The one virtue of the 12 volt battery is voltage. You are getting twice the voltage for half the weight. Two 6 volt GC's weigh about 120# for 12 volts while the 12 volt battery is 60#. If you want 24 volts two 12 volt batteries weigh 120 # but you need four GC's at 240# to get 24 volts. Usually sailors worry about weight. Unless you absolutely need the extra capacity from the GC's, 12 volt batteries have the weight advantage. Thanks. Steve S.
For sailboats there is no weight advantage when you are talking 12V batteries, unless the ONLY factor you include in the equation is the weight. You don't buy batteries by weight however, you by them by your consumption needs or Ah capacity for your planned use.
The the 120 pounds of 6V will be giving you 225 - 240 Ah's and the 60 pounds of 12V will be giving you just 105Ah's... That is less than half the capacity and roughly half the cycles. When you cut the capacity you also cycle the batteries deeper thus also shorten battery bank life.
When comparing weight it is best to compare Ah's to pounds based on price, consumption needs, cost, charging capacity and life cycles...
6V batteries are heavier because they have thicker lead plates. Because of this, they last longer. They also very often have more Ah capacity per pound than 12V making 12V batteries heavier for the same Ah capacity.. When you get to equal Ah's capcity a 12V battery the weight differences are negligible but the life span is not.
So for a 450Ah bank there may be 10 pounds difference, either way, but multiple years & cycles difference in battery bank longevity..
If you look at "foot print" a group 24 is roughly the same as a GC2 6V in the floor space it requires..
Four 6V GC2 batteries will give you 450Ah's and weigh roughly 260 pounds.
It takes SIX group 24 12V batteries to equal the Ah capacity of 4 6V batteries.
Those SIX group 24 batteries weigh 270 pounds or 10 pounds MORE for the same Ah capacity. You weigh more yet you will have a LOT shorter battery bank life with this ten pound heavier 12V bank than you would with 4 6V GC2's..
Weight to Ah's..
450 Ah's GC2 6V = 260 Pounds (4 batteries)
450 Ah's G-27 12V = 265 Pounds (5 batteries)
450 Ah's G-24 12V = 270 Pounds (6 batteries)
There are very few boats today over 30 feet that can get away with a single 60 pound/100Ah battery, (35 usable Ah's when cruising), and not be replacing it every other year or sooner. People are simply using a lot more power today than they did even 10 years ago..