The Mark 24 Mine (also known as FIDO or Fido) was a US air-dropped passive acoustic homing anti-submarine torpedo used during the Second World War against German and Japanese submarines. It entered service in March 1943 and continued in service with the US Navy until 1948. Approximately 4,000 torpedoes were produced, sinking 37 and damaging a further 18 submarines out of a total of 204 fired. The torpedo was also supplied to the British and Canadian forces.
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The US Navy began studies into an air dropped anti-submarine torpedo in the autumn of 1941. Based on a formal set of requirements, Harvard Underwater Sound Lab (HUSL) and Bell Telephone Labs began development in December 1941. These later projects later became Office of Scientific Research and Development project 61 (FIDO).
Both Bell Labs and HUSL proceeded with parallel development of torpedoes, with a complete exchange of information between them. Western Electric were to develop a lightweight, shock resistant, 48 volt Lead-acid battery capable of providing 110 amps for 15 minutes. General Electric were to design and fabricate propulsion and steering motors and to investigate an active acoustic homing system. David Taylor Model Basin was to assist with hydrodynamics and propulsion.
The guidance system consisted of four hydrophones placed around the mid-section of the torpedo connected to a vacuum tube-based sound processing array. A Bell Labs proportional and HUSL non-proportional steering system had been demonstrated by July 1942.
An existing Mark 13 torpedo provided the body of the torpedo, it was modified by shortening the hull, reducing the diameter, reducing the weight, and designing a hemispherical nose section to carry the explosive charge, and a conical tail section with four stabilizing fins and rudders and a single propeller. The effect of these modifications was to produce a relatively short "fat" torpedo.
In June 1942 the US Navy decided to take the torpedo into production even though there was still major testing work remaining on the project, including air-drop testing. The Bell Labs version of the guidance system was selected for production with proportional homing. Testing of the pre-production prototypes continued on into December 1942 and the US Navy received the first production models in March 1943.
Initially 10,000 torpedoes were ordered, but FIDO proved so effective the order was reduced to 4,000. The torpedoes ended up costing $1,800 each.
Upon water entry, FIDO performed a circular search at a predetermined depth controlled by a bellows and pendulum system. This continued until the potential target's 24 kHz acoustic signal detected by the hydrophones exceeded a predetermined threshold level, at which point control was then shifted to the passive acoustic proportional homing system. Initially the torpedoes were set to search for a target at a depth of 50 feet (15 m), this was later changed to 150 feet (45 m). To prevent the torpedo accidentally attacking surface ships, it resumed its circling search if it rose above a depth of 40 feet (12 m).
The torpedo's relatively low speed was kept secret since, while U-boats could not outrun the torpedo underwater, they could outrun it on the surface.
The first sinking with FIDO occurred in May 1943 and was possibly U-640 or U-657. During its career, the torpedo sank a total of 37 submarines, achieving an effectiveness of about 22%, compared with about 9% for depth charges.
from US Navy OEG Study No. 289, 12 August 1946 provides the following data related to Mark 24 effectiveness:
| Number of attacks in which Mark 24s were launched | 264 |
| Total number of Mark 24 torpedoes launched - all targets | 340 |
| Number of Mark 24s launched against submarines | 204 |
| Number of Mark 24 attacks on submarines by US aircraft | 142 |
| Number of submarines sunk by FIDO | 31 |
| Number of submarines damaged by FIDO | 15 |
| Number of Mark 24 attacks on subs by Allies (primarily British) | 62 |
| Number of submarines sunk by FIDO | 6 |
| Number of submarines damaged by FIDO | 3 |
| Total number of submarines sunk by FIDO (German & Japanese) | 37 |
| Total number of submarines damaged | 18 |
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