Complete Electronic shutdown

Once had several things stop working on previous boat. Turned out a main negative buss bar had its main wire, that was connected on the back of the bar out of sight, loose & corroded.

Sort of rings that there is a dedicated bank here that is weak and perhaps has its own main breaker that has tripped. I know I have a primary breaker on each of my battery banks in the vicinity of the batteries as well as on the DC panel...

Joel,
I too own a 2007 65 Marquis. Unless you or someone else changed the source(s), the bow and stern thrusters are powered by the port and starboard engine batteries. If the batteries and/or connections are of issue, voltage could have dropped enough to the engine brains (ECMs/SAMS, etc.) to shut down the engines. You mention you were using thrusters at the time. The amp draw is large. More amps lowers voltage. As others mentioned, not a good idea to run all the time in parallel. I'd check the power source and IF wired as from factory, load test both sets of starting batteries and all connections. Just one man's opinion.
Scott

It definitely points to a bad electrical design that you may want to revisit. Here are some guidelines:

1) Bow thrusters always on separate isolated batteries. There is no way around this. If you put them on the starter batteries, you will kill the starter batteries. If on the house, do not even think about it. Just think about the current draw. A 10 kW bow thruster running at 10.5V is drawing 1000A. This is way above the starter or the windlass.

2) The starting battery should only be connected to the starter. The engine controls should be connected to the house. If you really want to be independent of house, then a best practice design is to have a tiny 24V battery provide power to the engine electronics via diodes that can isolate the engine electronics from the starter voltage drop. But then, make sure you charge that tiny battery.

3) Ideally, you want completely independent starting battery banks for port and starboard including the engine controls, the throttles, the dash gauges, etc. but that standard is hard to maintain (i.e. in this case you would need 5-6 battery banks at 24V, which is not the most efficient). Still, totally separating port and starboard is a requirement for >200 miles offshore I believe.

4) Since most people will not be replacing that many batteries at the same time, put the freshest on the starter, the next on the house, the next on the bow thruster and the last pair on the stern thruster. Then rotate.

If it helps, I had two similar failures. In one case, one of the starter wet battery cells exploded - it had not been monitored for a long time. Lost all power on starboard. In the second case, the positive lug had corroded and while indicating voltage at low current, it could not provide high current. So, check all you batteries, that they are watered well, check the voltages to identify a potential bad cell and check connections.

I agree that thrusters should have their own battery source. However, the 2007 Marquis 65 has them powered by the port and starboard engine banks. Two 8D batteries for each engine/thruster. I think port engine bank powers bow and starboard powers stern. In my former Hatteras 58, we had a separate sole bank of power for each thruster. No so on the Marquis 65. Prolonged thruster use especially on weak starting batteries could cause a voltage drop enough to affect engine ECU.

Even the builders of "Premium" badged yachts have their share of odd arrangements that gets one wondering how they manage to get it through the scrutiny of various regulatory authorities.