I agree it is impossible.  If dropped from 18 feet, a torpedo would fall for under 1.2 seconds and travel horizontally about 227 feet if released at 121 knots.  Kennedy gives the rules for release as speed 90 knots, height 90 feet, range 900 yardsKennedy, Ludovic. Pursuit: The Sinking of the Bismarck p.161

  jpacobb