Unless you are planning to run submerged, I would not bother.
Every time you change energy from one form to another, you will suffer losses. Batteries, particularly lead-acid accumulators, are notoriously innefficient at storing energy, and recovering it again.
Diesel-electrics are often seen on rail locomotives, largely because of coupling issues as it is so difficult to get drive to multiple wheels with mechanical drive.
But on a sailboat??? Your diesel engine will give you about 40+% efficiency, and the tank fuel will take you far further than the most massive of battery banks, battery banks that slowly lose poke as they are cycled.
And what size of generator are you going to need for (say) 35 hp at the prop?
I warn you too, with seawater-wet hands, 48 V will make life very exciting for you. It is exciting enough with nominal 12 V. Try it.
Last edited by Rockter; 02-19-2012 at 03:24 AM.