Some general gameplay tips:
How to get renown: Sink ships, mainly merchants and with luck BB's and carriers. Forget about destroyers and other small stuff.
Attack the biggest merchants early war. Aim the torpedo at the bow or at the smoke stack. Only 1 torpedo / ship. Wait for it to sink. Or to be more realistic fire 2 torpedo's at a ship 1 bow; 1 stern/smokestack.
20k-60k patrols should be easy that way specially early war. Wich results in no need to cheat for a type IXB
Goodluck