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Join Date: Apr 2006
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Well, the first question would be "have you drawn up an energy budget for when you will be cruising?" as opposed to what you are using/doing now.
If you approach this logically, all the answers will fill themselves out and the only real question will be which price range you want to select the answer from.
For instance, with your existing battery bank "for a total of 420 amp hours" and a rule of thumb that you can usuaully safely charge batteries at 1/5th of their rating (i.e. 80+Amp charge rate) over five hours, your existing battery bank would call for an 80-85 Amp alternator for efficient charging, plus naother 10-15% for charging losses....but since battery banks get much better life when discharged only to the 50% level, you can get all the power you need from a 45-50A alernator, and still recharge your batteries in the shortest "safe" time. That's also assuming you have an external marine regulator, because the ones built into alternators are designed for cars, running all day and not OVER charging the batteries. Their logic cuts output very quickly--and won't charge deep cycle batteries properly without extensive hours.
So for what you've got...an external alternator might be the only upgrade you need. And as Balmar will tell you, once you get over 80A capacity, you cross into the land where you need either double belts, or a ribbed belt, and then you may need a pillow block to offset the load on the engine bearings. Cross that 80A line and your expenses and installation JUMP. For power that you will never need without a bigger battery bank.
But back to question #1: What will your energy budget be while cruising? Do you want an inverter that can occasionally power a hair dryer (1600 watts these days!) or just an electric drill or saw? (400-1000W)
With a 39' boat you have room for solar or a wind generator, there are threads about the pros/cons of each. Solar is a longterm investment--you've got to save a lot of fuel to pay for the installation, so just taking a year off may not return the investment.
One of the big hogs in the energy budget can be the masthead/nav lights, and paying the stiff fees for LED replacements can often make good sense because of the charging time and battery capacity you'll be saving.
But, you can't tell without pencilling in an energy budget. Where you need to guess, write down a "probably" and a "maximum" number, that's enough to start with.