If I had known it's called Papenberg and not Pappenburg I might have had more success in finding something.
But we've been on the right track. The german
Wikipedia article says that it was indeed used for submerging just parts of the boat. It was quicker to react to pressure changes than the normal depth gauges and was therefore better for the fine adjustments needed to keep the boat exactly at the required depth.
The guys at uboat.net were a bit more precise than us though:
http://uboat.net/forums/read.php?3,9...2541#msg-92541