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On May 24, 1969, a B-40 rocket hit Monitor M-92-1 of River Squadron (RIVRON 9) as it was supporting a reconnaissance mission on the My Tho River, wounding three crewmen with fragments. The monitor turned and returned fire with its 40mm and 20mm cannon while the embarked Army liaison directed artillery fire and a helicopter strike on the enemy position. Nine Viet Cong reportedly were killed during the 30-minute engagement. The monitor itself suffered no significant damage.

Introduced into service in September 1967, the monitors constituted the battleships of the Mobile Riverine Force. Heavily armed and armored, they suppressed enemy fire during ambushes and river assaults. Like the force’s armored transport carriers (ATCs), they were modified Landing Craft Mechanized 6 (LCM-6) boats from World War II. They differed from the ATCs in having the landing ramps removed and their bows rounded off to reduce water resistance.

Monitor gun turrets were armored against shrapnel and small arms fire. The hull and superstructure were better protected with an armor plate covered by 18 to 24 inches of polyurethane and welded hardened steel bars that either detonated or “shorted-out” enemy shaped charge rounds. The polyurethane absorbed the warhead’s blast stream. Below, water blisters protected against underwater damage and added buoyancy that reduced the boat’s draft.

Two sets of river monitors were built during the Vietnam War. Eight were converted into flamethrower variants called Zippos. The last 10 of the 24 built replaced their 40mm cannon with a Mk 49 105mm howitzer turret with a ring of bar armor and better superstructure protection.

The monitors were turned over to the South Vietnamese Navy on June 30, 1969. They had played a key role for two years, delivering firepower to support the troops operating along the riverbanks, but had no role in the U.S. Navy’s post-Vietnam “blue water” operations.


Crew: 11-13
Length: 18.5m/61ft
Beam: 5.3m/17ft 6 inches
Draft: 1m/42 inches/3ft 6 inches (full load)
Displacement: 76 tons (full load)
Propulsion: 2 x 220shp GM/Detroit Diesels – 2 propellers
Top Speed: 8.5kts
Fuel: 450 gallons diesel in two tanks
Max Range: 200km/110nm@6kts
Electronics: 1 x Raytheon 1900 Nav Radar, 2 x AN/ARC-46 & 1 x AN/PRC-25 radios
Armor: Hull/Superstructure: 20mm steel splash plate covered by urethane blocks & spaced XAR-30 22mm hardened steel bars; Turrets: 22mm hardened steel armor plate 
Armament: FWD Turret: 1 x 40mm Mk 3 & M2 .50 MGs; Aft Turrets: 3 x 20mm Mk 16 cannon; Well Deck: 1 x Mk 2 Mod 1 Combined 81mm mortar/ .50 M2 MG Mount, 4 x .30 M1917 initially/later M60 7.62mm MGs, 2 x Mk 18 40mm MGs (hand cranked), crew small arms (M16s, M79s)

This story appeared in the 2023 Autumn issue of Vietnam magazine.

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