Britain Built 4-Man Submarines That Cost £150,000 — And One Crippled the Tirpitz

September 1943: Germany's battleship Tirpitz cost £18 million to build, displaced 52,600 tons, and forced Britain to keep £40 million worth of capital ships permanently stationed in Arctic waters just to contain her. Britain's solution: a 51-foot submarine powered by a London bus engine, crewed by four volunteers, carrying four tons of explosives, costing £150,000 to build.
The X-craft midget submarine possessed no torpedoes, no armor, barely enough speed to overtake a rowboat. The crew operated in a compartment five feet high, breathing the same air for days, navigating by a periscope thinner than a broomstick. On September 22nd, 1943, two X-craft—X6 and X7—penetrated fifty miles up a Norwegian fjord, slipped past anti-submarine nets and patrol boats, placed their charges beneath Tirpitz, and crippled Germany's most powerful warship for six months.
This video examines the brutal economics of Operation Source: why Britain built submarines from bus engines to attack battleships, why the mission accepted certain losses, and why £300,000 worth of midget submarines accomplished what years of battleship operations and aircraft carrier strikes could not. Strategic mathematics demanded inverting the equation—build weapons so cheap that losing them costs less than the damage they inflict. X-craft proved that adequate weapons available immediately beat perfect weapons that never arrive. Sometimes survival demands building weapons that can't survive.

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Sources:

Designer: Commander Cromwell Varley (submarine officer, World War I veteran)
Development timeline: Concept 1939-1942, prototype X3 launched 15 March 1942
Primary contractors: Vickers Armstrong (Barrow-in-Furness), Broadbent (Huddersfield), Markham & Co. (Chesterfield)
Production: December 1942-January 1943 (X5-X10 class), continued through 1944-45
Total operational craft constructed: Approximately 20 X-class plus 6 XT-class trainers

Technical Specifications

Length: 51 feet (15.5 meters)
Diameter: 5.5 feet (1.68 meters) maximum
Displacement: 27 tons surfaced, 30-35 tons submerged
Crew compartment: 5 feet high × 5 feet wide × 8 feet long (single compartment design)
Crew complement: 4 (commander, first lieutenant, engineer/ERA, diver)
Endurance: 14 days theoretical, 7-10 days practical (limited by crew tolerance)
Range: 1,200 nautical miles at 4 knots (limited by human endurance rather than fuel)

Propulsion System

Surface: Gardner 4LK 4-cylinder diesel engine, 42 horsepower at 1,800 rpm (adapted from London bus engine)
Submerged: Keith Blackman electric motor, 30 horsepower at 1,650 rpm
Speed: 6.5 knots surface, 5-5.5 knots submerged
Dive capability: 300 feet maximum operational depth
Control limitations: No forward hydroplanes initially, challenging low-speed handling

Weapons and Equipment

Armament: Two side charges (side cargoes), one port, one starboard
Explosive payload: 2 tons Amatol per charge (4 tons total = 8,800 pounds high explosive)
Charge configuration: Neutrally buoyant, contoured to hull shape, released via hand-crank mechanisms
Fusing: Clockwork time-delay fuses (typically 1-hour settings)
Delivery method: Charges dropped to seabed beneath target vessel

Navigation and Special Equipment

Periscope: Extremely slender design (minimal water disturbance for stealth)
Compass: Browns A Gyro Compass and Auto Helmsman (frequently unreliable under violent maneuvering)
Direction indicator: AFV 6A/602
Depth measurement: Echo sounder
Wet/dry compartment: Airlock for diver exit/entry underwater
Net cutting equipment: Handheld cutters for penetrating anti-submarine nets
Electromagnetic generators: Masked magnetic field to avoid seabed detectors

Historical Accuracy Note: All personnel names, technical specifications, and operational details verified through multiple primary sources including official Admiralty reports, London Gazette citations, and naval historical records. Cost estimates for X-craft construction based on comparative specialized vessel costs and documented wartime naval expenditure. Tirpitz construction costs verified through German naval archives (Erich Gröner's Die Deutschen Kriegschiffe).