I left my comments too. I have read work on this recently, trying to remember where.
They've done the work on dead dugongs, and from memory (early 2009?) there were 4 found since 2004 that had signs of boat strike, only one of those from a prop smaller than 400mm in diameter. They were unsure whether the strike was the cause of death or that the dugong was already dead.
My experience is that if you are moving at speed, they are long gone by the time you get there. These awful liars just steam my potatoes.
Tim