Set intercept courses.
Use the layer.
Set silent running whenever possible, i. e. when not reloading or doing damage control.
Use decoys.
After firing, dive deep. Use the full depth capability of your boat.
Try to use passive detection to the maximum extent possible, even on the torpedoes. Don't launch wire-guided torpedoes with their onboard sonar already active. Activate that when the torpedo is away from your boat, if it all.
Try to fire your torpedoes into the enemy baffles and use only passive weapon sonar.
Avoid emissions such as radar and active sonar.
Hide under other vessels, or if you've sunk other vessels, use the creaking, noisy wrecks to hide.
Do not cavitate.
Use noisemakers, speed and rudder if a torpedo gets too close. Create knuckles.
Kill that which could kill you first. Even if the merchants sail on, you can catch up after killing the killers, or rise to periscope depth and use sensors to locate them, then launch missiles.
Run silent, run deep.
