Specifications
PT-109 belonged to the PT 103 class, hundreds of which were completed between 1942 and 1945 by Elco. PT-109's keel was laid 4 March 1942 as the seventh Motor Torpedo Boat (MTB) of the 80-foot-long (24 m)-class built by Elco and was launched on 20 June. She was delivered to the Navy on 10 July 1942, and fitted out in the New York Naval Shipyard in Brooklyn.
The Elco boats were the largest PT boats operated by the US Navy during World War II. At 80 feet (24 m) and 40 tons, they had strong wooden hulls of two layers of 1-inch (2.5 cm) mahogany planking. Powered by three 12-cylinder 1,500 horsepower (1,100 kW) Packard gasoline engines (one per propeller shaft), their designed top speed was 41 knots (76 km/h). For space and weight-distribution reasons, the center engine was mounted with the output end facing aft, with power directly transmitted to the propeller shaft. Because the center propeller was deeper, it left less of a wake, and was preferred by skippers for low-wake loitering. Both wing engines were mounted with the output flange facing forward, and power was transmitted through a Vee-drive gearbox to the propeller shafts. The engines were fitted with mufflers on the transom to direct the exhaust under water, which had to be bypassed for anything other than idle speed. These mufflers were used not only to mask their own noise from the enemy, but to be able to hear enemy aircraft, which were rarely detected overhead before firing their cannons or machine guns or dropping their bombs.
PT 109 could accommodate a crew of 3 officers and 14 enlisted, with the typical crew size between 12 and 14. Fully loaded, PT-109 displaced 56 tons.
The principal offensive weapon was her torpedoes. She was fitted with four 21-inch (53 cm) torpedo tubes containing Mark VIII torpedoes. They weighed 3,150 lb (1,429 kg) each, with 386-pound (175 kg) warheads and gave the tiny boats a punch at least theoretically effective even against armored ships. Their typical speed of 36 knots (67 km/h) was effective against shipping, but because of rapid marine growth buildup on their hulls in the South Pacific and austere maintenance facilities in forward areas, American PT boats ended up being slower than the top speed of the Japanese destroyers and cruisers they were tasked with targeting in the Solomons. Torpedoes were also useless against shallow-draft barges, which were their most common targets. With their machine guns and 20 mm cannon, the PT boats could not return the large-caliber gunfire carried by destroyers, which had a much longer effective range, though they were effective against aircraft and ground targets. Because they were fueled with aviation gasoline, a direct hit to a PT boat's engine compartment sometimes resulted in a total loss of boat and crew. In order to have a chance of hitting their target, PT boats had to close to within 2 miles (3.2 km) for a shot, well within the gun range of destroyers; at this distance, a target could easily maneuver to avoid being hit. The boats approached in darkness, fired their torpedoes, which sometimes gave away their positions, and then fled behind smoke screens. Sometimes retreat was hampered by seaplanes which dropped flares and bombs on the boats. The Elco torpedo-launching tubes were powered by a 3-inch (76 mm) black powder charge to expel the torpedo from the tube. Additionally, the torpedo was well greased so it would slide out of the tube. Sometimes, the powder charge caused the grease to ignite upon firing, and the resulting flash could give away the position of the PT boat. Crews of PT boats relied on their smaller size, speed and maneuverability, and darkness, to survive.
Ahead of the torpedoes on PT-109 were two depth charges, omitted on most PTs, one on each side, about the same diameter as the torpedoes. These were designed to be used against submarines, but were sometimes used by PT commanders to confuse and discourage pursuing destroyers. PT-109 lost one of her two Mark 6 depth charges a month before Kennedy showed up when the starboard torpedo was inadvertently launched during a storm without first deploying the tube into firing position. The launching torpedo sheared away the depth charge mount and some of the footrail.
PT-109 had a single, 20 mm Oerlikon anti-aircraft mount at the rear with "109" painted on the mounting base, two open rotating turrets (designed by the same firm that produced the Tucker automobile), each with twin, .50-caliber (12.7 mm) anti-aircraft machine guns, at opposite corners of the open cockpit, and a smoke generator on her transom. These guns were effective against attacking aircraft.
The day before her most famous mission, PT-109 crew lashed a US Army 37 mm antitank gun to the foredeck, replacing a small, 2-man life raft. Timbers used to secure the weapon to the deck later helped save their lives when used as a float.
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