Short answers, pulled from the story.
The Hedgehog weapon is a 1940s shipboard multi-barrel anti-submarine mortar of British origin. It was developed in 1941 by Lieutenant Colonel Stewart Blacker and adapted for naval use by Major Millis Jefferis.
The Hedgehog weapon uses contact fuzes that detonate only upon hitting a hard surface, unlike depth charges which rely on hydrostatic shockwaves. This design prevents the blind period where submarines escape undetected after an explosion because unsuccessful attacks do not disrupt sonar tracking.
The Royal Navy began operational use of the Hedgehog weapon in 1943 with the introduction of the Q attachment. Initial success rates hovered around five percent before training improvements increased effectiveness to one kill per every five attacks by the end of the war.
Lieutenant Colonel Stewart Blacker designed the original spigot mortar that formed the basis of the Hedgehog weapon. The Directorate of Miscellaneous Weapons Development adapted his infantry trench mortar design for naval application under the direction of Major Millis Jefferis.
The United States Navy kept the Hedgehog weapon in service until replaced by ASROC during the Cold War. A Soviet copy called the MBU-200 was created in 1949 and later evolved into the MBU-600, while Western navies eventually phased out these weapons in favor of homing torpedoes.