Assault Support Patrol Boat, Mark II (Sikorsky), ASPB 50AB691

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Assault Support Patrol Boat, Mark II (Sikorsky), ASPB 50AB691

Coastal vessel, fast patrol boat with a turret featuring a large howitzer and autocannons.

The Assault Support Patrol Boat, or ASPB, was one of the most powerful boats of the American riverine force during the Vietnam War, being one of the fastest and heavily armed. However as most of the force meandered around at about 6-8 knots, the ASPBs were unable to take advantage of their speed. While various issues with the design were ironed out by the Bureau of Ships, research began in 1967 on newer river vessels to replace the existing vessels, which were meant to be faster, more manoeuvrable, and with a shallower draft.

Two companies submitted proposals for a second-generation ASPB, one from Sikorsky Aircraft (1 of 3 non-aviation products they made, the others being an aerosani and a train), and one from Teledyne-Sewart. Sikorsky’s design had a fibreglass hull to keep weight down, and was powered by 3 gas turbine engines with waterjets. This allowed the boat to have both a shallow draft and high speed. Armament was contained in a turret with a 105mm army howitzer and 2 20mm autocannons, which allowed the boat to support troops with indirect fire while also being out of the accurate range of PVN and Viet Cong handheld rocket launchers. The vessel was delivered to the Navy in 1969, with the hull number 50AB691 (from what I understand, this means it was the first 50 foot “assault boat” of fiscal year 1969). It and Teledyne-Sewart’s boat both passed testing, though Sikorsky’s boat encountered some technical issues. Ultimately, as the US started to withdraw from Vietnam, neither would enter production nor would they see combat service. The ship was retained by the Navy for river warfare training until 1980, when it was discarded and scrapped.


Main turret, stabilized (sorry to the guy I told otherwise on the old forums):

  • 1x1 105mm M102
  • 2x1 20mm Mk.16

Mk.48? turret, remote controlled:

  • 1x2 7.62mm M60C
  • 1x1 40mm Mk.19 AGL

Slat and mesh armour around superstructure and turret
Armoured superstructure and turret with spall protection
Hull filled with flotation foam

33.8t full

Length: 15.18m

Beam: 6m

Draft: 1m

Propulsion: 3 Pratt & Whitney Canada ST6J-71 gas turbines, 1695 hp, driving 3 water jets

Speed: 43.4 knots (80.5 km/h)

Range: 250nmi (at 15 knots)

Crew: Unknown, at least 5 like the ASPB Mk.1

Raytheon Pathfinder 1900 radar


Official model:

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. 355–363). Naval Institute Press.

NH&HC pics: (lol the caption for this is wrong)


Absolute monster of a boat. I have a soft spot for anything Sikorsky makes so it’s a big +1 from me!