Gymnote (S655) was an experimental submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 of the French Navy

French Navy

The French Navy, officially the Marine nationale and often called La Royale is the maritime arm of the French military. It includes a full range of fighting vessels, from patrol boats to a nuclear powered aircraft carrier and 10 nuclear-powered submarines, four of which are capable of launching...

. She was a trials submarine for Submarine-launched ballistic missile

Submarine-launched ballistic missile

A submarine-launched ballistic missile is a ballistic missile capable of delivering a nuclear warhead that can be launched from submarines. Modern variants usually deliver multiple independently targetable reentry vehicles each of which carries a warhead and allows a single launched missile to...

s and powered by diesel electric engines. She is named in honour of the Gymnote

Gymnote

The Gymnote was one of the world's first all-electric submarines.Launched on 24 September 1888, she was developed in France following early experiments by Dupuy de Lôme, and, after his death, by Gustave Zédé and Arthur Krebs, who completed the project...

, the world's first all-electric submarine built in France in the late 19th century.

The French planned a nuclear propelled submarine in the late 1950s and laid down a hull (no Q244). Because France had not developed uranium enrichment facilities at the time, the planned power plant was to be a heavy water reactor

Heavy water reactor

A pressurised heavy water reactor is a nuclear power reactor, commonly using unenriched natural uranium as its fuel, that uses heavy water as its coolant and moderator. The heavy water coolant is kept under pressure in order to raise its boiling point, allowing it to be heated to higher...

, which could utilize natural uranium. But French engineers were unable to produce a reactor small enough to fit into the submarine, which led to the project being canceled in 1959.

In the early 1960s the French government decided to develop an independent nuclear deterrent based on SLBM's. Hull Q244 was redesigned as a trials submarine with 4 missile tubes in an extended casing. The Gymnote was commissioned in 1966 and fired the first M-1 Missile in 1968. The M-1 Missile was subsequently deployed aboard the Redoutable class submarines. She was extensively rebuilt in 1977-79 to enable trials of the new M-4 Missile

M4 SLBM

The M4 was a French Submarine Launched Ballistic Missile deployed on the nuclear Le Redoutable class submarines They entered service on the 1 May 1985...

. The Gymnote was decommissioned in 1986.