Patrol torpedo boat PT-109

PT-109 was an 80-foot (24 m) Elco PT boat (patrol torpedo boat) last commanded by Lieutenant (junior grade) John F. Kennedy, future United States president, in the Solomon Islands campaign of the Pacific theater during World War II. Kennedy's actions in saving his surviving crew after PT-109 was rammed and sunk by a Japanese destroyer earned him several commendations and made him a war hero. Back problems stemming from the incident required months of hospitalization at Chelsea Naval Hospital and plagued him the rest of his life. Kennedy's postwar campaigns for elected office often referred to his service on PT-109.

Hubert Scott-Paine of the British Power Boat Company had developed Air Sea Rescue fast motor boats in the UK. He took his PT boat to Elco in 1939, as there was a shortage of suitable engines in the UK at the outbreak of war. British Powerboat Company continued production of his designs in Britain throughout the hostilities. The seakeeping qualities of boats and ease of construction matched to the available Packard engines made a perfect combination.

PT-109 was an 80 ft (24 m), 40-ton Elco motor torpedo boat (MTB), one of hundreds built by the firm between 1942 and 1945 in Bayonne, New Jersey.

The seventh MTB of the PT-103 class, her keel was laid 4 March 1942, she was launched on 20 June, and delivered to the Navy on 10 July 1942 to be fitted out in the New York Naval Shipyard in Brooklyn. Fully loaded, she displaced 56 tons and could accommodate a crew of three officers and 14 enlisted men, with the typical crew size between 12 and 14.

The 80 ft (24 m) Elco MTBs were the largest PT boats operated by the U.S. Navy during World War II. They had strong wooden hulls, constructed of two layers of 1-inch (2.5 cm) mahogany planking, excellent for speed and reasonably adequate for seakeeping, but providing limited protection in combat.

PT-109 was powered by three 12-cylinder 1,500 horsepower (1,100 kW) Packard 4M-2500 marinized aero gasoline engines (one per propeller shaft), with a designed top speed of 41 knots (76 km/h; 47 mph).

To accommodate three engines in the boat's narrow beam (and improve weight distribution), they were staggered fore-and-aft, with the two outboard motors mounted with their output shafts facing forward and power transmitted through V-drive gearboxes to their respective propeller shafts. The center engine was mounted forward of the outboard pair in a conventional orientation, with power transmitted directly from its output end to an extended propeller shaft.

The engines were fitted with mufflers on the transom, both to mask the engines' noise from the enemy and to improve the crew's chance of hearing enemy aircraft. These directed the exhaust underwater at idle and extremely low speeds and were bypassed for anything faster.