After reading the install manual on my new genset I now understand why. Hydrolock. It happened twice last year.
My boat is a 1988 with twin CATS and an Onan Genset. The exhaust for the genset is piped through a wet muffler, to the exhaust of the port main engine. A big no-no it appears.
Why would the Onan be plumbed this way? The Onan install manual says the same thing!
Looks like I need a new hole in the stern of ASD!