On the hook charging both works pretty good. But at sea it don't because you don't have enough power with all the loads going in the boat. If you tried to charge both from "renewable" sources you'll end up with with both at 70% SOC most of the time. By alternating them the power it takes to charge a battery bank is more manageable. For instance if your total bank size is 800ah it takes 80 amps to bulk charge them @ C/10. Cut that down to 400ah and then it only takes 40 amps.
18-20 days at sea on a long passage and you'll gradually run your batteries down to where when you come into port they'll be at 50-60% SOC. Without running the diesel it's just plain hard to come up with enough onboard power to charge them every day. But that's what they make shore power chargers for.
You have to get the idea out of your head that a sailboat is a sustainable self-sufficient environment. It's not. It can carry enough food, fresh water, fuel and battery power to get you from Madagascar to Sydney, or whatever. But once you get to where you're going, you have to take on supplies, including battery power, to make it to the next one. All the solar panels, wind generators, hydro, etc. are is supplemental sources, just like your watermaker is for your fresh water tankage, to stretch what you can carry onboard.
If I'm looking at two different boats and one can carry two more solar panels but the other one carries 50 more gallons of diesel, I'll pick the one that can carry 50 gallons more diesel. When everything else goes to hell, that diesel in the engine room with a freaking big alternator on it is the one thing you can count on every single time.
18-20 days at sea on a long passage and you'll gradually run your batteries down to where when you come into port they'll be at 50-60% SOC. Without running the diesel it's just plain hard to come up with enough onboard power to charge them every day. But that's what they make shore power chargers for.
You have to get the idea out of your head that a sailboat is a sustainable self-sufficient environment. It's not. It can carry enough food, fresh water, fuel and battery power to get you from Madagascar to Sydney, or whatever. But once you get to where you're going, you have to take on supplies, including battery power, to make it to the next one. All the solar panels, wind generators, hydro, etc. are is supplemental sources, just like your watermaker is for your fresh water tankage, to stretch what you can carry onboard.
If I'm looking at two different boats and one can carry two more solar panels but the other one carries 50 more gallons of diesel, I'll pick the one that can carry 50 gallons more diesel. When everything else goes to hell, that diesel in the engine room with a freaking big alternator on it is the one thing you can count on every single time.
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