The batteries are in the aft lazarette underneath the cockpit. They are not easily accessible with very little overhead clearance. In this same space are the thruster batteries, DC electrical panel, and a diesel hydronic furnace. I can changed the form factor of the batteries if needed.
I would prefer to not go with the 8D size again.
Here are the options as I see them. I am also open to others I haven't thought of.
Wet GC2 batteries
8D AGM
Firefly Oasis:
AGM GC2?
I've studied on our upcoming bank replacement for about the last 5 years... and eventually decided that ease of service (essentially not much), lessened off-gassing (ditto), and the combination of cranking amps /amp-hours were my main criteria BUT our space is limited so I only had a few options. The Lifeline GPL-4CTs (~AGM GC2s) go in probably sometimes next week.
But for your case...
1) If AGM 8Ds are an option, then so should be wet 8Ds, basically a one for one replacement on your current system. A watering system, hire a mule to remove the old and drop in the new... WAY less expensive than AGM 8Ds.
2) If wet GC2s are an option, then so should be AGM GC2s. (I dunno what impact off-gassing might have with that diesel heater nearby, but... it's been there for years, yes? Maybe not much impact? Maybe not much argument in favor of AGM GC2s if you can do a watering system anyway?) I've gotten good and troublefree service from AGMs.
3) If GC2s are an option, maybe the taller versions would also be an option? (I dunno model numbers, maybe L16s or some such. I didn't study much on that idea myself, 'cause I don't have the vertical clearance...)
4) If Firefly Oasis are an option -- G31 form factor -- than any other G31 form factor batteries could be an option, too. As I understand it, the carbon foam advantage is all about PSOC operations... but if you can charge effectively during an outing and then completely recharge effectively after that outing, then the PSOC advantage might not be worth the $$$ premium. Which in turn leads to any decent wet G31s plus watering system, or decent AGM G31s, being options. Latter is what we've used; our oldest bank of 3x Odyssey PC-2150s (G31s) is what we're replacing... and those Odysseys were installed in March 2006. And we simply charge 2x/day when we're at anchor (coinciding with electric cooking times) and then recharge at the dock. Seems to have worked fine.
(We've also recently added an inverter/charger, so that our main banks are both now charged by a dedicated charger. Don't have results from that yet, but my expectation is that we're get even better results from charging while at anchor.)
5) Form factor might trump some of all that. Some number of GC2s (three, I think) will fit almost exactly into the space of a single 8D. If that's correct, then 9 GCs will fit where 3 8Ds fit... and 9 is maybe an inconvenient number, although maybe not a big deal. The G31/8D ratio is different, more like 2½ per 8D maybe (not sure); I know I can't put two 8Ds in the space I have, but I was able to put 3 G31s there...
-Chris