I just finished a similar scenario, terrible visibility AND at night, but with a single contact. The boat was only visible within about 100 yds. The boat wound up to be a single small freighter. I did manage to put it under with sonar only, but it is tough to update the info fast enough to keep the solution accurate. Remember, in real life, there was more than one person doing all the work. In this game you wear all the hats so to speak. In those conditions, pulling off a successful shot is very rewarding.