I remember in my early days of playing SH3... before I started realism game play...
In 1939 I could surface in the middle of an unescorted unarmed convoy use all my eels, every deck gun shell I had and sink em in order of high tonnage first... usually wiping out 90% of the convoy...
Once I upped the realism and started using the GWX mod this became err.. more difficult to do.
Toward the later part of the war 43-45 I stay away from convoys altogether and pray for singles.. and the end of the war.
But to answer your question .. Why bother..?
To SHOW EM I GOT CAJONES!!!