In spite of submarine warfare causing the British and French great distress in World War I, it wasn’t until 1915 that anyone came up with an effective means of attacking a submerged U-boat, the depth charge.
You’ve seen enough movies to have a basic grasp of what a depth charge is. A cylindrical container full of explosives rolled off the back of an escort ship that detonates when it reaches a preset depth, as determined by a hydrostatic firing device (know in the business as a “firing pistol” for some reason).
But simply rolling a few depth charges off the stern of a ship over the likely position of a submarine is very unlikely to yield any real effects on the target. Most depth charges weigh between 300 and 600 pounds. Roughly 1/2 to 2/3 that weight is explosive. And to be effective, a depth charge has to detonate within about 30 to 40 feet of the submarine. Given the extremely poor state of sensors in those days, coming that close would be more a matter of chance than tactics. Indeed, between 1915 and 1917, only 9 U-boats were sunk by depth charge.1 The linear pattern of depth charges meant a simple turn by the U-Boat could easily remove it from danger. The solution for the escort was to widen area covered by a single attack. Perhaps two ships could make parallel depth charge attacks? But there was seldom enough ships to allow this, nor were two ships likely close enough to be able to quickly coordinate an attack. Instead, the Y-Gun depth charge projector was invented.
The Y-Gun was basically a mortar with a single charge firing into two tubes arranged in a Y-shape. In each of the tubes was a piston that ended in a broad curved “lear” (leading to the pistons being know as arbors) that nestled a depth charge. Mounted on the centerline of a destroyer, when fired, a Y-Gun would send a depth charge about 40-50 yards to both port and starboard of the ship. Even such a modest increase in the square area of a depth charge pattern greatly increased the likelihood of a successful attack.
By the end of World War I, most destroyer types had at least one and and usually two Y-Guns aboard.
By the beginning of World War II, active sonar had improved to the point that, while not terribly effective as an area search weapon, it provided decent bearing and range information for an attacking escort. But ASW planners failed to understand the importance of determining the depth of a target sub. Some estimation could be made. The shape of the sonar beam and the way it angled through the water could provide a very rough trigonometric estimation of depth. The other serious improvement in technology was the rather simple idea of splitting a Y-Gun in half. The K-Gun fired one charge to one side. The advantage of this was that K-Guns could be mounted along the sides of an escort without displacing other weapons from centerline space. Even relatively small escorts could carry four, six, even as many as ten K-Guns. Combined with two chutes of depth charges, a pattern of charges could be laid on the suspected position of the target sub.
The uncertainty of the depth of the target meant that in addition to charges being delivered along the path of the attacking escort, and to the sides via the K-Guns, the attack had to be delivered at varying depths as well. Eventually the standard attack would evolve to be a “10 charge” attack. Essentially, two overlaying diamond shape patterns (with a fifth charge in the center) at two depths, above and below the suspected depth of the sub, to sandwich the target, or catch it as it attempted to turn away.
This double diamond attack was by far the most effective depth charge of the war. It had a whopping 5% success rate of sinking or seriously damaging its target.
One of the most serious shortcomings of the depth charge as an ASW weapon was that the attacking ship would lose contact with the target, depending on its depth, at a range of from 200 yards clear out to as much as 500 yards. Counting the time needed for the ship to travel that distance, and the further delay for the charges to sink, the target sub had significant time to maneuver to escape. And the explosion of the depth charges roiled the water, meaning reacquiring the target was problematic at best.
Later, we’ll look to weapons and sensors that addressed these shortcomings.