Historically and in-game, U-boats were most dangerous when used as "submersible torpedo boats." Attacking on the surface at night gives you a speed advantage over most merchant ships and makes you very hard to spot.
This is the heart of the game. Instead of relying on the "locking" feature, you manually input a target's range, bearing, angle on bow (AOB), and speed. Mastering this allows you to fire from long distances or difficult angles where the auto-targeting often fails.
While tempting to use against defenseless merchants to save torpedoes, the deck gun is useless in rough seas and leaves you vulnerable. Only use it when you are certain no escorts are nearby.
"Stiller Jäger 3" (Silent Hunter 3) is a legendary submarine simulation game focused on U-boat warfare during World War II. To master it, you need to move beyond simple "point and shoot" mechanics and embrace the technical realities of manual targeting and crew management.
When an aircraft is spotted, every second counts. Keying the crash dive command and immediately setting your planes to "hard dive" can be the difference between escaping or being destroyed by depth charges.