Originally Posted by prairieoyster
I'm trying to decide how big an alternator to get for the DD. how big a battery? 2-12volts and a seperate bank for cabin?
No generator...but considering building in a small Honda for power.
Our 26-foot trailer boat cruises the Inside Passage, anchoring 75% or more of the nights. We have no generator, and no high amp consumers of power like air conditioning or electric stove. We do have a windlass, but run it only with the engine running. Summer in the north doesn't even require much use of cabin lights. Refrigerator is by far our greatest AH user.
Your AH needs will be dependent on what equipment you use, and for how long between recharges, which also depends on your cruising area and habits.
When we designed our electrical system, we determined the amp draw of each device that would be run while at anchor, and estimated how long each of these devices would run. Multiplied together to get amp-hours, added all those up, and came up with a conservative (high) estimate of 60-70 AH per night at anchor. We used an Excel spreadsheet to make this much easier, as it does take some repeated re-calculation to get it reasonably correct. Would be happy to share if you like.
We have a Link 2000 battery monitor, so we have a good handle on actual AH usage: 30-60 AH per night. Our house bank easily handles one night, usually handles two in a row pretty well, and can squeeze out three on occasion if we need to.
Rule of thumb is that house bank AH capacity should be at least 3X the AH requirement between recharges. Our two g31 12V AGM batteries provide 210 AH, and were the most we could physically squeeze in (a pair of 6V golf cart batts could have worked). We'd like to have maybe twice as much AH capacity, but just cannot fit it in. Your beamier Tolly might have room.
Well matched with a 210 AH house bank, we have a 75A Balmar alternator, with MaxCharge external regulator. It easily recharges the house bank in 1-2 hours underway. It could handle a bank twice as big pretty well.
Our start bank is a pair of the smaller g34 AGM's, for a total of 1760 CCA, which work well to start our 260hp Volvo KAD44P diesel. They get recharged in a few (10-20) minutes, much like those in our diesel pickup.
We also have 30A shore power, so at marinas we usually plug in to keep batts fully charged, and allow use of a small electric heater, and the electric side of our hot water heater.