So we have seen how the crew addresses the Captain in this question; but how about the Chief? At about 3:03:00, Lt. Werner asks the Chief,
Kommen wir je wieder hoch, LI?
and calls him something like »LI«. What is he saying?
Also, the Kaleun said at about 3:07:30:
[edited to show what surely must be the answer from @Ansgar below and from listening again closely.]