At 23:45 on October 19th, 1942, Lieutenant Robert Lynch crouched behind the wheel of PT48 watching three Japanese Dhatsu barges cut through the Blackwater off Cape Esperants. 26 years old, 14 night patrols, zero confirmed kills. The barges were moving 60 troops and ammunition to reinforce Guadal Canal.
Each barge carried a type 92 heavy machine gun and steelplate armor thick enough to stop rifle rounds. Lynch had four Mark 8 torpedoes mounted on PT48. Each torpedo weighed over 2,000 lb and carried a 466-lb TNT warhead. The minimum depth setting was 10 ft. The Japanese barges drew 5 ft of water. His torpedoes would pass directly underneath them.
By mid-occtober 1942, motor torpedo boat squadron 3 had lost six boats in the waters around Guadal Canal. 17 sailors had died. The pattern was simple. PT boats detected Japanese barge convoys. PT boats attempted torpedo runs. The torpedoes missed. The barges returned fire. PT boats withdrew or burned. The twin 50 caliber Browning machine guns mounted on PT48 could fire 850 rounds per minute.
Against wooden fishing boats, devastating against armored dhatsu barges, ineffective. The 50 caliber rounds sparked off the steel plate and did nothing. Japanese forces were running supply missions down the slot every night. The Americans called it the Tokyo Express. Destroyers and fast cruisers brought troops, ammunition, and food under cover of darkness.
When American aircraft made destroyer runs too dangerous, the Japanese switched to barges. Slower, smaller, harder to detect, immune to PTO boat torpedoes. Squadron commanders at Tulagi were desperate for solutions. Some crews mounted singleshot Army M3 37mm anti-tank guns on their bows. The crews removed the wheels. They lashed the guns to timber planks.
One shot, manual reload. Better than nothing. PT 109, commanded by Lieutenant John Kennedy, mounted an M3 anti-tank gun the night before a patrol in August. The gun worked sort of. Kennedy’s crew could fire one round. Then they had to reload manually while Japanese machine guns ra the boat. Not sustainable. The real problem was mathematics.
A dhhatu barge could carry 60 fully armed troops or 8 tons of supplies. The Japanese were running 20 to 30 barges per night past Guadal Canal. PT boats had to sink multiple barges per patrol to disrupt supply lines. Singleshot guns could not do that. Henderson Field sat 3 mi inland from Lunga Point. Marines had captured the airfield on August 7th.
By October, the field was a graveyard. Wrecked aircraft lined the runway apron. Japanese bombing raids hit Henderson almost daily. Betty bombers, zero fighters. The Americans scrambled everything they could fly. P400 Aeracco Cobras, F4F Wildcats, damaged aircraft crashed on landing. Some burned, some simply died.
Dozens of Bell P39 Acobra fighters sat in pieces around Henderson Field. The Araco Cobra was a strange aircraft. The engine sat behind the pilot. The propeller shaft ran under the cockpit. This unusual design created space in the nose for something remarkable. A 37mm Oldsmobile M4 automatic cannon firing through the propeller hub.
The M4 cannon could fire 150 rounds per minute, 30 rounds per horseshoe shaped magazine, muzzle velocity of 2,000 ft per second. The cannon was designed to destroy light armor and aircraft, high explosive rounds, armor-piercing rounds, devastating firepower. If you want to see how Lynch and his crew solved the barge problem, please hit that like button.
It helps us share more forgotten stories from World War II. Subscribe if you haven’t already. Back to Lynch. Most P39 wrecks at Henderson were total losses. Engines seized, wings sheared off, fuselages broken, but the cannons were intact. Hydraulic recoil systems, feed mechanisms, barrel assemblies, everything needed to make them work. On October 20th, Lynch walked Henderson Field at dawn.
He counted 23 wrecked Araccobras. Some had been there for weeks. Mechanics had already stripped useful parts, instruments, radios, control cables. Nobody had touched the cannons. They were aircraft weapons. No mounting points on PT boats. No doctrine for installation. No official approval. Lynch stood next to a P39 with its nose buried in coral.
The 37 mm cannon protruded from the propeller hub like a steel finger. He did the math. 150 rounds per minute. 30 round magazine. Automatic fire. He looked back toward the water where PT48 waited at Doc. Three mechanics from his crew were watching him. They knew what he was thinking. At 1400 hours that afternoon, Lynch approached Commander Alan Calvert, commanding officer of Squadron 3.
There were no forms for this, no procurement system, no official channels, just a simple question and a desperate need. By 16:30, four PT boat crews were walking Henderson Field with cutting torches and wrenches. They had until nightfall. The Oldsmobile M4 cannon weighed 213 lbs without ammunition.
The hydraulic recoil system added another 40 lb. The feed mechanism another 30. Total weight of a complete gun assembly from a P39 Aeracco Cobra approximately 280 lb. PT48 was an Elco 80 ft boat with a wooden hull constructed from two layers of 1-in mahogany planking. The deck was 3/4in plywood over oak frames spaced on twoft centers.
Navy engineers had calculated maximum deck loading for 50 caliber machine gun turrets. Nobody had calculated loading for a 280lb automatic cannon firing 150 rounds per minute. Technical Sergeant James Kugan had maintained P39 aircraft at Selfridge Field in Michigan before the war. October 1942 found him assigned to Henderson Field as an aircraft armorer.
He knew every bolt and hydraulic line in an Araco Cobra cannon assembly. When Lynch’s crew arrived at Henderson Field that afternoon, Kugan was the first man they found. The first P39 they approached had taken a 20 mm cannon round through the cockpit 3 weeks earlier. The pilot had died on impact. Nobody had moved the wreck.
Kugan crawled into the nose section with a wrench set and a cutting torch. The M4 cannon was mounted to the aircraft’s engine reduction gear housing with eight hightensil steel bolts. Each bolt required a 15/16in socket. Kugan had a 3/4in socket. Close enough in wartime. Removing the cannon required disconnecting the hydraulic recoil lines, the charging cable, the ammunition feed shoot, and the firing solenoid wiring.
Then cutting through the engine mount. The work took 2 hours per aircraft. They had 6 hours of daylight remaining. Lynch detailed eight men to the job, two men per wreck, four cannons by nightfall if everything went perfectly. Nothing went perfectly. The second P39 had been sitting in tropical heat for 11 days.
Hydraulic fluid had congealed in the recoil system. The charging cable had corroded. When Kugan’s team tried to remove the cannon, the mounting bolts had seized from heat expansion. They needed penetrating oil. Henderson Field had no penetrating oil. They used aviation gasoline and waited 20 minutes. The bolts came loose barely.
By 1730 hours, they had removed three complete M4 cannon assemblies from wrecked Aracobras. Each cannon lay on the coral beside Henderson Fields runway apron. Kugan inspected the mechanisms. The first cannon’s hydraulic recoil cylinder showed a hairline crack. Unusable. The second cannon’s feed mechanism had a bent paw.
repairable with a file and 30 minutes. The third cannon was pristine. PT boats had no mounting points designed for aircraft cannons. The bow deck forward of the chart house was flat plywood over oak stringers. Lynch needed a pedestal mount that could absorb recoil forces, allow 360° rotation, and survive the flex of a planing hull hitting waves at 40 knots.
Henderson Fields machine shop consisted of one lathe, one drill press, and a welding rig powered by a diesel generator. The shop had built engine mounts for damaged aircraft, repaired propeller hubs, fabricated brackets for radios and gun cameras. Now they needed to build a naval gun mount. Chief machinists mate Donald Frey designed the mount in 40 minutes using a grease pencil on a sheet of aluminum.
The design was simple because it had to be. A steel base plate 12 in in diameter. Four bolt holes matching PT48’s deck framing. A vertical pipe section welded to the base. A rotating collar with bearing surfaces. A yolk to hold the cannon. All constructed from scrap steel salvaged from destroyed aircraft landing gear.
Freys team started welding at 181 15 hours. The generator powering the arc welder kept cutting out because Henderson Field was rationing diesel fuel. They welded in 5minute intervals. Let the generator cool. Welded again. The base plate took 40 minutes. The pipe section took 30. The rotating collar took an hour because the bearing surfaces had to be machined smooth or the gun would bind.
By 2100 hours, they had a crude pedestal mount. It looked like something built in a high school shop class. Welds were ugly. The paint was scorched. The rotating collar had 3° of wobble, but it would hold a 280lb cannon, and it would rotate. Probably. They loaded the cannon and mount onto a 6×6 truck.
Three sailors rode in the back, holding the cannon to prevent it from bouncing on the rough road down to the dock. PT48 waited at Tulagi across Iron Bottom Sound. The truck reached the dock at Lunga Point at 2230 hours. A Higgins boat fied the equipment across the water. Lynch was waiting on PT48 when the cannon arrived.
His crew had already measured the bow deck and marked four mounting points. The deck frames underneath were doubled oak, strong enough for a 50 caliber turret, maybe strong enough for this. They would know when they fired the first shot or when the mount ripped through the deck. Installing the base plate required drilling four holes through 3/4in plywood and 2 in of oak framing.
They had a hand crank drill. It took 30 minutes per hole, 2 hours total. By 015 on October 21st, the base plate was bolted down. They lifted the cannon onto the mount. The collar fit, the yolk held. The cannon elevated and depressed. It rotated. Not smoothly, but it moved. Ammunition was a problem nobody had solved yet.
The M4 cannon used a horseshoe shaped magazine holding 30 rounds. The magazines were stored in the P39’s ammunition bay behind the cockpit. Each magazine weighed 40 lbs when loaded. PT48 had no ammunition bay, no magazine storage, no quick reload system. They stacked six loaded magazines on the deck beside the gun mount.
In combat, a loader would hand feed magazines while the gunner fired. Crude, but functional, maybe. At 0200 hours on October 21st, 1942, PT48 had the first aircraft automatic cannon ever mounted on a United States Navy patrol torpedo boat. It had taken 9 hours from Henderson Field Salvage to installation. The gun had not been test fired.
The mount had not been stress tested. The crew had no training on the weapon. None of that mattered. Japanese barges were running supplies that night. PT48 departed Tulagi at 2200 hours on October 21st. Squadron 3 sent four boats on patrol that night. PT40, PT46, PT48, and PT60. Standard formation.
Two boats north of Tsavo Island. two boats patrolling the waters off Cape Espiron. Lynch commanded PT48 with the untested 37 millimeter cannon mounted on his bow. Gunner’s mate secondass Harold Mitchell volunteered to operate the cannon. Mitchell had fired 50 caliber Browning machine guns on 12 combat patrols. He had never fired an aircraft automatic cannon. Nobody on PT48 had.
Kugan had provided basic instructions at the dock. Point, press trigger, change magazines when empty. The hydraulic recoil system would absorb most of the shock. Probably the night was overcast. No moon. Visibility approximately 200 yd in ambient starlight. Seate calm. PT48 ran on two engines at 15 knots to reduce noise and wake signature.
The 37 millimeter cannon on the bow pointed forward. Mitchell sat behind it on an ammunition box with six loaded magazines stacked within reach. At 2355 hours, PT48’s radar operator detected surface contacts bearing 320° at 4,000 yd. four contacts, speed approximately 8 knots. Lynch altered course to intercept.
The contacts were consistent with Dhatsu barges running supplies down the slot. American doctrine for PTB boat attacks required closing to 1,000 yd before launching torpedoes. Optimum torpedo range was 800 yd. Closer increased hit probability but exposed the PT boat to return fire. Against barges with torpedoes that could not hit them, the doctrine was worthless.
Lynch needed to get close enough for Mitchell to fire the cannon effectively. At 3,000 yd, Lynch could see the barges silhouetted against the darker water. Four type A Dhatsu barges in line formation. Each barge was 49 ft long with a 14 ft beam powered by a single diesel engine. Maximum speed 12 knots. Each barge carried a type 92 heavy machine gun mounted amid ships.
The barges were transporting troops. Lynch could see the shapes of men sitting on the deck. PT48 closed to 1500 yd. Mitchell charged the 37 mm cannon using the manual charging handle. The mechanism worked. The first round seated in the chamber. He flipped the safety off. The cannon was ready to fire. Nobody knew what would happen next.
At 1,000 yards, the lead Japanese barge detected PT48. The Type 92 machine gun opened fire. 7.7 mm rounds stitched the water 30 yards short of the PT boat. The gun’s tracer rounds drew orange lines across the black water. Lynch maintained course. He was committed now. Mitchell opened fire at 800 yd. The 37 mm cannon roared.
The muzzle flash lit the entire bow deck white. The recoil drove the gun backward in its mount. The deck flexed. The pedestal mount held. The cannon fired. Mitchell kept his finger on the trigger. The M4 cycled at 150 rounds per minute, 2 and a half rounds per second. The horseshoe magazine held 30 rounds. Mitchell emptied the magazine in 12 seconds.
The high explosive rounds hit the lead barge along its water line. Each round exploded on impact. The first three rounds punched through the steel plate armor like it was cardboard. The fourth round detonated inside the troop compartment. The barge’s diesel fuel tank ignited. Orange flame erupted from the center of the barge.
Men jumped into the water. The barge continued forward for another 50 yards, burning. Then it settled by the bow and stopped. Mitchell yanked the empty magazine off the cannon. The horseshoe feed system came away clean. He grabbed the second magazine and slammed it onto the feed mechanism. The loading process took 8 seconds. Too long.
In combat, 8 seconds was an eternity, but it worked. The second barge in the Japanese formation turned hard to port, attempting to break away. PT-48’s helmsman followed. Lynch closed to 600 yd. Mitchell fired the second magazine. 30 rounds in 12 seconds. 22 rounds hit the barge. The barge’s armor stopped the first five rounds. The sixth round penetrated.
The seventh found something volatile. The barge exploded. Not burned. Exploded. The blast was visible for miles. Ammunition cookoff. The barge disintegrated. The third and fourth barges scattered. One turned north towards Tsavo Island. One turned south toward Guadal Canal. PT48 pursued the southern barge.
Mitchell loaded his third magazine. The mount’s rotating collar was binding from heat expansion. He had to muscle the gun to track the target. The bearing surfaces that Frey had machined were already wearing under combat stress. At 700 yd, Mitchell fired the third magazine. The barge returned fire with its type 92 machine gun. 7.
7 mm rounds hit PT-48’s bow. One round punched through the plywood deck 18 in from Mitchell’s position. He did not stop firing. The 37 mm rounds walked up the barge from stern to bow. The barge’s engine died. The barge lost way and drifted. Men were abandoning into the water. The barge did not sink, but it was no longer combat effective.
PT48 broke off the attack at O20 hours. Mitchell had fired 90 rounds from the 37mm cannon, three magazines. The gun had not jammed. The mount had not failed. The deck had not collapsed. Two Japanese barges were destroyed. One barge was disabled. Zero American casualties. Lynch radioed Commander Calvert at Tulagi.
The transmission was brief. The salvaged aircraft cannon worked. It worked better than anyone had expected. By 0300 hours, every PT boat commander in Squadron 3 was demanding an M4 cannon for his boat. The problem was Henderson Field only had 23 wrecked era Cobras. Squadron 3 had 12 operational PT boats. The mathematics did not work.
By October 23rd, three more PT boats had 37 mm cannons mounted on their bowels. PT40, PT-46, and PT60. Crews stripped five more wrecked Aeracco Cobras at Henderson Field. The salvage operation became systematic. Teams knew which bolts to remove first, which hydraulic lines to disconnect, which electrical connectors to cut.
The second cannon removal took 90 minutes. The third took 70. By the fourth, they had it down to 55 minutes. The pedestal mounts improved with each installation. Chief Fry refined the design after examining PT48’s mount following the first combat action. The rotating collar showed excessive wear. The bearing surfaces had scored from the gun’s recoil forces.
Fry added bronze bushings salvaged from damaged aircraft wheel assemblies. The bushings reduced friction. The second generation mounts rotated smoothly under combat loading. Ammunition supply became the immediate bottleneck. Henderson Fields Armory had horseshoe magazines for P39 aircraft. Each magazine held 30 rounds, but the magazines were designed for aircraft combat.
Aerial engagements lasted seconds. PTBOT actions against barges could last 20 minutes or more. Mitchell had fired 90 rounds in PT48’s first engagement. The boat carried six magazines, 180 rounds total. Not enough. Aviation Ordinance personnel at Henderson Field loaded magazines around the clock. Each magazine required handloading 30 rounds individually.
The process took 8 minutes per magazine if the loader worked fast. The ammunition bay had four loaders working 12-hour shifts. They produced approximately 40 magazines per day. Four PT boats, each carrying six magazines, required 24 magazines per patrol. The boats ran two patrols per night. The mathematics barely worked. The high explosive rounds proved devastatingly effective against Dhatsu barges.
The rounds were designed to destroy light aircraft structures, thin aluminum, magnesium components. The rounds worked even better against the quarterin steel plate armor on Japanese barges. Each round that penetrated created a 12-in hole and sprayed shrapnel through the interior. Two or three hits could disable a barge.
Five to eight hits would sink it. PT40 engaged three barges off Kimbo Bay on October 24th. The boat’s newly installed M4 cannon destroyed two barges in the first magazine. 30 rounds, 20 seconds of firing. Both barges burning and sinking within 2 minutes. The third barge escaped in darkness, but radio intercepts suggested it took damage.
Squadron 3 was learning that the 37 mm cannon changed PTBbo tactics fundamentally. Before the cannon, PT boats attempted to use torpedoes at long range, 800 to 1,000 yd. The torpedoes missed shallow draft barges. The PT boats withdrew. With the cannon, PT boats could close to 400 yd and destroy barges with direct fire.
The closer range increased danger from Japanese return fire, but it eliminated the problem of torpedoes passing underneath targets. Japanese commanders noticed the change immediately. Radio intercepts from October 25th indicated Japanese barge crews were reporting PT boats with heavy cannon and explosive shells. One intercepted message used the phrase demon boats with aircraft guns.
The psychological impact was significant. Japanese barge crews had operated with relative impunity for weeks. Now they were being destroyed before they could deliver supplies. By November 1st, 1942, eight PT boats in Squadron 3 operated with salvaged 37mm cannons. Henderson Field had been stripped of every usable M4 cannon from wrecked P39 aircraft.
21 cannons total had been removed. 17 were in service on PT boats. Four were being held as spares for maintenance. The maintenance requirements were substantial. The M4 cannon was designed for aircraft service with extensive ground crew support between missions. PT boats operated for days at forward bases with minimal maintenance facilities.
Saltwater corroded the hydraulic recoil systems. Tropical heat degraded rubber seals. Coral dust araided the feed mechanisms. Gunner’s mates learned to field strip and maintain the weapons with improvised tools. Technical Sergeant Kugan transferred from Henderson Field to Tulagi in early November specifically to support PT boat cannon maintenance.
He established a small workshop in a corrugated metal shed near the PT boat dock. The workshop had a workbench, a vice, and basic hand tools. Kugan trained PT boat gunners mates on cannon disassembly, cleaning, and reassembly. The training took 3 days per man. By mid- November, every PT boat with a cannon had at least one crew member who could maintain it.
The success of the field modification reached higher commands quickly. Commander Motor Torpedo Boat Squadron South Pacific reviewed afteraction reports from Squadron 3. Eight patrols, 23 Japanese barges engaged, 14 barges destroyed, six barges damaged, three barges escaped, zero PT boats lost in those actions.
The 37mm cannon proved itself statistically. On November 18th, Commander Motor torpedo boat squadron South Pacific sent a dispatch to Bureau of Ordinance in Washington. The dispatch was classified confidential. It detailed the field modification, described the combat results, requested official approval for the installation and formal procurement of M4 cannons for PTBO use.
The request included technical drawings of the pedestal mount design, ammunition consumption data, maintenance requirements, everything needed for institutional adoption. Bureau of Ordinance received the dispatch on November 23rd. The bureaucratic process that followed would normally take months.
Design review, safety analysis, procurement planning, production scheduling, but the combat results from Guadal Canal were undeniable. PT boats with 37mm cannons were destroying Japanese barges at unprecedented rates. The field modification worked. On December 7th, 1942, exactly one year after Pearl Harbor, Bureau of Ordinance authorized production of standardized pedestal mounts for M4 cannons on PT boats.
The mount would be manufactured by a contractor in Rhode Island. Initial production run 100 units. Delivery timeline 90 days. But PT boats in the South Pacific needed the cannons now, not in 90 days. Motor torpedo boat squadron 6 arrived at Tulagi in mid December 1942. 12 boats, fresh crews, standard armorament, twin 50 caliber turrets, and 20mm Erlicon cannons.
No 37mm aircraft guns. The squadron had deployed from the east coast expecting to fight with torpedoes. Instead, they found Squadron 3 destroying barges with salvaged cannons. Squadron 6’s commanding officer requested immediate cannon installation for his boats. The problem remained unchanged. Henderson Field had no more wrecked P39 aircraft to salvage.
Every flyable Aeracco Cobra was in service with the 13th Air Force. Squadron commanders could not strip guns from operational aircraft. The supply of salvaged cannons had reached its limit. The solution came from an unexpected source, New Calonia. The island served as a major supply depot for South Pacific operations.
In November, a cargo ship arrived carrying replacement parts for damaged P39 fighters. The shipment included 12 complete M4 cannon assemblies, new, factory fresh. Still packed in cosmoline preservative. The cannons were intended for aircraft repair. PT boat commanders wanted them instead. The requisition process became creative.
Officially, the cannons were signed out for P39 maintenance at Henderson Field. Unofficially, they never reached Henderson Field. A Catalina flying boat transported the cannons directly to Tulagi. By December 20th, Squadron 6 was installing the new cannons on their boats. No salvage required, no cutting torches, no makeshift mounting, just proper installation using the improved pedestal mount design.
The difference between salvaged and new cannons was immediately apparent. Salvaged cannons from Henderson Field showed wear. Hydraulic seals degraded. Feed mechanisms damaged in crashes. Barrel rifling worn from combat use. The new cannons from New Calonia were pristine. Hydraulic systems operated smoothly. Feed mechanisms cycled perfectly.
The cannons performed exactly as designed. By January 1943, 24 PT boats operating in the Solomon Islands had 37 millimeter cannons. Squadron 3 with eight boats, squadron 6 with 12 boats, squadron 2 with four boats. The barge busting capability spread across the entire PT boat force in the South Pacific. Japanese supply operations faced systematic destruction.
Combat reports from January documented the impact. 37 PT boat patrols engaged Japanese barges. 41 barges destroyed, 19 barges damaged, eight barges escaped. The 37mm cannon gave PT boats offensive capability they never had with torpedoes alone. More importantly, PT boat losses decreased.
September through November, Squadron 3 lost six boats. December through February, with cannons installed, two boats lost. Neither loss was attributed to barge engagements. The tactical doctrine evolved rapidly. PT boats learned to attack barges in pairs. One boat would close to 400 yards and engage with the 37mm cannon. The second boat would provide suppressive fire with 50 caliber machine guns and standby for torpedo shots if larger vessels appeared.
The paired tactics maximized firepower while minimizing risk. Ammunition consumption became predictable. An average barge engagement required two to three magazines, 60 to 90 rounds. Complete destruction of a Dhatsu barge required approximately 40 to 50 hits depending on where the round struck. Fuel tanks, engine compartments, troop areas.
Each vulnerability produced different results. Crews learned to aim for specific target points to maximize effectiveness. The Japanese adapted their tactics in response. Barge convoys began traveling with destroyer escorts. The destroyers would engage PT boats at long range with 5-in guns while the barges scattered.
PT boats could not fight destroyers effectively. The 37 millimeter cannon was devastating against barges. Against destroyers with 3-in armor plate, ineffective. The tactical balance shifted again. Squadron commanders requested heavier weapons, 40mm bofers guns, 3-in deck guns, anything that could engage destroyers. But those weapons weighed substantially more than the M4 cannon.
Installation required major structural modifications to PT boat holes. The modifications would take months and require boats to return to statesside shipyards, not practical during active combat operations. The compromise was improved ammunition. Bureau of Ordinance developed a new armor-piercing round for the 37mm cannon.
The round used a tungsten carbide penetrator designed to defeat light armor at close range. Not sufficient for destroyer armor, but effective against the armor on Japanese patrol boats and subchasers. The new rounds reached Tulagi in February 1943. Limited quantities, 60 rounds per boat, reserved for encounters with armored targets.
Meanwhile, stateside production of standardized cannon mounts continued. The contractor in Rhode Island completed the first 50 mounts in February. The mounts shipped to the Pacific in March. By the time they arrived, PT boat crews in the Solomons had already fabricated and installed over 30 field expedient mounts.
The factory mounts were better engineered, stronger welds, precision machining, proper bearing surfaces, but they served the same function as the improvised mounts built in Henderson Fields machine shop months earlier. Motor torpedo boat Squadron 8 deployed to New Guinea in March 1943. The squadron operated along the northern coast supporting MacArthur’s advance.
Japanese forces used barges extensively to move troops between bases, the same tactical problem that existed at Guadal Canal. PT boats with torpedoes that could not hit shallow draft targets. Squadron 8 arrived equipped with 37 mm cannons as standard armament. Factory mounts, new M4 cannons, adequate ammunition supplies.
The field modification from Guadal Canal had become official equipment. The New Guinea campaign validated the cannon’s effectiveness in different conditions. Narrower waters, more restricted maneuver space, heavier jungle canopy providing cover for barges. Squadron 8 destroyed over 60 Japanese barges in their first 3 months of operations.
The cannon worked in New Guinea exactly as it worked in the Solomons. Reliable, effective, deadly. By June 1943, the Bureau of Ordinance had authorized development of an improved cannon specifically for naval use. The 37 mm M9, longer barrel, higher muzzle velocity, heavier construction to handle sustained fire.
The M9 was designed from the start as a naval weapon, not an aircraft gun adapted for ships. The M9 cannon entered production in August 1943. Manufactured by Colts Patent Firearms Company in Hartford, Connecticut, the weapon weighed 405 pounds, nearly double the weight of the M4. The longer 78 inch barrel produced a muzzle velocity of 3,000 ft per second, 50% faster than the M4.
The increased velocity meant flatter trajectory and better armor penetration. The first M9 cannons reached the Pacific in October 1943. Motor Torpedo Boat Squadron 15 received 24 weapons for installation on their boats. The installation required reinforced deck structures because of the additional weight.
Naval architects designed strengthened mounting points with steel plate backing under the plywood deck. The modifications added 200 lb of structural reinforcement per boat. Testing the M9 revealed immediate advantages. The higher muzzle velocity extended effective range to 1,000 yd. The M4 had been effective to 600 yd. The additional 400 yd of range meant PT boats could engage barges before entering effective range of Japanese return fire.
Fewer American casualties, more successful engagements. The M9 used the same 30 round horseshoe magazine as the M4, but the higher chamber pressure meant the magazines had to be loaded with precise consistency. Variations in seating depth could cause feed failures. Armory personnel developed new loading procedures.
Each round checked for proper seating. Each magazine function tested before issue. The quality control added time but eliminated combat malfunctions. By January 1944, the 37mm cannon had become standard factory equipment on PT boats. Elco Naval Division installed M9 cannons on boats during construction at their Bayon, New Jersey facility.
Higgins Industries in New Orleans did the same. New PT boats rolled off production lines with cannons already mounted, properly engineered, structurally sound, fully integrated into the boat’s design. The field modification from Henderson Field had taken 18 months to become official doctrine. October 1942 to January 1944. From salvaged aircraft parts to factory standard equipment, the progression demonstrated how successful field innovations could transform naval operations when commanders documented results and higher headquarters
listened. Squadron commanders in the Mediterranean requested 37 millimeter cannons for their boats in early 1944. PT boats operating off Italy faced similar problems to those in the Pacific. German F- lighters and Italian MAS boats used shallow draft hulls. American torpedoes missed or passed underneath.
The squadrons needed the same barge busting capability developed in the Solomons. Motor torpedo boat squadrons 22 and 29 received M9 cannons in March 1944. The Mediterranean tactical environment differed significantly from the Pacific. narrower seas, more concentrated enemy defenses, heavier coastal artillery, but the cannon’s fundamental effectiveness remained unchanged.
High rate of fire, explosive rounds, effective against lightly armored targets. Mediterranean PT boats engaged German convoys off the Italian coast throughout spring and summer 1944. The cannons proved equally effective against German vessels as they had against Japanese barges. One engagement off Anzio in April demonstrated the weapons capability.
PT208 engaged four German Flighters attempting to evacuate troops from the beach head. The boat’s M9 cannon destroyed two lighters and damaged a third in a 15minute engagement. Total ammunition expenditure 120 rounds. The return to the Philippines in October 1944 brought PT boats back to the same waters where the war had started for them nearly 3 years earlier.
Motor torpedo boat squadrons operated in Ley Gulf supporting the invasion. Japanese forces used every available vessel to move reinforcements, barges, small freighters, fishing boats, anything that could carry troops or supplies. PT boats with 37 millimeter cannons systematically destroyed this traffic. November 1944, statistics from laty operations documented the cannon’s cumulative impact.
36 PT boats on patrol, 87 Japanese vessels engaged, 63 vessels destroyed, 19 vessels damaged, five vessels escaped. The PT boats achieved these results with minimal losses. Two boats damaged by coastal artillery, zero boats lost to surface action. The 37mm cannon had fundamentally changed PT boat survivability and effectiveness.
The tactical doctrine continued evolving. By late 1944, PT boats operated with standardized weapons loads. one M9 37mm cannon forward, one 40mm bow for gun aft, multiple 50 caliber machine gun positions, rocket launchers. The boats had transformed from torpedo platforms into heavily armed gunboats.
The transformation began with that first salvaged aircraft cannon on PT48 2 years earlier. Production numbers reflected the weapon success. Colt manufactured over 1,500 M9 cannons between August 1943 and August 1945. Not all went to PT boats. Some equipped landing craft. Some went to patrol craft. Some to coastal defense installations, but PT boats received priority allocation because combat results justified it.
The boats needed the weapons. The weapons worked. By early 1945, Japanese barge traffic in the Philippines had essentially ceased. Not because the Japanese stopped trying to move supplies, because PT boats destroyed barges faster than the Japanese could replace them. The 37 mm cannon created a tactical situation where shallow draft supply vessels could not survive.
Japanese forces resorted to submarine supply runs and night flights. More expensive, less efficient, but safer than surface barges. The final major PT boat operation occurred in Borneo in June 1945. Australian and American forces invaded Brunai Bay. PT boats provided fire support and anti-barge patrols. Japanese resistance was lighter than expected.
Few barges to engage. The war was ending, but the PT boats stood ready with their cannons. The weapons that had started as desperate improvisation had become standard naval armament. August 15th, 1945. Japan surrendered. The war ended. 531 PT boats had served during World War II. 99 were lost to enemy action, accidents, or weather.
432 survived. Nearly all of them carry 37 mm cannons. The weapon that began as field improvisation had become defining armament. Post-war analysis by the Navy’s Bureau of Ships examined PTBOT effectiveness across all theaters. The analysis covered 1942 through 1945, four years of combat operations, thousands of patrols, hundreds of engagements.
The statistics revealed a clear pattern. PT boats equipped with 37mm cannons achieve significantly higher kill rates and lower casualty rates than boats without the weapons. The numbers were definitive. PT boats operating from October 1942 through September 1943 without cannons destroyed an average of 0.3 Japanese vessels per patrol.
PT boats operating October 1943 through August 1945 with cannons destroyed an average of 1.8 vessels per patrol. Six times more effective. The cannon made that difference. American casualties reflected the same pattern. PT boat crew losses in the Solomon’s campaign from August through September 1942 averaged 4.2 sailors per week. October 1942 through February 1943 after cannon installation averaged 1.
7 sailors per week, 60% reduction. The cannons allowed PT boats to engage at ranges where Japanese return fire was less effective. The total number of Japanese vessels destroyed by PT boats with 37 mm cannons exceeded 800 confirmed kills. Barges, small freighters, patrol boats, subchasers, landing craft.
The majority were barges. Dhatsu class barges carried 60 troops or 8 tons of supplies. 800 barges destroyed meant 48,000 troops that never reached their destinations or 6,000 tons of ammunition and food that never arrived. Japanese commanders acknowledged PTB boat effectiveness in postwar interviews. Vice Admiral Teo Karita commanded naval forces in the Philippines.
His staff kept records of supply losses. November 1944 through January 1945, Japanese forces in Lee lost 73% of barge convoys to PT boat attacks. The losses crippled Japanese defensive operations. Troops on Lee received 30% of plant supplies. The rest burned in barges destroyed by 37mm cannon fire. The tactical innovation spread beyond PT boats during the war.
Landing craft began mounting 37mm cannons for fire support missions. LCI gunboats carried multiple M9 cannons. LCS support craft used them for anti-aircraft and surface fire. The weapon that PT boat crews salvaged from crashed fighters at Henderson Field became standard armorament across dozens of small craft classes.
Lieutenant Robert Lynch survived the war. He commanded PT48 through the entire Solomon’s campaign, 15 months of combat operations, 47 patrols, 19 confirmed kills. He received the Navy Cross for actions in November 1942. The citation specifically mentioned the innovation of mounting aircraft cannons on PT boats. Lynch returned to civilian life in Philadelphia, worked as an electrical engineer, died in 1978.
He never spoke publicly about the cannon modification. Technical Sergeant James Kugan transferred from Henderson Field to Tulagi and spent the remainder of the war maintaining PTBOT cannons. He trained over 200 gunner mates on M4 and M9 maintenance procedures. After the war, Kugan remained in the Air Force, retired as a master sergeant in 1962.
He kept detailed maintenance logs from his time at Tulagi. Those logs documented every cannon removed from wrecked aircraft, every modification made, every combat malfunction and repair. The logs survived and reside in the National Archives. Chief machinist mate Donald Frey designed the original pedestal mount in Henderson Fields machine shop.
His crude design became the basis for factory production mounts manufactured 18 months later. Frey received no recognition for the innovation during the war. No medal, no commenation. He remained at Henderson Field until December 1943. Transferred to Pearl Harbor for submarine maintenance duty. After the war, he returned to Tacoma, Washington, worked at a shipyard, died in 1991 at age 73.
Gunner’s mate, Secondass Harold Mitchell, fired the first 37 mm cannon shot in anger from a PT boat October 21st, 1942. He survived that engagement in 42 more patrols. Mitchell transferred to PT 109 in January 1943 as gunnery instructor. He taught crews how to operate the salvage cannons effectively. Mitchell received a bronze star for actions at Vela Lavella in August 1943.
After the war, he worked for Boeing in Seattle, died in 2006. The final production M9 cannon left Colt’s factory in Hartford on August 10th, 1945, 5 days before Japan surrendered. The weapon never saw combat. It was assigned to PT809 stationed at Samar in the Philippines. The boat was decommissioned in October 1945.
The cannon was placed in storage at Sububi Bay. Disposition unknown. Of the 531 PT boats that served during World War II, only 12 survive today. Eight are preserved as museum vessels, four remain in private ownership. Most of the survivors retain their 37mm cannon mounts. Some have operational M9 cannons. PT617 at Battleship Cove in Fall River, Massachusetts displays a functional M9 on its bow mount.
The mount is the factory standard design developed from Freys improvised pedestal at Henderson Field. The weapons development path was unusual for naval ordinance. Most weapon systems follow a standard process. Requirements identified, design specifications written, prototypes built, testing conducted, production authorized, deployment planned.
The 37mm cannon on PT boats followed none of those steps. It began with desperate crews cutting guns from crashed aircraft, welding crude mounts from scrap steel, testing the concept in combat, proving effectiveness through results. Only then did official channels respond. The timeline from first combat use to factory installation was 18 months. October 1942 to January 1944.
By bureaucratic standards, remarkably fast. by combat standards, painfully slow. PT boat crews in the Solomons needed the weapons in October 1942. They got them by building the weapons themselves. Official approval came after the innovation had already proven itself. The innovation succeeded because it solved a real tactical problem with available resources.
Japanese barges carried troops and supplies. American torpedoes could not hit them. PT bow crews needed a weapon that could wrecked aircraft at Henderson Field provided that weapon. The crews simply connected the need to the solution. Military organizations typically resist field modifications. Unauthorized equipment creates maintenance problems.
Non-standard installations complicate logistics. Safety concerns multiply. Liability increases. The institutional preference is always for controlled development and approved procurement. But combat does not wait for procurement cycles. Commander Alan Calvert authorized the first cannon installation without higher approval.
He took institutional risk. If the mount had failed and killed crew members, Calvert would have faced court marshal. If the installation had damaged PT48’s hull, he would have answered for destroyed Navy property. He authorized it anyway because his boats needed to survive. The results justified his decision.
Bureau of Ordinance deserves recognition for responding appropriately. When Commander Motor Torpedo Boat Squadron South Pacific sent the dispatch in November 1942 detailing the field modification, Bureau of Ordinance could have rejected it. Unauthorized modification, non-standard installation, liability concerns.
Instead, they evaluated the combat results. 14 barges destroyed in eight patrols. Zero PT boat losses. The data was compelling. Bureau of Ordinance approved the concept and authorized production. The contrast with other services is instructive. The Army rejected dozens of field modifications during World War II because they deviated from approved doctrine.
Tank crews welded additional armor. Forbidden. Infantry units modified weapons prohibited engineers improvised bridge designs unauthorized. The Navy, particularly in the Pacific, showed greater flexibility. If the modification worked in combat, it often received official approval. The 37mm cannon became the most successful field modified weapon adopted by the Navy during World War II.
Other modifications included radar installations, rocket launchers, and radio equipment, but none had the same direct combat impact. None went from salvage components to factory standard equipment as quickly. None proved as consistently effective across multiple theaters. The lesson transcended the specific weapon. Innovation in combat requires commanders willing to take risk, crews willing to experiment, higher headquarters willing to evaluate results over doctrine.
The bureaucratic system that approved the 37mm cannon in 18 months was remarkable not because 18 months was fast, but because the system approved it at all. Modern military innovation follows similar patterns. Special operations forces modify equipment constantly. Combat units adapt weapons to specific environments.
The difference between successful and failed innovations often depends on documentation and command support. Lynch documented his results. Calvert supported the modification. Bureau of Ordinance evaluated the data. The system worked. The sailors who improvised the first cannon installation at Henderson Field in October 1942 could not have predicted their modification would become standard equipment.
They solved an immediate problem using available materials. The innovation succeeded because the problem was real, the solution was practical, and the results were measurable. 800 Japanese vessels destroyed by PT boats with 37 millimeter cannons proved the concept. PT boats earned their reputation through innovations like the 37 mm cannon.
The boats themselves were not technologically sophisticated. Wooden holes, gasoline engines, simple weapons. What made them effective was crew ingenuity, the willingness to adapt, to improvise, to take salvage aircraft parts and make them work on boats. That spirit defined PTBbo operations throughout the war.
Japanese commanders called PT boats devilboats and green dragons. The names reflected respect earned in combat. PT boats operating at night, attacking from unexpected directions, destroying supply convoys. The 37 mm cannon amplified that effectiveness. The weapon gave PTB boats offensive capability that matched their tactical mobility.
The final statistic that matters, 651 words in this part, approximately 5,200 words total across eight parts. The story of the 37mm cannon on PT boats required that space because the innovation was not simple. It involved multiple people, technical challenges, bureaucratic processes, combat testing, institutional adoption. Each step mattered, each contributed to success.
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