Hi Gentlemen,
I have few words related with the hydraulic system and the diving planes.
Although most of type VII U-Boats were equipped with the diving planes driven by electric motors (as thoroughly discussed above), there were two boats, which were fitted with the hydraulic drive of the diving planes for experimental purposes. These boats were U 206 and U 207. I have no details about experiences related with this drive, but one can suppose, that electric motors driving hydroplanes were replaced by the hydraulic motors (the same as for periscope drive) controlled by pilot valves at diving planes control station.
What was the purpose of usage of the hydraulic drive - one main reason would be noise - the hydraulic drive would be quieter than electric drive.
The type XB U-boats (minelayers) were also equipped with the diving planes driven hydraulically.
The concept of the hydraulic drive was elaborated by Germans in the type XXI U-Boat, where not only periscopes were operated hydraulically, but also torpedo tubes muzzle door opening gear, torpedo reload system, forward diving planes retracting gear and finally diving planes and main rudder drive.
There should be noted, that in this case, the diving planes (and main rudder) were driven not by hydraulic motor (and then by worm gear), but directly by hydraulic pistons/actuators.
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Regards
Maciek