Simon.
Hydraulic system schnorchel.
The system is hooked up to the navigation periscopesystem as indicated on the sketch below. High pressure from the hydraulic system enters the schnorchel maneuvering slide via the stop/ speed throttling valve B. The slide allows the hyd. pressure oil to enter, via the boardvalve, the rising/lowering cylinder on either side as required. The return oil from the appropriate side of the cylinder passes the boardvalve to the schnorchel maneuvering slide and then to the stop/ speed throttling valve A before it enters the returnpipe from the periscope winch IMO motor. For some strange reason the B valve housing is painted red and the valve handwheel is removed on the valve shown on the photo from the U 995.
There are several sketches of the schnorchel system, in my opinion the sketch shown is closest to the real thing as on U 995. A and B valves are used to adjust the rising and lowering speed of the mast giving it a smooth homing in both endpositions, whereas the maneuvering slide distributes the pressure- return oil to the appropriate side of the rising/lowering piston.
Tore