As I discussed in Part I and Part II, depth charges had several drawbacks. Chief among the drawbacks was the requirement that an escort prosecuting a submarine contact had to lose contact prior to launching her depth charges.
The Royal Navy had the lead on this problem, once again. They developed a "spigot mortar" that would launch a pattern of mortar bombs, each of which had a warhead of approximately 35lbs of high explosive. This became known as the Hedgehog. The mortar bombs were loaded onto rods; the bombs each had a cylindrical well along its center axis so that the bombs slid down and rested on the rods.
The early Hedgehog mounts were roll-compensated, but they could not be trained more than a few degrees to either side (moved from left to right) by tilting the mount, so the ship herself had to be aimed at the submarine. Later mounts were fully trainable. The mortar bombs themselves were launched in pairs at very short intervals, back to front, so that the bombs that fired at higher arcs were launched first (they had a longer flight time) and thus all of the mortar bombs would hit the water nearly simultaneously. The launchers' rods were set so that the bombs hit in a pattern, usually oval or circular, at a distance of 200 yards. The bombs would sink fairly rapidly.
Unlike depth charges, Hedgehog mortar bombs were contact weapons. Unless the submarine could hear the sound of the bombs being launched, she would not know that an attack run was underway, as escorts would also make non-firing runs to refine their targeting solutions. The time it took to reload the Hedgehog mount was usually less than it took to reposition for another firing run.
Hedgehog did have some of the same drawbacks as depth charges, in that the ships had quantities of high explosives on the weather decks. The launcher crews had to work topside, sometimes in far less than ideal conditions. At least one warship was lost when the stored Hedgehog bombs blew up because of faulty fuzing. They were also dumb weapons with a fairly long time between firing and impact, often close to half a minute or more, depending on the depth of the submarine.
Hedgehog nonetheless was a very lethal weapon when employed by a skilled crew. The USS England (DE-635) sank six Japanese submarines in 1944 in a period of 12 days.
Hedgehog was effective, but it still required that the escort close to the submarine's position in order to attack it. World War II sonars operated on sound frequencies around 14-30 KHz; many were "searchlight" sonars, such as the QGB sonar that transmitted a beam on one bearing; the sonar head was steered by the operator. The high frequency meant that the sonar head was small, it could be mounted on smallish ships, but because it was a searchlight system, searching for submarines was a matter of luck. The high frequency also limited the range.
Sonars developed into true search sonars that could transmit an omnidirectional beam. The QHB sonar transmitted an omnidirectional beam, but because it operated between 20 and 26 KHz, its range was limited. The early 1950s-vintage AN/SQS-23 sonar operated at 5 KHz, which greatly increased the detection range, but which also required a much larger sonar dome. The increased detection range of sonar sets like the SQS-23 meant that the escorts could track submarines far outside of the range of Hedgehog.
Clearly a longer range weapon was needed.