 Time and again, U-boat captains tracked British targets and fired, only to watch the ships sail on unharmed as the torpedoes exploded prematurely (due to the influence pistol), or hit and failed to explode (because of a faulty contact pistol), or ran beneath the target without exploding (due to the influence feature or depth control not working correctly).Not a single British warship was sunk by a U-boat in more than 20 attacks.As the news spread through the U-boat fleet, it began to undermine morale.