- Mark 37 torpedo
The Mark 37 torpedo is a torpedo with electrical propulsion, developed for the US Navy after World War II. It entered service with the US Navy in the early 1950s, with over 3,300 produced. It was phased out of service with the US Navy during the 1970s, and the stockpiles were sold to foreign navies.
Its engineering development began in 1946 by Westinghouse-ORL. It was based on the active homing system tested on modified Mark 18s, with added passive homing and a new torpedo body. Between 1955-56, thirty torpedoes were produced for development testing, with large-scale production commenced shortly afterwards. 
Due to its electric propulsion, the torpedo swam smoothly out of the launch tube, instead of having to be ejected by pressurized air, therefore significantly reducing its acoustic launch signature.
The guidance of a Mk37 mod 0 torpedo was done by a gyroscope control during the initial part of its trajectory, where the gyro control achieved a straight run, a passive sonar homing system, and at the last 700 yards (640 m) by a Doppler-enabled active sonar homing, with magnetostrictive transducers operating at 60 kHz. The electronics was based on miniature vacuum tubes, later on solid-state semiconductor devices.
The efficiency of Mk37 torpedoes was high for targets with speed lower than 20 knots (37 km/h) and depth less than 1,000 ft (300 m). As submarines with higher speeds and operating depths appeared, new torpedoes were developed. Of them, NT37C, D, E, and F are based on the Mk37 design.
In 1967, the mod 0s started being refurbished as mod 3, and mod 1 as mod 2. These modifications involved many changes including replacement of magneto-constrictive transducers with piezoelectric ones, and resulted in target acquisition range increased from 700 yd (640 m) to 1,000 yd (910 m) without loss of sensitivity with increasing depth.
The torpedoes used Mark 46 silver-zinc batteries. These had a known tendency to overheat, occasionally causing fires or explosions. Training torpedoes used reusable rechargeable secondary batteries.
For a long time, the Mark 37 was a primary U.S. submarine-launched ASW torpedo. It was replaced by the Mark 48 starting in 1972. The remaining inventory was then rebuilt and sold to several countries, including Israel, as the NT-37C after the vacuum tube guidance systems were replaced by solid-state electronics and the electric propulsion was replaced with a liquid monopropellant.
Note: the Mk 67 Submarine Launched Mobile Mine  is based on a Mark 37 torpedo body. It entered service in 1983 and is capable of swimming as far as 10 miles[clarification needed] through or into a channel, harbor, shallow water area and other zones which would normally be inaccessible to the vessel laying it. After reaching the target area it sinks to the sea bed and acts like a conventionally laid "influence mine". The exploder in the Mk 67 warhead is computerised and incorporates magnetic, acoustic and pressure sensors.
- Power plant: Mark 46 silver-zinc battery, two-speed electric motor
- Length: 135 inches (340 cm) (mod.0), 161 inches (410 cm) (mod.1)
- Weight: 1,430 pounds (650 kg) (mod.0), 1,660 pounds (750 kg) (mod.1)
- Diameter: 19 inches (48 cm)
- Range: 23,000 yards (21 km) at 17 knots, 10,000 yards (9.1 km) at 26 knots
- Depth: 1,000 feet (300 m)
- Speed: 17 knots (31 km/h), 26 knots (48 km/h)
- Guidance system: active/passive sonar homing; passive until about 700 yards (640 m) from target, then active; mod.1 with wire-guidance
- Warhead: 330 pounds (150 kg) HBX-3 high explosive with contact exploder
- Date Deployed: 1957 through 1987
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