 The engine were two-stroke diesels made by the French constructor Schneider, which proved unreliable and noisy to the point where the engine section became difficult to man at full power.From 1966 to 1970, the "Narval"s underwent extensive modernisation, where their engines were replaced by a diesel-electrical design based on the SEMT-Pielstick 12PA4-185.The two stern torpedo tubes were deleted, electronics were replaced, and the conning tower was replaced by a more modern sail plan from the .