Exhaustpipe expansion.
I had another look at the divingvideo of U-251 provided by Christopher and located the exhaust pipes outside the pressurehull. What strikes me is the relatively complex arrangement of connections between the group exhaust valve, 90 degrees bend, muffler and sparkarrestor, all connections have a short visible part of an uncooled smaller pipe entering the next component.
This is not without any reason. As I briefly mentioned before, free heat expansion has to be provided for and the connections have an flange for a "stuffingbox" allowing the pipe to expand freely into the next component. This uncooled exhaustpipe stub has a small diameter (no coolingjacket) which makes it is easier to provide such an expansion stuffing box . I guess you find the same arrangement for the exhaustmanifold.
Tore