I sank the wrong ship

[pmcmull]

I'm on the career campaign. My second patrol for the 1941 asiatic fleet start. It is around the 1st of May 1942. After completing a patrol objective in the Celebes sea with no targets, I set off in the direction of the Solomons to see if I could find action there. Cruising along about 300 miles North of Rabaul, I detect a ship. It is a merchant traveling alone, going North. I need to chase it to get in range. With no escorts to worry about, I can run on the surface until close to firing range, then submerge. I focus on getting to a good range and angle to fire torpedoes. The light is poor so can't see all the details. After getting in position, I fire 2 torpedoes at him from about 1500 yards. Torpedoes run straight ad true and get 2 hits. Yea! After waiting for a minute or 2 I look to see if I need to fire another torpedo. It is only then that I see the American flag. Dammit. Nothing I can do so I sail away back to my patrol route. The ship sinks before I leave the area.

I have not completed the mission yet due to difficulty finding merchants to sink (Japanese merchants). what damage to renown or other penalty will I see when I get back and dock? Also what was an American merchant doing alone in enemy waters. Not an excuse but just not what I expected.