Magui:
http://www.subsim.com/radioroom/showthread.php?t=166320
It does away with the stock notepad manual targeting method, and replaces it with a emulation of periscope functionality that was atleast present on some Uboats at that time. So it can also allow you to measure AOB. It is based on the OLC gui mod, so both work similar. But Magui has other tools too to help do various manual targeting and tracking tasks.