[Aktungbby]

Originally Posted by Onkel Neal

U-boat underwater speed averages at 4kn.
U-boats more often go into silent running at 2-3kn.
U-boats may achieve a maximum underwater speed of 6kn, although rarely and briefly.
U-boats dive to medium and deep more often, but due to lower U-boat speeds the hit probability has increased significantly.
Minimum turning radius for U-boat evasive maneuvers has been corrected.

The underwater speed factor also reflects the fact that the extended periscope would vibrate above 4 knots anyway, reducing optic efficiency and target calculation in real life. The 5 meter tube, when extended from the housing, would be almost useless above 6 knots due to excess vibration which could, in turn, lead to major problem #2: irreparable leakage or loss of air-tightness in the tube which would cause fogging from the damp submersible's always moist atmosphere.