Finally had some success, but not without assistance.
I managed to set myself up for this attack and the Tribal Destroyer passed me without incident. I knew I had to keep my stern tubes as defence once I had launched my attack
My two prime targets were a T3 Tanker and a Liberty Cargo. I fired two torpedoes at each target and destroyed the T3 immediately. The Liberty was disabled and dead in the water.
You can see the 4 fish running, the impact on both ships was almost simultaneous.
Obviously this drew the attention of the Tribal Destroyer. Depth charges and pinging galore.
As I had already anticipated trouble once my fish detonated I had called in assistance from the Auxiliary Cruiser to the south. I just had to stay alive until it arrived if I could.
I went deep and followed a track to intercept the cruiser so that the Destroyer would be dragged along to meet its doom. The Cruiser arrived just in time and I blew ballast to arrive safely on the surface where I finished off the Liberty with one of the fish from my stern tubes.
Destroyer getting the business end of the Cruiser supporting me.
I stayed alive and now have to allow my DC party to fix me up before I head off to seek more prey
