Armament
The primary anti-ship armament was two to four Mark 8 torpedoes, which weighed 2,600 pounds (1,179 kg) and contained a 466-pound (211 kg) TNT warhead. These torpedoes were launched by Mark 18 21-inch (530 mm) steel torpedo tubes. Mark 8 torpedoes had a range of 16,000 yards (14,630 m) at 36 knots (41 mph). These torpedoes and tubes were replaced in mid-1943 by four lightweight 22.5-inch-diameter (570 mm) Mark 13 torpedos, which weighed 2,216 pounds (1,005 kg) and contained a 600-pound (270 kg) Torpex filled warhead. These torpedoes were carried on lightweight Mark 1 roll-off style torpedo launching racks. The Mk13 torpedo had a range of 6,300 yards (5,800 m) and a speed of 33.5 knots (38.6 mph).
PT boats were also well armed with numerous automatic weapons. Common to all US PT boats were the two twin M2 .50 cal (12.7 mm) machine guns. Early PT boats (Elco PT20 through PT44) mounted Dewandre plexiglas enclosed hydraulically operated rotating turrets. Almost immediately after the attack on Pearl Harbor, the Dewandre turrets were replaced on the entire PT boat fleet with open ring twin mounts. The ring mount was designed by both Elco and Bell, and designated Mark 17 Twin 50 caliber aircraft mount. Part of the Mark 17 Mod 1 and Mod 2 ring mount consisted of the Bell Mark 9 twin cradle.
Another automatic weapon commonly mounted on PT boats was the 20 mm Oerlikon cannon. On early series of boats, this cannon was mounted on the stern. Later in the war, several more of these 20 mm cannons were added amidships and on the forward deck.
Forward of the chart house of some early Elco 77-foot (23 m) boats (PT20 through PT44) were twin .30 cal (7.62 mm) Lewis machine guns on pedestal mounts. Beginning in mid 1943, some boats were fitted with one or two .30 cal Browning machine guns on the forward torpedo racks on pedestal mounts.
Occasionally, some front line PT boats received ad hoc up-fits at forward bases, where they mounted such weapons as 37mm aircraft cannons, rocket launchers, or mortars. When these weapons were found to be successful, they were incorporated onto the PT boats as original armament. One such field modification was made to Kennedy's PT-109 which was equipped with a single-shot Army M3 37mm anti-tank gun that her crew had commandeered; they removed the wheels and lashed it to 2x8 timbers placed on the bow only one night before she was lost. The larger punch of the 37mm round was desirable, but the crews looked for something that could fire faster than the single shot army anti-tank weapon. Their answer was found in the 37mm Oldsmobile M4 aircraft automatic cannon cannibalized from crashed P-39 Airacobra fighter planes on Henderson Field, Guadalcanal. After having demonstrated its value on board PT boats, the M4 (and later M9) cannon was installed at the factory. The M4/M9 37mm auto cannon had a relatively high rate of fire (125 rounds per minute) and large magazine (30 rounds). These features made it highly desirable due to the PT boat's ever-increasing requirement for increased firepower to deal effectively with the Japanese daihatsu barges, which were largely immune to torpedoes due to their shallow draft. By the war's end, most PTs had these weapons.
The culmination of larger bore cannons resulted in the installation of the 40mm Bofors gun on the aft deck. Starting in mid-1943, the installation of this gun had an immediate positive effect on the firepower available from a PT boat. The Bofors cannon had a firing rate of 120 rounds/min (using 4 round clips) and had a range of 5,420 yards (4,960 m). This gun was served by a crew of 4 men, and was used against aircraft targets, as well as shore bombardment or enemy surface craft.
Towards the end of the war, beginning in 1945, PTs received two eight-cell Mark 50 rocket launchers, launching 5 in (130 mm) spin-stabilized flat trajectory Mark 7 and/or Mark 10 Rockets with a range of 11,000 yards (10,000 m). These 16 rockets plus 16 reloads gave them as much firepower as a destroyer's 5 in (130 mm) guns. By war's end, the PT boat had more "firepower-per-ton" than any other vessel in the U.S. Navy.
PT boats also commonly carried between two and eight U.S. Navy Mark 6 depth charges in roll-off stern racks. Additionally, a few PT boats were equipped to carry Naval Mines launched from mine racks, but these were not in common usage.
Although not a weapon, U.S. Navy PTs also were fitted with Raytheon SO radar, which had about a 17 nm range. Having radar gave Navy PTs a distinct advantage in intercepting enemy supply barges and ships at night.
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