Coastal vessel, hydrofoil fitted with the Sheridan’s gun, otherwise similar to Tucumcari
In 1965, the US Navy sent specifications for a hydrofoil patrol boat to 7 companies. Only 2 responded, Grumman and Boeing, who were already designing hydrofoils. Rather than choose a design, the Navy decided to order both in 1966 and test them together. The Grumman design became USS Flagstaff, and the Boeing design USS Tucumcari. The two vessels differed mainly in their foil configuration. Flagstaff had a conventional design where the front foils supported the majority of the weight, and Tucumcari had the opposite.
USS Flagstaff was completed in 1968, it and Tucumcari were deployed to Vietnam in 1969 and participated in the coastal patrols of Operation Market Time, rotating between each other every 5 days. The next year, both hydrofoils were recalled because of high maintenance costs and used for other duties. Tucumcari went on a trip to Europe as a demonstrator and Flagstaff used as a test ship. It spent a year being refitted with a 152mm Gun/Launcher. A modified Sheridan turret with a bubble canopy to save weight was installed and in 1971 firing trials were held. Tests were successful, with 38 shots fired in total, 20 of them while foil-borne, showcasing that small hydrofoils could mount heavy weaponry. In 1974, Flagstaff was transferred disarmed to the Coast Guard as they were interested in high-speed craft, but due to the high maintenance cost, was eventually returned to the navy. The ship was then sold off in 1978.
1x1 152mm M81E1 gun/launcher
2x2 12.7mm M2
~72.5 tons full
Draft: 1.3m (foils up) or 4.1m (foils down)
Propulsion: 1 Rolls-Royce Tyne Mk.621 gas turbine + 2 GM C6V-53N diesel engines, 3620 + 300 hp, driving 1 shaft (foils down) + 2 waterjets (foils up)
Speed: 8 (foils up) or 51 knots (foils down) (14.8 and 94.5 km/h)
Range: 400-1500 nmi
Flagstaff with its foils up in 1974, note the turret ring for the Sheridan turret
[The hydrofoil USS Flagstaff (PGH-1) tests the 152mm gun and turret from an M551 Sheridan light tank. - GIF on Imgur] Video of it during 152mm firing trials
Friedman, N. (1987). Motor Gunboats and PTFs. In U.S. Small Combatants: Including PT-Boats, Subchasers, and the Brown-Water Navy: An Illustrated Design History (pp. 257-263). Naval Institute Press.
Moore, J. ed. (1974). United States. In Jane’s fighting ships 1973-74 (p. 468). Janes.
Sayers, K. (2021). Patrol Gunboat-Hydrofoils. In U.S. Navy Patrol Vessels (pp. 235-240). McFarland.
Someone else made a Flagstaff suggestion on the old forums, but their account is long gone.