Lifeline AGM is all I buy. I have seen them stand up for years of abuse on other boats and is why I bought them for my boats. Completely sealed, can take vibration and can take a beating. Excellent amp/hours. downside: cost , weight.
Moving 8D's in a bilge is like next to impossible. I had to build a scoffing like structure to get them in place. The have both deep discharge and starting types for your needs. IMO it is probably one of the top boat batteries. The deep discharge 8D's are popular with the solar panel crowd living off grid.
Check out
http://bdbatteries.com/ . I got free shipping from Co to the boatyard and no tax. I will use them again in my next boat. You should get an honest 7+ years of hard use if you keep them charged. Repeated discharges to super low levels will reduce the life expectancy . If you keep them above 50% they will last a loooong time..
Also, make sure you set your charger to charge for an AGM battery. They prefer a slightly different charging profile than the lead acid battery you get at napa. That said they can take a hell of an initial charge...pretty much all your charger can dish out so it charges pretty quick, then steps down to a trickle. You can leave the charger on all the time if set up for agm types.
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