US riverine monitors

The comparatively small type of monitor intended for use on rivers and larger wetlands is well protected and, in general, carried the largest-calibre guns of any riverine warship. On 18 December 1965, as its major commitment to the Vietnam War was in its early stages, the US Navy decided to create a ‘brown-water navy’ for deployment on the extensive waterways characteristic of the southern region of South Vietnam, and in July 1966, authorisation was granted for the establishment of a Mobile Riverine Force, which would re-create the heavily armoured river monitor fitted with a single gun turret and closely related in conceptual terms to the river gunboat.

River monitors are intended for service on inland waters such as rivers, river estuaries and deltas, canal networks and lakes. In general such vessels are characterised by the shallow draught which allows them to operate effectively in enclosed waters, but the displacement, size and draught of such craft varies quite considerably depending on the precise nature of their operational theatre. In Asia, large river monitors had first been employed on the Amur river by the USSR and Japan, and were of a displacement of as much as 1,000 tons and armed with 130-mm (5.12-in) guns. However, over the course of the Vietnam War, the US Navy commissioned riverine craft of several types, generally based on the ‘LCM-6’ class of landing craft. These included 24 Monitors, of which 10 carried one 40-mm Bofors cannon in a Mk 52 turret, eight carried one 105-mm (4.13-in) M49 or M101 howitzer in a T172 turret, and six carried two M10-8 flamethrowers in a pair of M8 turrets located on each side of the vessel’s 40-mm cannon.

For Vietnamese operations, the US Navy’s ‘brown-water navy’ deployed its monitors as part of the River Assault Flotilla One, which at first comprised four River Assault Divisions: RAD 91 had three monitors, RAD 92 two monitors, RAD 111 three monitors, and RAD 112 two monitors.

Old and new

The riverine monitors used in South Vietnam were divided into two groups of steel-built craft: Program 4 comprised the 40-mm gun monitors converted from ‘LCM-6’ class landing craft built in World War II and therefore fitted with a ramp bow intended for lowering, and Program 5 comprised the eight 105-mm (4.13-in) howitzer monitors and the eight flamethrower monitors based on new-build hulls derived from that of the ‘LCM6’ class with a rounded rather than ramp bow. All of the monitors were powered by two Gray Marine Model 64NH9 water-cooled Diesel engines delivering 330 hp (246 kW) to two shafts for a speed of 8 kt in the ramp-bowed Monitor Mk IV and 8.5 kt in the round-bowed Monitor Mk V. With a full-load displacement of 75 or 75.5 tons, the craft of the two monitor classes each carried about 10 tons of armour protection, including screen and bar armour designed to defeat the hollow-charge warheads of the rockets fired by recoilless rifles and rocket-propelled grenade launchers, and were manned by 11 men.

The Monitor Mks IV and V had a beam of 17 ft 6 in (5.33 m) and draught of 3 ft 6 in (1.07 m), but at a length of 60 ft 6 in (18.44 m) the Monitor Mk V was 6 in (0.15 m) short than the Monitor Mk IV. The Monitor Mk IV was armed with one 81-mm (3.2-in) mortar or two M10-8 flamethrowers in a well between the conning position and the 40-mm cannon turret, one 40-mm cannon in a forward turret, one 20-mm cannon in a small turret on the rear of the conning position, two Mk 18 grenade launchers, three M79 grenade launchers, two 0.5-in (12.7-mm) Browning heavy machine guns in port and starboard positions just forward of the 20-mm cannon, and four 7.62-mm (0.3-in) M240 medium machine guns. As noted above, the Monitor Mk V was developed in two forms. That carrying a howitzer as its primary armament for the direct-fire engagement of communist bunker complexes on the banks of the waterway had one 105-mm (4.13-in) howitzer in the main turret, two 20-mm cannon in a bow turret, three M79 grenade launchers, two 0.5-in (12.7-mm) Browning machine guns as in the Monitor Mk IV and one 7.62-mm (0.3-in) M240 machine gun, while that equipped for flamethrowing had two 20-mm cannon, two flamethrowers each with a range of 220 yards (200 m), three M79 grenade launchers and two 0.5-in (12.7-mm) Browning machine guns.