A series setup, all things in consideration equal, will weigh more and generally cost more, then a single battery of the required voltage and capacity, or a parallel setup of the required voltage and 1/2 required capacity.
I fly UAVs commercially, using lithium polymer batteries as fuel, and series battery arrangements are almost never seen except out of economy (ie, I already have 2x 3 cell batteries, and I need a 6 cell battery), because of the weight penalty involved. Series wiring does not increase capacity, just voltage. Parallel wiring batteries of the required end-state voltage increases capacity. I cannot reconcile how your mechanic can say (think) a series-wired 6v bank of "X" capacity would outperform a stand-alone 12v battery of the same "X" capacity or a parallel-wired bank of 12v batteries at 1/2 "X" capacity. I can see him recommending this based on cost; ie, two 6v batteries wired in series gets you to "X" at "Y" price, whereas one 12v battery gets you to "X" at "Z" (a higher $).
You also lose any redundancy as compared to parallel wiring, because one dead battery in a series configuration is as good as a dead bank altogether.
Obviously boats aren't aircraft, where weight is a prime consideration, but I'm still not tracking with what he said, or what your experience was. Were the 12v type 27's of the same capacity, chemistry, and make, as this 6v series bank you've been using? Were they used and cared for the same (charged and discharged?).
Not an inquisition, and you're free to ignore this post as it really doesnt have much to do with your original post at this point.
. A new tool is always a good thing.
