will this patrol ever end?

[VonHammer]

on my first patrol, i have to patrol in the celbes sea. the mission was to to deploy there and engage enemy merchant shipping. i did so and managed to sink about 10 ships i think, but they were either junks or fishing boats. would this have any effect? so after doing that for 3 full days i still have not completed the 2nd objective, i decided to head back to port.