A PT boat (short for Patrol Torpedo boat) was a torpedo-armed fast attack craft used by the United States Navy in World War II. It was small, fast, and inexpensive to build, valued for its maneuverability and speed but hampered by ineffective torpedoes, limited armament, temperamental airplane engines, and comparatively fragile construction that limited it to coastal waters.
The PT boat was very different from the first generation of torpedo boat, which had been developed prior to World War I and featured a "displacement" hull. It rode low in the water, displaced up to 300 tons, and had a top speed of 25 to 27 kn (29 to 31 mph; 46 to 50 km/h). Instead, World War II PT boats exploited advances in planing hull design borrowed from offshore powerboat racing to reduce displacement to only 30–75 tons and increase top speed to 35 to 40 kn (40 to 46 mph; 65 to 74 km/h), with a corresponding improvement in mobility.
During World War II, PT boats engaged enemy warships, transports, tankers, barges, and sampans. As gunboats they could be effective against enemy small craft, especially armored barges used by the Japanese for inter-island transport.
Primary anti-ship armament was four 2,600 pounds (1,179 kg) Mark 8 torpedoes. Launched by 21-inch Mark 18 (530 mm) torpedo tubes, each bore a 466-pound (211 kg) TNT warhead and had a range of 16,000 yards (14,630 m) at 36 knots (41 mph). Two twin M2 .50 cal (12.7 mm) machine guns were mounted for anti-aircraft defense and general fire support. Some boats shipped a 20 mm Oerlikon cannon.
Propulsion was via a trio of modified Packard 3A-2500 V-12 derived aviation gasoline-fueled, liquid-cooled aircraft engines.
Nicknamed "the mosquito fleet" – and "devil boats" by the Japanese – the PT boat squadrons were heralded for their daring and earned a durable place in the public imagination that remains strong into the 21st century.