1945-1991: Cold War world Wiki
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21 inch Mark 20 Bidder torpedo.
Category. Data.
Type. Conventional Cold War Torpedo.
Nationality. British.
Made in. 1955.
Retired. 1980s.
Designer. N/A.
Manufacturer. N\A.
Variants. Non.
Diameter. 21 inches.
Speed. 20 knots.
Warhead. N\A, probably TNT or torpex.
Operational range. 12,000 yards (11,000 m).
Guidance system. Autonomous passive sonar.
Launch platform. Submarines, ASW frigates and escort frigates.
Weight. N\A, but probably akin to those of comparable role, configuration and era.
Sources. https://www.wikiwand.com/en/British_21_inch_torpedo.

Developed under the codename "Bidder", the Mark 20 was a passive-seeker battery-powered torpedo for use by surface ships (the Mark 20E - for "Escort") and submarines (Mark 20S). The E variant was not long in service due to problems with its programming. This led to several of frigates intended to use them (Rothesay and Whitby classes) never being fitted with torpedo tubes or having them removed.

It was replaced in the submarine service in the 1980s by Tigerfish.