Oberleutnant zur See (Lieutenant) Walter Köhler climbed the ladder from the control room of U-451 on December 22nd 1941. Climbing through the hatch atop the conning tower he was greeted by the calm night air of the western approach to the Strait of Gibraltar. The sound of the Type-VIIC U-Boat’s diesel engines chugged through the air as they charged the batteries used to power the submarine when submerged whilst under the cover of dark.
A World War II submariner’s life was often a singular one. There was very little news of the war outside the confines of the submarine’s hull and most crews didn’t know their country’s fortunes or failings until returning to port. One man in Nazi Germany’s U-Boat force who had an unparalleled view of the war however, at least compared to others in the service, was Konteradmiral (Rear Admiral) Karl Dönitz, the head of Germany’s submarines. The last quarter of 1941 had brought him cause for both celebration and concern. On November 13th one of his U-Boats, U-81, had torpedoed and sunk the British aircraft carrier HMS Ark Royal. As well as being a major combat loss for the British the vessel’s sinking was also a major hit on British morale.
This sapping of the British morale however would be shortlived because less than a month later the Japanese attacked the US Navy at Pearl Harbour and despite being advised against it by many of his top leaders, Adolf Hitler decided to honour the almost entirely “paper” alliance with Japan and declare war on America also. With German forces heavily committed to the war in the Soviet Union it was seen by nearly everyone as an almost suicidal move. For Dönitz this meant his U-Boats were now subject to attack by American warships anywhere in the world but he was still confident in his men and their submarines.
In truth he had every right to be. His U-Boats were still an elusive enemy and were inflicting savage losses on the Allied effort to keep Britain in the fight. The convoy system had done much to improve the odds in the favour of the Allies but they were still taking losses especially when the U-Boats operated at night. Contrary to popular belief, World War II submarines spent around 80-90% of their time on the surface to make sure their batteries were fully charged for the attack or for when they in turn were attacked. This was always safest at night and it also gave the German lookouts more of a chance of finding a target. It was always easier for a U-Boat lookout to spot a ship’s large silhouette on the horizon than vice-versa because like an iceberg most of a surfaced U-Boat remains below the waves. A crew on a destroyer or an aircraft might be lucky enough to spot the wake of a surfaced U-Boat in the moonlight but the chances were very slim. The night had been the refuge of the U-Boat since the war began.
With that in mind, Walter Köhler spent most of his watch scanning the horizon looking for a target confident that U-451 was as good as invisible and they would have plenty of time to react should they come under attack from a roaming Allied warship. An aircraft flying overhead might spot the wake of the submarine but Köhler and his lookouts would hear any multi-engined aircraft before it had time to attack and thus allow them time to dive away. Even radar hadn’t helped the RAF and Royal Navy hunt U-Boats at night…yet. The problem was that while ASV radar could vector an aircraft to within a mile of the U-Boat the pilots often couldn’t see the target especially if the U-Boat had dived. The radar sets simply weren’t sensitive enough to give a truly precise location of a U-Boat for an effective attack.
U-451 was operating in a rich hunting ground with the Strait of Gibraltar acting like a bottleneck for Allied ships in to and out of the Mediterranean. Gibraltar itself had a very heavy British military presence including a large number of aircraft. As Köhler’s watch continued a strange sound appeared to come from the diesel engines. Maybe it was a cylinder misfiring?
One of the lookouts screamed at the top of his voice; aircraft! The British Swordfish torpedo-bomber, an almost antiquated design, with it’s single engine being drowned out by the U-Boat’s own diesel engines had sneaked up on the surfaced submarine. Its ASV radar had got them into the area and its crew had plotted the direction their target was travelling in and worked out where it would be when they arrived in it’s immediate vicinity. It was a mix of radar information coupled with a highly educated guess. The Captain of U-451 ordered the submarine to dive quickly and Köhler along with the three other lookouts rushed for the hatch. The buzz of the Swordfish’s engines was now a roar and he glanced upwards, his night-oriented eyes spying the Swordfish for the first time as it’s depth charges began to drop from under it’s wings.
The weapons detonated ahead of the submerging U-Boat mortally wounding the vessel. The explosions sent pillars of water in to the conning tower forcing the captain, unaware that his vessel was already doomed, to take the drastic action of closing the hatch before Köhler could get inside to prevent further flooding. The unfortunate German officer found himself alone outside of a sinking submarine in a torrent of swirling water and so with little choice left to him he leapt in to the sea and started swimming away so as to not get caught in the swell of the sinking U-Boat. Within a few minutes U-451 was gone although he was unsure if it had dived or had been destroyed. Indeed he would not know for certain the fate of his shipmates until some time later. It would be nearly an hour and a half before a British ship found the exhausted Köhler and took him prisoner. Under interrogation he displayed hostility towards U-451’s captain unaware that the captain’s order to close the hatch effectively saved Köhler’s life.
Reports of the attack filtered back to London and Berlin. While it has to be taken in to account that the Swordfish crew’s skill and a certain degree of luck had a part to play in the sinking, the possibilities ASV radar offered could not be ignored. Interestingly, the radar used by the Swordfish crew had not been designed for this role but in the following year new radar sets built specifically for hunting submarines at night meant that Dönitz’s U-Boats had lost the cover of the dark. This would eventually force the Germans to introduce primitive radar warning receivers to give the U-Boat crews advance notice but it was a short term solution and this resulted in the development of the snorkel to allow the U-Boat to charge it’s engines while remaining at periscope depth; a slow and frustrating process.
By the end of the war a U-Boat on the surface could expect attack in either day or night from overwhelming Allied air power that had both a numerical and qualitative superiority. One final note; the Swordfish that sank U-451 was attached to No.812 NAS operating out of Gibraltar. The only reason the squadron was flying from there was because their carrier, Ark Royal, had been sunk by a U-Boat the previous month. The Swordfish crew had taken their revenge.