Don.
As you know we had 3 VIICs in service U995, U-926, and U-1202 the latter 2 were VIICs. U995 is VIIC/41. As far as I remember they were equipped with the same external exhaust system. The external systems were with two external main exhaust flap valves which had a double functions
1. The outer main exhaust valve being a shut off flapvalve with rotating valvedisk for grinding off carbon deposits on the valve seatings. This valvedisk was driven by a pneumatic motor in the engineroom with rod extension through the pressurehull to the outer main exhaust valve.
2. The inner main exhaust flap valve being a shut off valve with a large drain to the bilge, the rotating valvedisk could be manually turned with a wrench on a stubshaft outside the valve casing.
At diving the outer exhaustvalve was shut and the pneumatic grindingmotor started, as soon as we descended through the watersurface we got a sligth back pressure on the valvedisk. When we came deeper the grinding came to a halt at some 4 meter as the back pressure created a torque larger than the pnuematiq motor could manage. A few times this grinding was not sufficient and we drained the leakwater down to the chamber in the inner shutoff valvecasing and further to the bilges. The deeper we came the higher back pressure on the outer exhaust valvedisk. I cannot recall we ever had to surface because of this system.
When surfacing you know we used the exhaustgases for residue blowing. As there is a limit as to the back pressure the engines can take, as far as I remember the limit was 6-8m wc . This was monitored by the engineers and you had a procedure for not exceeding the back pressure. For controlling the exhaust back pressure the outer mainexhaust valve was used. The low pressure residue blowing (exhaustblowing) followed a certain sequence by starting with the highest ballasttanks and as the submarine ascends further blowing the deeper tanks. A drawback with the pneumatic grinding was the increase of the atmospheric pressure of the submarine and a few time we had to start the E-compresor to get the pressure down.
Tore
