30 years ago the automotive industry switched to serpentine belts. Single-V-belts concentrate the tension in one small area on the pulley, which stresses the bearings, slips, creates a ton of dust, and regularly snap.  Industrial 10 groove serpentine belts do not slip, and consequently they do not wear, and one belt will typically last the life of the engine so you typically will never need to buy a new one. They also have the strength to handle the loads of our high output alternator, and can be retrofitted onto most marine engines in an hour or two. Balmar produces an awesome one.

Approx cost, $3000.