Assault Support Patrol Boat, Mark I, ASPB A 92-1 (50AB6710) - "Alpha" of the Mekong

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Assault Support Patrol Boat, Mark I, ASPB A 92-1 (50AB6710)

Coastal vessel, armoured patrol boat with turrets featuring light automatic weapons.

In 1965, the United States entered the Vietnam War, and with it came a large riverine fleet, Task Force 117 or the Mobile Riverine Force, to take control of the densely populated Vietnamese rivers. Most of its craft were variants of existing vehicles like the Landing Craft, Medium, but there was one new design, the Assault Support Patrol Boat. It was meant to be a successor to the French/South Vietnamese STCAN patrol boats, a shallow draft armoured boat that could be used in a variety of roles. The Bureau of Ships began design work in 1966 with the main parameters being: higher speed than existing river ships, armour to protect crew and machinery, low draft and high manoeuvrability, and enough fire power to support troops. By June of that year, the Bureau had completed its design, but was unable to stay within the original size limits. The final design featured a strong hull, with spaced armour able to withstand 57mm recoilless rifles and the hull small mines, while also having the space to carry 8 troops. It was armed with 2 turrets, which could mount a variety of automatic weaponry and a grenade launcher, and a mortar in the stern.

The first boats were ordered in October of 1966, constructed by the Gunderson Brothers Engineering Corporation, with the first boat completed in May 1967 and arriving in Vietnam in September. A total of 87 boats were produced in 2 series (Program 4 and Program 5). The Program 4 ASPBs suffered from a low freeboard due to a mistake during lofting, with a few boats sinking simply from the wake of passing boats, and had to have foam filled sponsons attached to them, which incidentally also offered some protection. Program 5 ASPBs corrected this mistake. ASPBs were nicknamed “Alpha Boats” and “River/Delta Destroyers”, thanks to their higher speed compared to the LCMs, though that often wasn’t put into full use as it had to escort the much slower monitors and transports. Many were modified by their crews, some had extra grenade launchers attached to the superstructure, others had rocket launchers fitted to the forward turret, and many had extra machine guns mounted.

A 92-1 (standing for the first ASPB of the 92nd River Assault Division), hull number 50AB6710, was the 10th? ASPB built. It’s not very special, seeing the same general service as all ASPBs, though somewhat notably it was hit multiple times with RPGs directly in its turrets during an action on 4 April 1968, part of a series of events which led to the removing of machinery armour to reduce top weight. However, a weirdly large amount of photos feature it so I’ve suggested it as a representation of a “standard” early ASPB, without special weapons like rocket launchers or extra machine guns. Interestingly the turrets were interchanged at various points, as pictures show a 20mm or twin 7.62mm in the forward turret. Probably like most other ASPBs it was given to South Vietnam after the US withdrawal from Vietnam.


Mk.48 Mod 0 turret:

  • 1x1 20mm Oerlikon Mk.16 (500 ready rounds, 4350 total)
  • 1x1 40mm Mk.19 AGL (50 rounds)

Mk.48 Mod 2 turret:

  • 1x2 12.7mm M2 (4260 total rounds)
  • 1x1 40mm Mk.19 AGL (50 rounds)

1x1 81mm Mk.2 mortar (40-60 shells)

6.35mm aluminium plate + 16mm hardened steel plate, spaced armour for superstructure and machinery
5.6mm steel hull, tapering to 3.2mm on top end
Hull filled with flotation foam

34.3t full

Length: 15.3m

Beam: 4.6m

Draft: 1.3m

Propulsion: 2 General Motors 12V-71N diesel engines, 860 hp, driving 2 shafts

Speed: 16 knots (29.6 km/h)

Range: 130nmi (at 10 knots)

Crew: 5

Raytheon Pathfinder 1900N radar



With additional AGLs, note the hole for a coax AGL is covered.
Internal diagram of earlier ASPB design, hatched parts are armour

Branfill-Cook, R. (2020). Riverine Craft of the Vietnam Wars (pp. 61-62). Seaforth.
Friedman, N. (1987). Vietnam: The Riverine Force. In U.S. Small Combatants: Including PT-Boats, Subchasers, and the Brown-Water Navy: An Illustrated Design History (pp. 349–357). Naval Institute Press.
Rottman, G. (2010). Vietnam Riverine Craft 1962-75 (pp. 46-49). Osprey.