Hi,
right again. There was none on the exhaust side other than the previously mentioned one at the pressure hull used to secure from snorkeling and go deep. Snorkeling, the Diesels could handle the backpressure of an occasional undercut.
The flapper or more correctly the T-valve head would close when undercutting much to the discomfort, pain, bulging eyes and busted eardrums, but after all the crew did not sign on for a pleasure cruise. We will probably never know for certain how many boats were lost to snorkeling accidents, when the entire crew got the air sucked out of them - diving into oblivion. The type XXI had an autopilot installed, which could keep station much better than a helmsman at the hydrplanes. Was developed from an aircraft autopilot. Rgds, RB