I don't see the physical numbers as you mention. I'm about to pull the trigger on adding 2 group 31 batteries to my house bank, but maybe i should reconsider.
Trojan T-125 - 6 volt 240 AH,
Weight: 66 lbs
Size (inches): L-10.38 W-7.13 H-10.88
Volume: 803 cu-in
Group 31 dimensions
Volume 822.7 cu-in.
The advantage for the 6 volt is only 2.5% (803 cu-in / 822.7 cu-in.) however most group 31's are rated for 115 amp hour so 2 will give you 230. another 4% in favour of the 6 volt, but combined its only 6.5%
Am i missing something? I would love to get the capacity you're talking about.
No need to do the height, only length and width.
2- Group 31- 175 115 usable ah
2-Golf cart- 148 120 usable ah
3- Group 31- 262 172 usable ah
4- Group 31- 350 230 usable ah
4- Golf Cart- 298 240 usable ah
Remember you only get to use 1/2 of the usable ah. Thats why its important to do an electrical diet for a day. The big difference is when you get to 3 and 4 batteries.
Ours is almost 78 ah per day, but we have refrigeration ( 48 ah/day), chartplotter, but we have almost all LED lights now 9 .1 a/hr).
115 ah ( 2 group 31) can go quickly in one overnight or day of sailing. Also you engine will charge up to 85% 98 ah qwuickly but then will slow charge past there so most who use the engi8ne to charge dont get much above 85%. Different with a good 3 stage dock charger it can get back to 100% charge. Thats why I am saying you may want more capacity for peace of mind. Especially if you do more than one overnight.
My recommendation on the golfcarts were two fold. First the heavier plates give you more deep cycles therefore your battery will last longer. Second, if you decide to increase you capacity the space is considerably less (20%) when you get to having 220 usable amps.
For a boat which the diet is 70 ah like ours thats three days. For a boat with 40 ah/ day thats 5 days.