Hi Ludovic,
Essentially the snorkel valves were operated the same way as on the non-snorkel boats, where the E/R air intakes were hight up on the CT. The duct leading down to the E/R had a watertight valve at the point where the duct leads into the pressure hull. When opening this valve after a dive a certain amount of seawater (contents of the duct) poured into the E/R bilge and was pumped out. Same with the snorkel ducts. Remember, the ducting did not go directly into the engine, but into the E/R. Since the ducts were not part of the pressure hull, they must fill with water before deep diving, otherwise they would have been crushed by the water pressure. Ofc, a similar set of valves were also installed on the exhaust side leading either to the snorkel or the the surface exhaust lines.
I do not have the time it took to hoist and lower the folding hydraulic snorkels of the type 7 and 9, but the piston throw was less than 1 meter. Thus my guess that up and down movement was less than 10 sec. each.the early type with the wire rope and hydraulic winch/pulley system took much much longer and was quickly discarded.
I do not know how long it took to hydraulically hoist and lower the vertical system of the types 21 and 23. My guess, at 1/2 meter per sec. about 8-10 sec. Rgds, RB