Although U-boats certainly did play a role in Rheinübung's planning, the German submarines only played a marginal role in the actual operation. The inactivity of the U-boat arm was due to a confluence of factors that meant whatever U-boats that were available were out of position to make any attack on British naval forces.

Part of the problem was the lack of any useful coordination between the different German military arms. Although the Bismarck did have both a Luftwaffe and U-boat liaison officer as part of Lütjens's staff interservice and interbranch coordination was never the Wehrmacht's strong suit in the war. This meant that while there was planning for air and undersea elements for Rheinübung, it was in accordance to a pre-envisioned operation in which the Bismarck and Prinz Eugen could break into the Atlantic. There was a line of U-boats detached from an attack on convoy HX-127 south of Greenland. As with the rest of Rheinübung, the main target for these boats was enemy shipping, Lütjens's orders made explicit note that heavy units should be engaged only when the primary mission was endangered.

The problem for the German plan was that the Battle of the Denmark Strait compounded what was already quite unfeasible plan. In a move the Bundeswehr's official history of the war "reveals both dilettantism in planning and a lack of foresight," none of the planning for Rheinübung anticipated the need to refuel Bismarck. The damage at the Denmark Straits and fuel burned by maneuvering constrained Lütjens's course of action when he decided to sail for France on 23/24 May. Lütjens initially anticipated a more circuitous route to St. Nazaire that would have been closer to U-boats south of Greenland and those in the Bay of Biscay. The Bismarck radioed a request for a concentration of U-boats along that route as well as air cover. Yet the fuel situation coupled with a growing pessimism in the Bismarck's bridge that they were hopelessly shadowed by the British forced Lütjens on the morning of 24 May to abandon this route for a more direct course to France.

Lütjens's vacillating and inconstant course made it nearly impossible for German forces to be in a position to aid the battleship. There were only two U-boats even close to Bismarck during her torpedoing by the Ark Royal, the U-74 and U-556. The latter submarine had actually sighted both Ark Royal and her task force launching its raid, but the boat had no torpedoes. The U-556 was actually able to rendezvous with U-74 and relay BdU's orders to assist Bismarck or take on her war diary, but it was too late for either. U-74 reached the combat zone only in its terminal stages and could not set up an attack on the British forces. Bad weather meant she could not position herself to intercept, let alone pursue the heavy units and only managed to find three German survivors that had not succumbed to the elements.

The travails of both U-556 and U-74 underscore two of the great difficulties in planning operations in the Atlantic: size and weather. The breadth of the Atlantic as a military theater makes it really difficult to move units together in a coordinated fashion. Rheinübung's somewhat slipshod approach to coordination only compounded matters. Lütjens's double changing of courses threw the even limited degree of cooperation with other units out of whack. But even had the Kriegsmarine had a much greater sense of cooperation, the speed limitations of the Type VII and Type IX U-boats meant the pickets of U-boats would not have been able to help the Bismarck that much. Submarines of this era had great difficulty hunting down and stalking enemy warships, which were much faster and adapted various tactics like zig-zagging that made a successful attack at sea a mixture of luck and skill. American experience in the Pacific showed submarines had the best success against warships when they were athwart the surface ships' line of advance, and even here, USN skippers had the advantages of clear weather conditions, radar, and intelligence derived from code-breaking. These were not advantages the Kriegsmarine possessed in 1941. The poor weather of the Atlantic in spring meant that visual sighting was difficult and it also socked in the few aircraft the Luftwaffe devoted to coastal operations.

The Bismarck was very much alone when the Prinz Eugen departed her company. Lütjens made critical errors, especially when he believed he was still being shadowed on the morning of the 25 May, but the larger failing of the Naval Staff in anticipating the battleship's fuel needs put him in an unenviable situation. There was little for the Kriegsmarine to do given the constellation of these events; the Naval Staff did not anticipate the Bismarck to return to France so early and its attempts to improvise after the fact meant it had to fight an uphill battle with the geography of the Atlantic while reacting to events. These challenges proved to be insurmountable and the net result was Rheinübung closed on a very one-sided note.

Sources

Baron Von Mullenheim-Rechberg, Burkard. Battleship Bismarck: A Survivor's Story. New York: Naval Institute Press, 2012.

Lichtfuss, Gregor. "U-74 and the Bismarck tragedy." http://uboat.net/history/u74-bismarck.htm .

Militärgeschichtliches Forschungsamt. Germany and the Second World War: Volume VI The Global War. Clarendon Press: Oxford: 2001.