, November 1967 The ASPB was approximately long, its hull was constructed of steel, from the
gunwale up it was steel, its superstructure was aluminum which provided protection against
57mm recoilless rifle rounds and armor-piercing bullets up to
.50 caliber in size. The first 36 boats were ordered from
Gunderson Brothers Engineering Corporation of Portland, Oregon on 25 October 1966. The initial boats were armed with two Mk 48 turrets with either
20 mm cannon or
.50 cal machine guns, two single
M60 machine guns or
Mk 21 machine guns and one Mk 2 Mod 0/1 .50 cal machine gun/
81mm mortar in a well in the aft of the boat, but this well was eliminated in later models. The boat had a complement of five crewmen and could carry up to eight soldiers. The boat was powered by two 430 hp
General Motors 12V71 diesel engines giving a top speed of 14.8 knots and it expelled its exhaust directly into the water reducing noise and smoke emissions, making it the quietest river patrol craft. The boat carried some 650 gallons of diesel fuel, allowing an operational range of at 10 knots. The ASPBs first saw action on 28 September 1967 during
Operation Coronado V when they were used as minesweepers for an assault on An Dinh village, the "birthplace" of the
Viet Cong. The Stewart Mark II had three gun turrets and a mortar well in the extreme bow and bar armor around midships superstructure. ASPB Mark II prototype, c. 1969
Sikorsky Aircraft also built a Mark II prototype mounting a 105mm gun and two 20mm cannons in a central turret. The prototype was delivered to the Navy at the end of 1969. It was powered by three
Pratt & Whitney PT-6 engines connected to three
water-jet pumps giving a maximum speed of . The prototype was never adopted into service but remained in use with special forces until 1980. ==Operators==