Hi Glen;
Of course if you add a relay you can switch just about anything. You could open the yanmar and redirect either of the field wires through the relay. Warranties are for wusses anyway.
I would connect both alternators directly to their battery / battery banks. I realize most authorities don’t like that. So;
Presumably if your start battery is switched off, the engine can’t be started and your Yanmar is therefore safe without field coil intervention.
The big alternator is more of a problem; being [I presume] battery sensed, if it’s run while disconnected from the battery it will destroy itself.
I just had a rotor melt down from that myself... luckily where I am right now I could get it rewound [right on spec too] for $15.
Why not switch them the other way; Yanmar on the simple switch [since it won’t start when off] and use the clever switch with field cutout on the house side where you can set it up to protect the Leece-Nevile.
If connected to “Both”, your house battery bank will still start the engine.
Anyway, just wanted to say to be sure you wire you alternators so they can’t power up when disconnected.
Regards, Mark Holden


My adjustable alternator regulator; www.amsterdamhouseboats.nl\voltage_regulator.htm