I think YXM is on track, I saw this happening on a yawl a few years ago. The APU would start and promptly shut down and everything was dead.
It took some doing to find, but the final diagnosis was that the house bank was asking too much of the cold generator. Modern converter/charger/inverters can hit the AC supply hard when first fired up. We finally trapped the sequence:
1 - The generator (APU) would start (autostart in this case)
2 - The converter would grab the output and lug the poor APU down and at the same time is would pop the DC breaker at the system main and if it didn't it would pop the AC breaker on the APU. The the converter would pop the battery main breaker.
3 - It would be suddenly quiet and it took less than a second for this to happen.
We were able to get the owner going again by having him shut off the autostart and open the APU breaker. Then, fire the APU and let it get rolling unloaded. Close the APU breaker and cross fingers.
If it stayed on line great.
If no, disable the convert/charger/inverter monstrosity and get the APU stable - this gave the house bank time to recover from the starting load of the APU and then when the converter was enabled, it didn't pull the APU to its knees.
More fun on the water.
Matt
