I was surprised to find that there are some alternators that do not require excitation. I have a modified GM alternator added to my boat, and the shop that modified it sold it as a ONE WIRE alternator, but of course the "other" wire is the frame ground. There are NO one wire circuits.
I also know that disconnecting the battery with a running alternator can create horrendous voltage "spikes". Switching the batteries through the off position on a running system on your twin engine boat can fail two voltage regulators, a little over $1500 on my twin engine Mercruiser. You can guess how I know this. The other part of the story is that these Mercruiser engines are a weird design. I designed HVAC systems for GM. We had to do a lot of surge protection work on Audi systems, because they start them without a battery to drive them onto the boats for transshipment. The best answer for the original question would be to use a minimal alternator, and a small battery of the high quality types that have very low impedance. Hawker Genesis series comes to mind Ron and Martha Freiberger mail to ronandmar...@earthlink.net
