So far so good but you've got to know the loads and times before you can make sure you're on the right side of the battery situation. You need to know how much current
each item draws and how many hours/day they run. Once you know those numbers you need to divide the loads into cruising/passagemaking and on the hook/mooring. The highest consumptiion should be your guide.
I've always preferred 6V deep cycle "golf cart" batteries because if one fails, I can generally get another as easily as a 12V, and if not, I've got a spare 6V in case. They're also lighter in weight than a comparable 12V battery.
The current wisdom seems to indicate one huge battery bank, the reasoning being, as far as I can determine, that the battery depletion will be less on a big bank than two banks. For a long time I sailed with 2 house battery banks, alternating days of operation. If you know your wiring
, and one battery goes down you can either disconnect it from the bank or throw the switch and isolate it.
Some folks lump the engine
start battery into the house but I'm not sure I want the unfortunate experience of having every battery on board unable to start the engine so I keep my engine start battery isolated from the house bank.
Your 2 17A alternators will probably put out a realistic 25A for about 300AH charge capability. 640AH batteries at 75% capacity would be about 480AH (figuring a 180AH load, which isn't unrealistic on a long passage), but you've got a 30% charge penalty. That indicates that you'll need 620AH to charge them back to full (which generally never happens unless you've got one monster charging
system). I tend to think you've got the charging portion of the cruise
You've got two engines and if you go with2 battery banks, chances are you're going to need some kind of combiner/controller to allow both alternators to work together yet be separated in case one fails. That generally means a dual bank charger
No doubt the biggest load is going to be the windlass, but on a cat, chances are you'll be anchoring
a bit shallower than us monohulls. I'd want to make sure the voltage drop from battery to windlass is less than 3% and wires properly sized and fused.
I'd ditch the toaster unless it's a deal killer.
I don't know about cell phone
coverage but the power draw is small, ditto the LCD screen
and associated hardware
if you're aware of the loads. One of the biggest loads I have is my battery operated power tools and a good, strong 120VAC drill.