G'day
Drivesafe and Blacknight would be probably the men to answer this correctly technically but from a laymans terms this is my interpretation.
Firstly let me say that this is no longer a necessary option. With 12v Batteries giving out up to 900 Cold cranking Amps that I know of dual sixes are just not necessary.
Having said that, I recently sold an ll metre cruiser. This was powered by a 120 HP Sabre Marine converted ford 2715e 6 Cylinder diesel. At about 365 cubic inches and with a compression ratio at about 16 / 1 it need some serious cranking capacity to start it.
Now we are not talking about average available amphers here BUT what is know now days as "Cold Cranking Capicty or CCA.
from 1958 - 1973 when these engines were built the ol 12 volt battery just wasn't up to it, so they used 2 x 6 in series to get the necesesary cranking capacity.
The other issue they faced was that these engines DIDN'T have any glow plugs. They used a mechanical system which is known as "Flood Starting, Cold Feed, etc etc etc. No doubt Black Night or one of the other experts can give u the technical details but the upshot of it all was that when you used cold feed start it effectively put a measured amount of diesel into each cylinder to flood it and there by raise the compression ratio even further to get the bugger to start.
The upshot of this approach was its dead simplicity. An engine in good nick simply didn't need cold feed to start it unless its injectors were dodgy or it was down on compression.
In th 4 years I had Knot Accountable, https://www.aulro.com/afvb/ I only once used the cold feed start switch on th injector pump and that was becasue it was around 0 degrees :eek: in the engine room.
Hope this helps, Cheers
