The word "Rudel" literally translates as "pack" into English, but the meaning wolf pack is implied, as only wolves and dogs form packs. The term "Wolfsrudel" exists, but is uncommon. The use of the word to describe group tactics by submarines is colloquial as far as I know. Official documentation almost invariably refers to such U-boat operations as "Gruppentaktik", "gruppenweiser Ansatz" or "U-Bootgruppenoperationen". The term "Wolf" for submariners is likely a result of German propaganda and plays on the Rudel - Gruppe analogy. It became even more popular in post-war Germany (West), as a process of glorification and mystification of the German submarines' deeds set in after 1945. One of the most popular books on the WW2 u-boat campaign was called "Die Wölfe und ihr Admiral" (The Wolves and their Admiral"). The label has stuck ever since - although Timothy Mulligan has recently published a book called "Neither Wolves nor Sharks", which tries to shed some light on the people behind the label.