Our CN 35 Hull 68 Alegria has two 6 volt batteries in series underneath the
port watch seat and two more 6 volt batteries in a specially glassed in area
in the hanging locker adjacent to the watch seat for a total of 450 amp hours.
We have a single 12 volt starting battery mounted in the forward part of the
starboard lazarette. The battery switches are set up so that starting can be
accomplished from either bank of batteries. For charging, we have a Heart
inverter/charger mounted on the bulkhead underneath the chart table and a 100
amp alternator. Our power requirements are very similar to yours except we
don't have a SSB and we sometimes run a small 120 volt microwave from the
batteries by way of the inverter.
Judging from a recent three week cruise, I would guess our 450 amp hours would
last about 2 days. However, we generally charged the batteries when we were
at a 50% level which means we charged almost every day. It takes about 2 to 3
hours of engine time at about 900 rpm with our 100 amp alternator to top the
batteries off from a 50% discharge level. We have one of those E meters by
which you can monitor your battery discharge and charging. You can also
determine exactly how much drain each of your electrical devices take by
alternatively turning them on an off and checking the meter. These meters
aren't real cheap but I think they are worth it.
Regarding your question on 6 volt batteries in series, one marine electrical
reference book that I have says that 6 volt batteries generally have heavier
and more durable plates than 12 volt batteries.. The one provisio is that
batteries in series should be of the same make and age and if one needs to be
replaced, all should be replaced.
As to your means of charging, ideally I don't know if I would want to depend
only on the engine for charging if I was headed offshore. We talked to
several cruisers headed south from California to Mexico for the winter who
had fitted both a wind generator and solar panels. The thinking was you could
have wind and fog on the northern part of this trip and then no winds and sun
once you get farther south but that you would seldom have no sun and no wind.
Hope some of this helps