Dead Wake by Erik Larson

• Loved this. [Goodreads link]
• "THAT FIRST TURMOIL, that first bubble of foam, was the expulsion of compressed air from the submarine’s launching tube as the torpedo exited. The torpedo itself was 20 feet long and 20 inches in diameter; its nose, shaped like the top of a corn silo, contained 350 pounds of TNT and an explosive called Hexanite. Though German commanders typically set the depth at 15 feet, this one traveled at 10 feet. It moved at about 35 knots, or 40 miles an hour, powered by compressed air stored in a tank toward its nose, just behind the compartment that contained the explosives. The air rushed against the pistons in its engine, geared to spin two propellers, one clockwise, the other counterclockwise, to keep the torpedo from rolling and veering. The air was then exhausted into the sea, where it bubbled to the surface. These bubbles needed a few seconds to rise, which meant the torpedo itself was always well ahead of the track that appeared above.
  As the torpedo advanced, the water rushing past its nose turned a small propeller, which unscrewed a safety device that prevented detonation during storage. This propeller slipped from the nose and fell to the sea bottom, thereby exposing a triggering mechanism that upon impact with a ship’s hull would fire a small charge into the larger body of explosives. A gyroscope kept the torpedo on course, adjusting for vertical and horizontal deflection.
  The track lingered on the surface like a long pale scar. In maritime vernacular, this trail of fading disturbance, whether from ship or torpedo, was called a “dead wake.”"