To understand depth charges, you need to know that active sonar (the Brits called it ASDIC, for some obscure reason) did not exist in World War I. The British and American navies had been working on it for years, but it was not ready by Armistice Day. The ASW escorts charged the datum and began putting depth charges into the water. As submarines could not go very fast on batteries, if a destroyer got to the last known position fast enough, it was possible to catch the sub.
When active sonar was installed on ships between the world wars, the sets were at what now would be regarded as very high frequencies. That permitted small lightweight sonar domes, but also meant limited range. Those sonars were probably not good submarine detection sets; they had to know about where the submarine was.
There are four basic stages in ASW: Search, localize, track and attack. World War I destroyers were pretty deficient in the first and third stages, as they had no sensors, other than the Mk 1 eyeball, to detect submarines and no way to track them after they submerged. The early sonar sets were not much better on the search aspect, but once the crew knew where to look and got close enough, they could localize and track.
Depth charges had huge deficiencies. First off, they were horribly unsafe. They were often stored in racks on the weather decks, so each ship had thousands of pounds of high explosives just sitting on deck. They were vulnerable to shipboard fires, enemy fire and for getting loose in heavy seas.
Second, they were dangerous in use. If a depth charge that was rolled off the fantail detonated early, it could blow the stern off the ship. Even if it didn't do that, the shock could blow out the stern tube seals around the propeller shafts and sink the ship by flooding the engine rooms.
Third, blowing up a number of depth charges created a lot of regions of disturbed water, which obscured the sonar signals.
Fourth, sonars could not look straight down or to the rear. When the destroyer which was tracking the submarine commenced its depth charge run, it lost track of the submarine just before it got into position to launch the depth charges.
Imagine, if you will, that you are at a trap or skeet shooting range. You call "pull" and a clay bird is launched. You swing your shotgun towards the clay bird and just as the barrels start to swing into alignment, you close both eyes, try to maintain your swing and then fire.
Now imagine that the clay bird has the ability to alter its flight path and that the clay bird knows that just before you fire, you are going to be blind. The sub commanders knew that a depth charge run was underway and, as the destroyer came into position, the sub commanders would order a sharp turn. The destroyer commanders had to guess which way the sub might go and try to compensate. The sub might alter its speed, anything to mess up the depth charge run.
Something that was not known early in World War II was that the German boats could dive deeper than American or British submarines, in some cases, three times as deep. The Germans could go below the maximum setting of the early depth charges. Even when depth charges would detonate that far down, it took a bit of time for them to sink that far, which meant that actually hitting a deep sub was more a matter of sheer-assed luck.
Something had to be done.
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