That's exactly what I do. I calculate their course with the ruler and get into a 90 degrees angle about 700 meters away from me. When their bearing is less than 5 degrees to zero, I fire one torpedo per merchant and wait whether they sink. If not, I wait if they stop or can still make speed to escape. In this case I chase them and fire another torpedo in their back (difficult, 'cos they don't follow a straight course), if they stop I surface and fire my guns at them. Now that you say merchants can get armed lateron in the war, I may prefer staying submerged. Thanks for that hint
