Alternator Amperage
If you looking to just run then engine. 40+ amps would be enough. Add up what you have, Electrical Fans, Fuel Pump and anything else you have.
Most GM cars and trucks now a days have 100 - 140 amp alternator to handle the load of all of the electrical devices and load, which includes the engine, lights, seat warmers, AC, USB ports, etc..
Having said that I would think any alternator you put on there will be more than large enough. Even the old carb'd motors had alternators that would produce 45+ amps.
If your worried about size.. you can try a Japanese Import Alternator.. they are small and can produce enough amps for your boat.





