[Yurek]

@ GoldenRivet

Thanks ! You're right about that.
In the meantime I got the answer from U-boots Archive in Cuxhaven:

Wooden pieces fixed to the upper front hull at port and starboard side in 1943 and after were so-called "rubbing strakes" (= "Scheuerleisten") to protect the body of the hull from the scratching by the mooring lines. I have added a photograph taken in July 1945 at Lisahally, Northern Ireland, where after the surrender of Germany more then 60 still operational U-Boats were moored side by side. The pic shows (from right to left) in the front formation U-1109, U-1058, U-278 and U-901. In the explanatory text below is points at the "Scheuerleisten" (= rubbing strakes), which I circled on the pic and the text.

So, mistery is solved at last...