High capacity alternators
Wayne, you really have little to fear from upgrading your alternator so long as you keep it a small frame unit and don''t expect huge gobs of output from it. The reservations you ''should feel'' IMO have to do with having an overall system that allows you to really benefit from upgrading the alternator - the ''ripple effect'' from the HiCap alternator that moves to external charging controller and a battery monitor (e.g. Link) system of some kind.
When this conversion is done poorly and/or a huge alternator is mounted with a single belt, problems with belts and and pump shaft seals (not to mention alternator mounting brackets) can emerge...but these are not common with the less ambitious installation you plan (assuming it''s done well), and especially given your description of your type of sailing. The kind of loading you have computed for your engine seems reasonable to me, and your engine is clearly up to the task given its rating. I fear that Fernando''s 10 belts is symptomatic of his installation rather than a predictor of yours.