How exactly did bridge watches work for the U-boats in WWII? David Miller writes in his book that the 1WO and 2WO each did two four-hour shifts per day, and the Bootsmann and Obersteuermann each did a four hour-shift. He then goes on to say that men from the seaman branch (Matrosengefreiters and Matrosenobergefreiters?) individually did one four-hour watch on the bridge per day.

Now, if it's true that the bridge watch included two men from the seaman branch per 4-hour watch, that would mean a boat would need at least twelve Matrosen among the crew's complement. Many rosters at Uboatarchive.net, however, list crew rosters with far fewer Matrosen than that.

So, how exactly did this all work out in reality?