Plongeur was the first practical mechanically powered submarine, launched by France in 1863.
Designed by Siméon Bourgeois and Charles Brun, Plongeur used a compressed-air engine rather than human muscle or a conventional surface engine. Its name means “Diver” in French. The submarine was launched at Rochefort on 16 April 1863 and entered French service in 1864.
Plongeur displaced about 420 tonnes and was powered by a two-cylinder reciprocating engine. Compressed air stored in tanks drove the machinery, giving it a limited submerged endurance. A small auxiliary boat supplied air and carried equipment when the submarine operated near the surface.
Plongeur did not become a successful fleet weapon, but it marked a major transition from experimental hand-powered craft to mechanically propelled submarines. Earlier vessels such as Turtle and Nautilus had demonstrated underwater navigation, but they lacked Plongeur’s scale and mechanical propulsion.