I can definitely confirm that in GWX!
I only have to send one torp into one of those whaler ships and if there's even a glimmer of fire all I have to do is wait around and the thing turns into a total inferno and sinks. It was actually the only moment in the game where I honestly felt sorry for anyone on board.
I suspect if I even hit the deck of one of those things with an explosive round it would sink from the fire.
I'm not sure if I've ever had to fire more than one torp at a whaler to sink it...
__________________
|