You can see my little U mark just beside the merchant I hit. I found a convoy and followed submerged all day and then rose at night to fire from the surface. One torp struck the prop and she stalled. 3 warships chased me for an age and I finally snuck under the merchant and lay there for over an hour real time, as I reckoned they wouldn't fire on their own just to get me.
The boats didn't move off as I thought they would... and then the merchant began to sink, so I dropped to 180 and crept outta there, finally managing to steal away. The boat sank and I didn't get a credit for it, no little sunken ship sign...I was furious after all that time and evading...nothing to go down in my log. Damn.