Ok guys, some good comments there, and some slightly of topic as well
Just for a bit more clarification, I have a Princess 32 motorboat, it has 2 x GMC 6.2litre v8's with a 70A alternator on each engine
At present I am not happy with the battery setup, it has 3 x 110aH batteries all connected seperatly over 2 (1/2/both/off) rotary switchs.
What I am looking for is a solution so I can have 2 x 110aH batteries as my starter batteries and another bank of domestic batteries.
Now I have seen this http://sterling-power.com/products/volt-drop-alternator-splitting-system-pro-split-r?variant=882767643
but I don't know if there are any better options out there?
Hope this adds a bit of clarity to the proceedings?