As with any U-Boat, it's best to conduct stationary (silent) sound checks. The same is true for the Type 21.
The 21 was designed for extended spells submerged, to avoid Aircraft and HK patrols, however, to enable yourself to hear sound contacts, I'd suggest 40m depth and 5kts (XXI only).
Think about using the 21 as a hit & run boat. Locate your target, attack quickly BUT escape/evade quicker.
Also, if your 21 is fully pimped, don't be affraid to spend some time surfaced as you should get adequate warning via radar of any approaching contacts (use high TC at your own risk).
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