“Most of the Allied bombing raids aimed at submarine pens inflicted only superficial damage.”
By Robert Schreiner
ON THE night of April 9, 1945, two German submarines were berthed within the cavernous concrete enclosure of the Kriegsmarine’s Kilian submarine pen in the German port of Kiel.
One was the newly-launched U-4708, which was undergoing final outfitting in preparation for her first commission on coastal patrol duty. Moored beside her was the much larger U-170, a veteran ocean-going U-boat with one confirmed merchant kill to her name.
Work crews attendant to both submarines toiled into the night under flickering electric lighting, as air raid sirens blared and Allied bombs began to fall upon the port. A massive Royal Air Force bomber force of more than 300 aircraft was attacking the key German naval base. Among the Allied targets were multiple German surface vessels moored dockside, including the heavy cruisers Admiral Scheer and Admiral Hipper and the training cruiser Emden.
The laborers working on the U-boats may not have been particularly concerned by the bombardment. The partially submerged walls of the Kilian pen were 2.5 meters (8.2 feet) thick, and the pre-stressed concrete roof above them was a staggering 4.8 meters (15.7 feet) thick. The entrances to the pen were secured by pairs of thick steel blast doors. Although the Allies had repeatedly bombed German sub pens in Germany, France, and elsewhere, none of those raids had done significant damage either to the concrete structures themselves nor to any submarines within. On that night, however, one Allied bomb would change that perfect safety record.
In World War II, German U-boats were a prime target for Allied naval and air forces. From 1939 onward, Germany’s reliance on unrestricted submarine warfare and “wolfpack tactics” had been devastatingly effective. During the course of the conflict, U-boats sank more than 2,700 Allied ships, which was more than two-thirds of Allied shipping lost in all theaters of the war.
Allied countermeasures — convoy escorts, radar, advancements in depth charges and other ASW weaponry, and the decoding of Enigma codes to track U-boat deployment — all helped to mitigate the threat posed by the German submarines. What’s more, mounting and relentless attacks by Allied warships and aircraft had halved Hitler’s 1,200-vessel U-boat arm.
However, the Allied bombing campaign targeting the Kriegsmarine’s submarine pens was a series of frustrating failures. Even advanced bomb sights of that era, such as the American Norden and the British SABS, could not deliver precise results. Often, entire payloads of bombs would miss their intended targets by hundreds or thousands of yards. The submarine pens themselves, although massive, were difficult to pinpoint from the air. And even when they were struck, the bombs themselves were simply not powerful enough to penetrate the thick layers of reinforced concrete shielding the pens. Most of the Allied bombing raids aimed at submarine pens inflicted only superficial damage.
The two German submarines secured in the Kilian pen the night of the April 9 raid were of distinctly different types. The smaller submarine, U-4708, was a relatively new Type XXIII submarine, a revolutionary Elektroboot. These were designed to operate almost entirely underwater rather than more conventional submarines of the era, which mostly operated on the surface and would only submerge to attack or evade pursuit. Type XXIII boats were small enough that they were equipped with only two torpedoes. These short-range boats were designed to operate in coastal waters, and U-4708 was one of fewer than 70 such vessels laid down before the end of the war. That evening, she was undergoing final fitting out before her first deployment.
The other, larger boat in the Kilian pen was U-170, a conventional IXC/40 ocean-going attack submarine. U-170 already had one confirmed kill to her name and had recently undergone extensive repairs in preparation for her fifth patrol. With a submerged displacement of more than 1,200 tons, she was nearly five times the size of the smaller U-4708 beside her. As the thunderous Allied bombardment raged outside the pen, U-170’s captain ordered the hatches of his vessel to be closed and secured – a decision that would prove to be critical.
If the Kriegsmarine’s overbuilt submarine pens had an Achilles’ heel, it was their seaward entrances. Although shielded by heavy steel doors, those openings were the only vulnerable part of the structure. As the RAF bombers rumbled high overhead, releasing their deadly payloads, one bomb – by chance – fell precisely outside the doors to the Kilian pen. The resulting detonation caused the huge steel doors to implode with extraordinary force. Shards of jagged steel raked the interior, striking the submarines and killing multiple personnel working in the pen. The intense pressure of the blast caused a rolling wave of seawater that crashed heavily over both U-boat hulls. Unlike U-170, the smaller U-4708’s hatches were still open, as workers and crew toiled within the vessel. Rushing water cascaded down through the hatches, flooding interior compartments and almost instantly swamping the small submarine. It sank almost immediately, carrying multiple crewmen with it to the bottom of the pen. The larger U-170 survived the blast, and her crew was able to help save a few of the survivors who had been aboard the ill-fated U-4708.
The aerial assault on Kiel that evening, just one month before Germany’s unconditional surrender, was extremely successful. The naval base itself suffered extensive damage. RAF bombs capsized and sank the Admiral Scheer at her moorings, and both the Admiral Hipper and Emden were heavily damaged in the raid and were subsequently scuttled. Although unknown to the Allies at the time, the bombardment also resulted in the sinking of U-4708 within the Kilian pen – the only confirmed instance of Allied bombs sinking a German U-boat inside a submarine pen during the war.
ROBERT SCHREINER is the author of The Wolves and the Greyhounds: A Novel of the Great War. He is a former CIA Intelligence Officer, a consultant and executive in the global private security industry and an amateur military historian who has traveled the world, routinely sneaking in side-trips to visit ancient fortifications and battlefields. He lives in the Blue Ridge Mountains of Tennessee with his wife, two spoiled cocker spaniels, and an amusingly musical cockatiel.
