I would thnk that, if you alternator has a smart changer; and, you don't run them down below 50% and not return them to charge, then they might last a lot Ilonger in an adequate state.

I know that you keep your boat a mooring -- so using a shore power unit is tough; however, you only sail the boat for 4 months out of the year. What do you do in the winter.

ALSO, if you have i) an inverter/charge with a smart chager (and a control panel that will allow you to do so) and ii) a powerful alternator (relative to the size of your bank); then you can boost the voltage charging voltage higher by using he inverter/charge while motoring for a longer period of time.