Some key points you should know about

            alternators:

 

• The alternator that came with your boat will almost certainly not be good enough for extended cruising, if it doesn't have an external regulator it definitely won't be!
• If you run your engine in neutral to charge the batteries it's doubly important to have a large alternator to load the engine, diesels hate running without load (best to avoid running the engine in  neutral if you can)
• If you go for an alternator that delivers about 25% of house bank amp hours, then you can expect to get from 50% charge to 80% (the point at which the internal resistance of the batteries will limit the charging anyway so the size of the alternator doesn't matter any more) in about an hour and a half.* So motoring in and out of an anchorage should set you up nicely even if you sail all day.
• Alternators without external regulators are chronically bad at keeping the voltage high enough to charge your battery fully on no wind days when your motoring. Because they're mounted in the alternator they get hot and throttle back. Manufacturers go over the top with this as they're not really interested in giving you the best performance and unless it's a dedicated boat alternator they're not expecting a big house bank. Remember working your batteries* between 50 and 80% is your goal while cruising, but the occasional top up to 100% is really good too. If you can get that from solar, great. But really good to have your alternator able to do it too!
• If you go for an alternator bigger than your requirements you can de-rate it slightly (Balmar call it 'belt management') This will make your alternator last longer.

 

*I'm referring to lead acid type batteries here, i.e. Gel, AGM, Flooded. Lithium Ion batteries are a different matter, they can take MUCH bigger charge currents. But that's a whole different topic.

 

 

This Video takes you through sizing an alternator for a cruising sailboat. Wiring and fitting including dealing with the fanbelt arrangement ( you will need twin vee's or serpentine for anything over 100Amps)