destroyers have four modes of detection
1. visual - the DD crew observes your surfaced u-boat, or a part of your u-boat as it penetrates the surface
2. Radar - they detect your radar signals or your radar signature of your surfaced boat.
3. Passive Sonar - they hear your engines running / props turning or repair work being conducted / torpedoes being loaded (if not silent running) they dont hear anything else. (motors, dive planes, rudders etc)
4. Active Sonar - sound signal hits your boat and reflects a target... this depends heavily on your aspect ratio and depth
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