24 volts are better than 12 volts because...
24 volt is better than 12 volts, because the wiring is cheaper and you don't spend as much money heating wires. These engines have onboard computers and electronic injection. The engines use 12 volts to run, and won't run without it. I consider this the simplest approach, actually. I would make the whole boat 24 volt, if only the engines came with a 24 volt option.
And, I am having no trouble finding 24volt gear, except for 2 or 3 low draw electronic items, which I will power with a DC to DC converter, which costs about $85.
As far as I am concerned, 12 volt systems are an obsolete relic of the days when boats had much, much less electrical gear. Just price the cables to run a big electric anchor windlass with 12 volts versus 24 volts, and you will see my point. I'd go with manual, but this is a 65' boat we are talking about, and it will operate in coral waters, where a rope-chain rode isn't a practical option.
And, if it so common to do this, why can't I find one lousy wiring diagram for the idea?
I am designing my own boat, as I have been an amateur designer since the days of planimeters, splines, and French curves. I had a bad experience in 1971 sailing from Hawaii to Seattle, and decided to learn yacht design in self-defense. The construction method will be modified Kelsall, with a radius chine design. The radius chine will have a 2' diameter throughout, hand laid up, with the rest of the boat made by resin infusion on a flat table.