Weapon Systems
Stoner Page 3

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Weapon Systems
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Weapons Systems on the PTFs - Page 3

[More PTF Weapons information from Bob Stoner when asked about 81mm Mortars.]

11/21/01 PTF Weapons Systems

PTF1 and PTF2 were holdovers from the 1950s. They had gasoline engines and were cranky. They were soon discarded for the Norwegian "Nasty"-class boats (PTF-3 through PTF-16). PTF-17 through PTF-22 were copies of the "Nasty"-class boats built in the USA. PTF-23 through PTF-26 were the "Osprey"-class. With the exception of PTF-3 and PTF-4, all other boats were the same in their weapons arrangement.

PTF-3 and PTF-4 had 40mm/L60 Bofors guns on the bow and were surrounded by 40mm ready-service lockers. All PTFs carried a 40mm/L60 Bofors gun aft. These guns were air-cooled and could fire 120 to 160 rounds/minute. They came in two distinct mountings: Mk 3 (Navy) and M3 (Army). The Mk 3 gun had power drives for elevation and traverse; the M3 gun got this accomplished manually. I believe all the 40mm guns used on PTFs were of the M3 type because of the KISS (keep it simple and stupid) principle involved. (We used M3 guns on PTF-13, PTF-17 through PTF-19 that I was on.)

RVN PTFs carried the Mk 4 20mm Oerlikon machineguns (ala WW2 with steel splinter shields). These 20mm guns used 60-round snail drums with a cyclic rate of 450 to 550 rounds/minute and an effective range of 2,000 yards. Postwar PTFs used the Mk 16 Mod 5 20mm machinegun on a Mk 67 or Mk 68 gun mount. The Mk 16 Mod 5 machinegun (actually an M24 or M3 aircraft gun) used linked M95 AP-T, M96 INC, or M97 HE-I ammunition at a cyclic rate of 650 to 800 rounds/minute. The M67 Mod 1 mount carried 385 rounds of linked 20mm ammunition in a single box. The Mk 68 Mod 1 mount carried 400 rounds of 20mm ammunition in two boxes.

PTF-5 through PTF-26 had the Navy Mk 2 Mod 1 81mm/.50 BMG mount in place of the 40mm. It was felt that the 81mm mortar was very useful for indirect fire application and the firing of parachute flares for illuminating targets at night. The .50 M2HB Browning machinegun was effective out to 2,000 yards. The 81mm projectile ranges varied due to the number of increment bags used and the angle of elevation. (There is also a Mk 2 Mod 0 mortar. The difference is that it does not have the machinegun mounted on it.)

The Mk 2 Mod 1 could traverse 360 degrees, elevate to 71.5 degrees, and depress to -30 degrees. It fired in one of two modes: DROP FIRE or TRIGGER FIRE. Rate of fire at 45 degrees for DROP FIRE was 18 rounds/minute and 10 rounds/minute in TRIGGER FIRE.

Cartridges used by the 81mm mortar are: 

  • M43-series HE - Light (as used in WW2 81mm M1 mortars) 
  • M362-series HE - Heavy (as used in 81mm M29 mortars) 
  • M374-series HE - Heavy (as used in 81mm M29 mortars) 
  • M301-series HE - Heavy (as used in 81mm M29 mortars) 
  • M57-series WP Smoke - Heavy (for M29 mortars) 
  • M375-series WP Smoke - Heavy (for M29 mortars) 
  • Mk 112 Mod 0 Leaflet (used only by Mk 2 mortars) 
  • Mk 115 Mod 0 Chaff (used only by Mk 2 mortars) 
  • Mk 133 Mod 0 Chaff (used only by Mk 2 mortars) 
  • Mk 134 Mod 0 Chaff (used only by Mk 2 mortars) 
  • Mk 135 Mod 0 Chaff (used only by Mk 2 mortars) 
  • Mk 120 Mod 0 APERS (used only by Mk 2 mortars) 


The HE rounds came in either point detonating, proximity, or mechanical time fusing. The WP Smoke rounds were point detonating or mechanical time fused. The Illumination rounds were mechanical time fused. The chaff and leaflet rounds were mechanical time fused.

The Mk 120 Mod 0 cartridge was fuseless round. Its body resembled a WW2 German "potato masher" hand grenade with a large can-shaped container for 1,200 13-grain flechettes; a petal-shaped drag ring in the middle of the shaft; and fins on the end of the shaft. The Fin assembly Mk 4 Mod 1 contained the ignition cartridge M6, primer M34, and attachment point for seven increment charges M2A1. No explosive filler was used. The effective range of the cartridge was 10 to 600 feet. The cartridge is painted olive drab with a 1/2-inch brown band marked with white diamonds and carries the following description stenciled in white: "CARTRIDGE, 81MM, ANTI-PERSONNEL, MK 120 MOD 0." Overall length of the loaded cartridge was 12.62 inches.

When fired, the burning propellant gases would melt a fusible plug in the bottom of the flechette carrier end. the gas would expand into a chamber topped by a plate (piston) the piston would move forward, carrying the flechettes with it, break the plastic cover, and be expelled in a cone-shaped pattern effective out to 600 feet. Drop at 600 feet was approximately 6 feet (or nil) which made it perfect for the direct fire role.

The Mk 120 Mod 0 cartridge turned the 81mm mortar into a giant shotgun. It was typically used in the TRIGGER FIRE role where the devastating effect of its 1,200 flechettes could be a tactical advantage. You wouldn't want to stick your head up within 600 feet of this nasty swarm of killer-bees.

I have only anecdotal information on the nature of PTF missions up North. The PTFs dropped off LDNN raiding parties for sabotage of North Vietnamese radar or other military sites; did prisoner snatches of NVA people or fishermen; shot-up NVA shore activities (including P-4 and P-6 motor torpedo boat bases). The P-4 and P-6 boats were the natural enemies of the PTF. They carried multiple 12.7mm, 14.5mm, 23mm, or 37mm automatic weapons. Whereas the PTFs usually operated in alone, in pairs, or small groups the P-4 and P-6 boats operated in multiples. This meant the PTF was usually outnumbered and its safety usually lay in its great speed (which could approach 45 mph). The PTFs were very aggressive and carried the war to the NVA's backyard many times. The VNN and MST sailors who ran them out of DaNang are virtually unknown outside of old Naval Special Warfare types.

PTFs were not painted black. They were painted a dark, lusterless marine green. The paint did not reflect-back light even when painted by a search light. The human eye interprets this as the color black. Because they were mostly wood, PTFs did not have a very good radar signature; however, visual sights remained unaffected by radar lock problems. Not a good thing if those sights are on a ZPU-14.5-4 heavy machinegun battery!

I hope this has clarified some of your questions.