Was a US Navy Anti-Submarine Warfare Operator: The Soviet Alfa-Class Was the Fastest and Deepest-Diving Submarine Ever Built -- 40-Plus Knots and a 900-Meter Design Depth. X-Rays Found Cracks in the Titanium, So They Capped It at 400. The Reactor Was So Loud They Could Not Use the Speed Either.

A former US Navy Aviation Anti-Submarine Warfare Operator describes the Soviet Project 705 “Alfa-class” as the fastest and deepest-diving submarine ever built, citing briefing details from the early 1980s. The vessel, under 2,500 tons on the surface, was described as exceeding 40 knots submerged and having a design depth of 700 to 900 meters. Its titanium-alloy hull reportedly could not be welded in open air, leading the Soviets to use a sealed, argon-filled welding hall. The writer also notes that deep dives produced micro-cracks in the hull and that the lead-bismuth reactor’s noise could make speed a liability by revealing location. The piece then broadens to Cold War submarine detection and attack concepts.