65ft Patrol Boat Mark III “Sea Spectre” (40mm Bofors Mod.7)

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65ft Patrol Boat Mark 3 “Sea Spectre” with 40mm Bofors

Coastal vessel, asymmetrical patrol boat, this one has a 40mm gun.

After the end of the Vietnam War, naval requirements for small craft changed drastically, and a new generation of patrol boats were developed by the Norfolk Division of the Naval Ship Engineering Center. The 65ft Patrol Boat Mark III was a successor to the Vietnam-era Swift boats and the unsuccessful CPIC patrol boat prototype. It was an asymmetrical vessel, with the bridge on the starboard side, while the port side had a superfiring platform that allowed for the installation of an autocannon, recoilless rifle, mortar, or twin torpedo tube. Additionally the hull was designed to have a small radar cross-section, though installing weaponry would render that feature useless. The ships had 3 engines, which made the interior space quite cramped, but allowed it greater survivability during engagements. The first Mark IIIs were ordered during Fiscal Year 1973, with 8 built at the Peterson Builders in Wisconsin, and were operated by the Navy SEALs. Additionally 18 more were ordered in FY 75 and 77. None of them had names, being known only by their hull number (65PBYYX, ie 65PB751). The ships in service were rarely fitted with the heavy weaponry it was designed for, making many design choices meant to accommodate them rather pointless. Though a few new small craft weapons were tested on the Mark IIIs: a 40mm Bofors in a new Mod.7 mounting, which was widely adopted, the Norwegian Penguin anti ship missile, which was rejected for being too heavy, and a navalized 25mm Bushmaster, which in 1985 was adopted as a new standard small craft gun. All the Mark IIIs would be discarded by 2000.

The 40mm Bofors Mod.7 mounting uses the Mark 3 gun but featured a power drive and an autoloading drum, which allowed a single crew member to fully operate the gun. These guns were commonly installed on Mark IIIs, though it was meant to be an interim weapon until Hughes’ 25mm Bushmaster or GE’s 25mm EX-35 were ready for service (funnily enough neither of these would be commonly mounted on any Mark IIIs). Also a note, I’ve seen this mounting be referred to as the Mod.9 too.


1x1 40mm Bofors L/60 Mk.3 Mod.7 mount (48 round magazines)
2x1 20mm Mk.16
2-4x1 12.7mm M2

41.3 tons full

Length: 19.8m

Beam: 5.5m

Draft: 1.8m

Propulsion: 3 GM 8V-71TI diesel engines, 1800 hp, driving 3 shafts

Speed: 26 knots (48.2 km/h)

Range: 450nmi (at 26 knots)

Crew: 5

LN66 navigation radar


This one showing off the name, 65PB - 65 foot patrol boat, 758 - fiscal year 1975, 8th boat
A preserved Mark III

Friedman, N. (1987). Post-Vietnam Small Combatants. In U.S. Small Combatants: Including PT-Boats, Subchasers, and the Brown-Water Navy: An Illustrated Design History (pp. 390–411). Naval Institute Press.
Gardiner, R., Chumbley, S., & Budzbon, P. (1995). USA. In Conway’s All the World’s Fighting Ships 1947-1995 (pp. 626). Naval Institute Press.