Trim-tanks to affect longitudinal balance, especially after torpedoes are fired, or with changes of depth, are I think the missing component here. From the little I've read - and I'm no expert - one of the chief causes of boats being forced to the surface, or deeper than they intended to go, was the boat becoming out of trim. This could occur from depth charges exploding beneath the boat, causing the front or aft end to decend rapidly into the aerated water beneath. The crude pitch changes required to regain control necessitating major trim changes. If this process got away from the dive officer, they could find themselves unable to cease a rise or descent of the boat.
A need to operate such trimming would make attaining and maintaining periscope depth harder and more time-consuming, and to potentially make vertical operation of the boat more demanding.
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