ok.. i had to replay a saved position for a nearly dozen of times for getting an idea of the premature explosions of the torpedos..

and no setting seemed to influence the matter (sent them deep or shallow & contact or magnetic).. when i was just up to give şt up i've read here at some other thread that the fast torpedo setting has a bug.. i'm using slow torpedos eversince and did not have any of the prematures for 2 patrols.. however, if it is a bug its a quite big one..