Thank you folks. I went back in today and got the same result and CTD. I think it might have been something to do with the damaged tug as it always happened as the external view got nearer to it - I even tried travelling closer to it in Rotbile and peering at it through the peri, and the same thing happened. I think it was some sort of calculation conflict with the game engine because the boat was reckoned to be stationery, and it was, but at the same time it sounded like it was full speed ahead on the sonar.
Anyway, I left it alone, found another merchant and sunk that - and had no problem flying over that to see it go down, so reckon it was just a one- off problem.
