Hi Karel
I am not 100% sure on the drip channels, I have not research this yet. But several things make my think about it. Why would you need a drip channel that sent 90% of it time under the water.
I think what looks like drip channels are attachment strips for the bow/stern casting cap for the saddle tank & hull casting. In some pictures you can clearly see holes that are for the rivets to fix the casting cap on to the saddle tank, and you can see a fixing strip on the pressure hull.
Picture from U-BOOTE CREWS: Daily Life, 1939 - 1945 by J DelizeIn this pictures you can see the fixing strip (with rivet holes) along the pressures hull to fix the hull casting to. You can also see the fixinging strip around the casting cap of the saddle tank.