Project 1231

Project 1231 was a Soviet hybrid surface combatant and submarine developed in the 1960s, known as "Dolphin" (Дельфин) or "diving missile boat" (ныряющий катер-ракетоносец). It represented a new type of ship capable of high surface speeds while also diving and moving underwater. The concept originated with Soviet Premier Nikita Khrushchev, who envisioned a fleet that could submerge for secrecy during nuclear war after seeing submarines and missile boats at a naval base in Balaklava. Initially, he sought a vessel that could fly, swim, and dive, but designers convinced him to focus on a practical ship instead.

Project 1231 built on lessons from Project 662, an earlier submersible ship, particularly using hydrofoils to achieve greater surface speed. However, work on the project ceased after Khrushchev's removal in 1964, and no prototype was built. According to E. A. Aframeev, the design had no realistic chance of practical implementation.

Earlier attempts at similar diving boats included Valerian Brzezinski's M-400 Flea, developed in 1939. This torpedo boat had a surfaced speed of 33 knots but slowed significantly when submerged. Its development was halted during World War II due to damage from artillery fire. Project 662 in the late 1950s also explored hydrofoils for speed, achieving up to 50 knots on the surface, but its underwater performance was inadequate for combat.