Mark 24 FIDO Torpedo
The Mark 24 Mine was a US
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 air-dropped passive acoustic homing anti-submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...

 used during the Second World War against German
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 and Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

ese 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 and Canadian
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...



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
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 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
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 were to develop a lightweight, shock resistant, 48 volt Lead-acid battery
Lead-acid battery
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...

 capable of providing 110 amps for 15 minutes. General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

 were to design and fabricate propulsion and steering motors and to investigate an active acoustic homing system. David Taylor Model Basin
David Taylor Model Basin
The David Taylor Model Basin is one of the largest ship model basins — test facilities for the development of ship design — in the world...

 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
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

-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.

Combat history

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

General characteristics

  • Diameter: 19 inches (46 cm).
  • Length: 84 inches (2.13 m).
  • Weight: 680 lb (308 kg).
  • Warhead: 92 lb (41.7 kg) Torpex
    Torpex is a secondary explosive 50% more powerful than TNT by mass. Torpex is composed of 42% RDX, 40% TNT and 18% powdered aluminium. It was used in the Second World War from late 1942. The name is short for Torpedo Explosive', having been originally developed for use in torpedoes...

     high explosive.
  • Propulsion: 5 hp (3.7 kW) electric motor driving a single propeller, powered by a 48 volt lead acid battery.
  • Speed and endurance: 12 knots for 10 minutes, giving a range of about 4,000 yards (3,700 m)
  • Homing system: 4 piezoelectric hydrophones operating at 24 kHz and vacuum tube signal processing system with proportional steering.
  • Maximum drop altitude: 200 to 300 ft (60 m to 90 m)
  • Maximum aircraft launch speed: 120 knots (220 km/h).


  • Mark 27 torpedo
    Mark 27 torpedo
    The Mark 27 torpedo was the first of the United States Navy 19-inch submarine-launched torpedoes. This electrically propelled torpedo was 125 inches long and weighed 1174 pounds . The torpedo employed a passive acoustic guidance system and was intended for both submarine and surface targets...

    (Cutie) was developed for submarine use against surface vessels. It saw service in the Pacific war from the summer of 1944. Lieutenant Commander Carter L. Bennett's Sea Owl
    USS Sea Owl (SS-405)
    USS Sea Owl , a Balao-class submarine, was a ship of the United States Navy named for the sea owl, a lumpfish of the North Atlantic....

    achieved the Mark 27's first combat success, damaging a Japanese patrol vessel in the Yellow Sea in November.


  • Blair, Clay, Jr. Hitler's U-Boat War: The Hunters, 1939-1942. ISBN 0-679-64032-0
  • Blair, Clay, Jr. Silent Victory. Bantam, 1976.
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