The hedgehog consisted of 24 spigot mortar bombs each35lb, angled to form a circular pattern about 50yds diameter. The spigot mounting resembled a hedgehog ! There was no depth setting, it relied on one impact detonation to countermine the rest. It was a stopgap weapon which could be retro-fitted to existing frigates and corvettes. The real ATW (ahead throwing weapon) was the Squid, which was a six barrelled mortar of 11 inch bore, firing projectiles shaped like aerial bombs but with a flattened nose, some 300 yards ahead. They had a fast sinking time and could be set (by asdic 147b) at the moment of firing to explode at any depth to 1000ft. Three exploded 50ft above the sub and three 50ft below the target, which was thus sandwiched by the simultaneous explosion of about one ton. Each projectile weighed 390lbs. The Squid could not be retrofitted to existing frigates because of the complicated magazine and handling arrangements. The Loch class frigates were designed around the weapon. The Castle class corvettes carried a 3 barreled version.
Ron C