Sunderland 250lb Depth Charges
The RAF and Coastal Command faced significant challenges in their early attempts to combat German U-boats. They initially relied on 500-pound anti-submarine aerial bombs that had been developed during World War I. These bombs, however, proved more hazardous to the attackers than to the U-boats. The bombs had to be dropped at low altitudes and speeds, which increased the risk of the explosion impacting the aircraft itself, especially if the bomb was not accurately dropped.
Early Development of Aerial Depth Charges
In 1940, in an attempt to address these issues, the British developed an improvised aerial depth charge based on the Mark VII naval depth charge, which weighed 450 pounds and was drum-shaped. This aerial version was modified with a conical fairing and stabilizing fins to improve its performance when dropped from an aircraft. The depth charge used a hydrostatic pistol, which was designed to detonate the charge at a specific water pressure (indicating a certain depth) and prevent premature explosions if the charge bounced off the water surface.
Despite these improvements, these aerial depth charges proved largely ineffective. From September 1939 to June 1941, Coastal Command calculated that only about 1% of U-boats attacked with these depth charges were actually sunk. This led to frustration among aircrews, who often saw U-boats survive even when bracketed by explosions.
The Role of Operational Research
In early 1941, the British established the Operational Research Section within Coastal Command, led by Dr. Patrick Blackett and Dr. E.J. Williams. This team conducted in-depth analyses of U-boat attacks, using records and photographs to identify how to increase the effectiveness of depth charges. By the summer of 1941, their research indicated that the best chance of damaging a U-boat was to strike it while it was still on the surface or just starting to dive.
To achieve this, the depth charge needed to be modified in two ways:
More Powerful Explosive: The first step was to increase the explosive power of the depth charge. The initial charges were filled with Amatol, but by April 1942, they were replaced by Torpex, a more potent mixture of RDX, TNT, and aluminum. Torpex was 30 to 50% more powerful than Amatol, making it the preferred explosive for depth charges.
Improved Detonation Timing: The second modification involved the depth at which the charge would detonate. Initially, depth charges were set to detonate at 100 to 150 feet, based on the assumed depth of a U-boat during a crash dive. However, this was often too deep, as the U-boat would typically have moved ahead of the explosion. The solution was to set the detonation depth much shallower – around 25 feet – to ensure the charge would explode closer to the U-boat.
Challenges and Further Modifications
The existing hydrostatic pistols, designed for shipboard use, could not safely detonate the charge at depths shallower than 50 feet. To address this, the Mark VIII detonator was introduced in the spring of 1942, allowing for a minimum detonation depth of 34 feet. However, this was still not ideal for U-boats that were surfaced or just beginning to dive, as the lethal range of the Torpex-filled depth charge was only 19 feet. Additionally, a coating of bubbles often formed on the depth charge when it hit the water, delaying the pressure needed to activate the detonation.
To further improve the effectiveness, by July 1942, the Mark XI depth charge was adapted with the Mark XIII Star pistol, which allowed for detonation at a depth of 15 to 25 feet. This version also featured a break-away tail and a concave nose spoiler to improve the depth charge’s stability and effectiveness upon impact, significantly increasing the chances of damaging or sinking U-boats.