Mark 24 mine
The concept of the acoustic torpedo existed by World War I, but was impractical due to the insufficient understanding of underwater acoustics. Germany was developing electrically-driven acoustic torpedoes by 1933. The resulting G7e and G7es anti-ship torpedoes entered service on submarines in 1943. These weapons ran at a preset depth and relied on early passive homing technology. The United States Navy received a similar anti-ship torpedo, the Mark 28, in 1944. ASW torpedoes must detect and respond to greater changes in depth and azimuth than anti-ship torpedoes. FIDO was designed within the additional constraint of being air-dropped.
USN studies for Fido began in late-1941. Requirements were issued to Bell Telephone Labs and Harvard Underwater Sound Lab (HUSL). Development began in December that same year. The project became the Office of Scientific Research and Development's Project 61, codenamed "FIDO". Bell and HUSL worked in parallel with complete information sharing. Western Electric developed a lightweight, shock resistant, 48 volt lead-acid battery capable of providing 110 amps for 15 minutes. General Electric designed and fabricated propulsion and steering motors. They also investigated an active acoustic homing system. David Taylor Model Basin assisted with hydrodynamics and propulsion. The guidance system used four hydrophones on the torpedo's midsection. These connected to a vacuum tube-based sound processing array. A Bell proportional navigation and HUSL non-proportional steering system were demonstrated by July 1942.
The body was a modification of the Mark 13 torpedo. Length, diameter, and weight were reduced. The nose section was hemispherical for the explosive charge. The conical tail section had four stabilizing fins, rudders, and a single propeller. Fido was relatively short and fat. Diameter measured 19 inches or 48 cm. Length reached 84 inches or 2.13 m. Weight totaled 680 lb or 308 kg. Warhead contained 92 lb or 41.7 kg HBX high explosive. Propulsion came from a 7.5 hp off-the-shelf General Electric Washing Machine electric motor driving a single propeller. This motor powered a 48 volt lead acid battery. Speed and endurance lasted for 10 minutes, giving a range of about 4,000 yards or 3,700 m. Homing system used 4 piezoelectric hydrophones operating at 24 kHz. Vacuum tube signal processing handled proportional steering. Maximum drop altitude ranged from 200 to 300 ft or 60 m to 90 m. Maximum aircraft launch speed hit 120 knots or 220 km/h.
The torpedo entered service with the Allies in March 1943. The United States Navy used it until 1948. Approximately 4,000 were produced. Each cost $1,800. Initial order of 10,000 FIDOs was later reduced due to excellent performance. Upon water entry, FIDO performed a circular search at a predetermined depth controlled by a bellows and pendulum system. Passive acoustic proportional homing took over once target's 24 kHz acoustic signal exceeded threshold. Initially torpedoes set to search for targets at depths of 50 feet or 15 m. This changed later to 150 feet or 45 m. Circular search resumed above depths of 40 feet or 12 m to prevent attacks on surface ships. Fido's relatively low speed remained secret. It could outrun submerged but not surfaced U-boats. Testing of pre-production prototypes continued into December 1942. USN received first production models in March 1943.
Two engagements in May 1943 have claim for first U-boat sunk by FIDO. On the 14th of May, a USN Catalina sank Kemp p117 or Neistle p78. On the 13th of May, RAF Coastal Command Liberator B/86 damaged submarine that sank following day. In all, FIDO sank 37 submarines for effectiveness about 18%. This compared with 9.5% for aircraft-launched depth charges. Number of attacks in which Mark 24s launched totaled 264. Total number of Mark 24 torpedoes launched against all targets reached 340. Number launched against submarines hit 204. Attacks on submarines by US aircraft numbered 142. Attacks by Allied primarily British aircraft counted 62. German U-boats sunk by FIDO reached 31. German U-boats damaged by FIDO totaled 15. Japanese submarines sunk by FIDO reached 6. Japanese submarines damaged by FIDO totaled 3. Total submarines sunk by FIDO combined German and Japanese reached 37. Total submarines damaged reached 18.
The United States Navy used the weapon until 1948. The Mark 27 torpedo Cutie developed for submarine use against surface vessels. It saw service in Pacific war from summer of 1944. Lieutenant Commander Carter L. Bennett's Sea Owl achieved Mark 27 first combat success. This event damaged a Japanese patrol vessel in Yellow Sea in November. Blair, Clay Jr. Silent Victory Bantam 1976 page 788 documents this achievement. References include Torpedo MK 24 Fido First American ASW Acoustic Homing Torpedo. S NAVY TORPEDOES by Frederick J Milford Part Four covers WW II development of homing torpedoes 1940-1946.
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Common questions
When did the Mark 24 mine enter service with the Allies?
The Mark 24 mine entered service with the Allies in March 1943. The United States Navy received first production models during this month after testing of pre-production prototypes continued into December 1942.
How many submarines did the Mark 24 mine sink during World War II?
The Mark 24 mine sank a total of 37 submarines combining German and Japanese targets. This figure includes 31 German U-boats and 6 Japanese submarines among the total count.
What were the physical dimensions and weight specifications for the Mark 24 mine?
The Mark 24 mine measured 84 inches or 2.13 meters in length with a diameter of 19 inches or 48 centimeters. It weighed 680 pounds or 308 kilograms while carrying a warhead containing 92 pounds or 41.7 kilograms of HBX high explosive.
Who developed the propulsion system for the Mark 24 mine?
General Electric designed and fabricated the propulsion and steering motors for the Mark 24 mine. A 7.5 hp off-the-shelf General Electric Washing Machine electric motor drove the single propeller using power from a 48 volt lead acid battery.
When was the first combat success achieved by the Mark 24 mine against a submarine?
Two engagements in May 1943 claim the first U-boat sunk by the Mark 24 mine. On the 14th of May 1943, a USN Catalina sank Kemp p117 or Neistle p78.