First guess would be a failed injector.. allowing unatomized fuel to enter a cylinder.. this produces a loud, mechanical sounding knock and smoke from unburnt fuel. We've had this occur before, the offending injector was 'breaking over' (spraying) at 500 psi instead of the required 1200 or more. As a result it was more of a 'peeing' of fuel than a misted spray. Sounded frighteningly loud, esp as this was right off a fresh engine rebuild. A new injector cleared it up.
Hopefully it's as simple as that....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)