The Battle of the Atlantic, fought between the Allied navies and the U-boats of the German Kriegsmarine, was the longest single battle of the Second World War, lasting from the very first day of the conflict to the very last. And early on, it seemed as though the Germans might actually win. By early 1941 the U-boats had reduced Britain’s yearly imports from a pre-war total of 68 million tons to a mere 26 million tons. Admiral Karl Dönitz, head of the U-boat arm, calculated that if he could inflict sustained losses of 700,000 tons per month, he could starve Britain into submission.
But for all their menace, the U-boats had a key weakness. Like all submarines of the era, they were powered by a combination of diesel engines on the surface and electric motors underwater. However, submerged speed and maneuverability were poor and battery capacity limited. Consequently, U-boats spent most of their time on the surface, using their superior speed and the cover of darkness to infiltrate Allied convoys at night and only submerging to escape enemy attack.
But such tactics soon became untenable as the introduction of radar and increased Allied air cover made the surface a dangerous place for a U-boat to be caught. One stopgap solution, introduced in 1943, was the Schnorkel, a retractable air intake and exhaust duct that allowed U-boats to run their diesel engines and recharge their batteries while running at periscope depth. But soon even this became inadequate, as advances in radar technology allowed antisubmarine aircraft to detect even objects as small as a periscope protruding above the water. It soon became clear that an entirely new type of U-boat was needed, one which could spend the majority of its time underwater like a true submarine.
On this front, the Germans pursued two main lines of research. The first, the Elektroboot concept, used a combination of improved hydrodynamics and increased battery capacity to produce submarines which were actually faster submerged than surfaced and which could remain submerged for days on end. Two such designs reached production before the war’s end. The larger of the two, the Type XXI, was 76 metres long and had a displacement of 1,621 tons. It featured a radically streamlined hull and conning tower free of the deck guns, guard rails, and other drag-producing protuberances common to previous designs, and a battery capacity nearly three times that of the Type VII, the workhorse of the U-boat fleet. This allowed the type XXI to reach underwater speeds of up to 17 knots or remain submerged for up to 75 hours while cruising at 5 knots. The type also sported other advanced features, including quieter electric motors for silent running, an automatic torpedo-loading system that allowed it to launch up to 18 torpedoes in 20 minutes, and improved amenities for the 57-man crew including a freezer for storing perishable food.
The type XXI’s smaller sibling was the Type XXIII, designed for operations in the shallow coastal waters of the Mediterranean, Black, and North Seas. Measuring 35m long and displacing 234 tons, the Type XXIII could achieve 12.5 knots submerged or remain submerged for 48 hours cruising at 4 knots. As the boat was designed to be transported cross-country by rail, its size was dictated by the dimensions of standard German railway tunnels. This in turn limited the Type XXIII’s armament to two torpedoes with no reloads, the tubes having to be reloaded from the outside while the boat was docked.
118 Type XXI and 63 type XXIII U-boats were completed before the war’s end, though they entered service too late to have any major impact. Only two Type XXIs and six Type XXIIIs were deployed on war patrols, with the Type XXIIIs managing to sink 14,601 tons of Allied shipping. After the war, however, captured Elektroboote were operated by the navies of several countries, and the types strongly influenced the design of post-war submarines like Soviet Whiskey-Class and American Tang-class. Today, only one Elektroboot survives: the type XXI Wilhelm Bauer – formerly U-2540 – preserved as a museum ship in Bremerhaven, Germany.
The second and more radical line of submarine development was the Type XVII Walterboot. This submarine was powered by a unique air-independent propulsion system designed by Dr. Hellmuth Walter, perhaps most famous for designing the HWK 109-509 engine used in the Messerschmitt Me-163 Komet rocket-powered interceptor – and for more on this, please checkout our previous video The German Rocket Fighter That Dissolved its Pilots Alive. Like these engines, Walter’s submarine propulsion system was based on the decomposition of High-Test Hydrogen Peroxide, or HTP, known in German parlance as T-Stoff. When brought into contact with a suitable catalyst, hydrogen peroxide decomposes violently into high-temperature oxygen and steam. In Walter’s submarine propulsion system, the peroxide was decomposed using a sodium permanganate catalyst, with the oxygen-rich exhaust being mixed with fuel oil to increase its energy before being passed through a turbine connected to the propeller shaft. This system produced a tremendous amount of power while not requiring external oxygen to operate, theoretically allowing a submarine to reach unheard-of underwater speeds in excess of 20 knots.
In order to prove the concept, Walter obtained funding from the Kriegsmarine to build the V-80, a small 22-metre, 80-ton submarine with a 4-man crew. The V80 was innovative not only for its propulsion system but also its hull shape, which was optimized for underwater performance rather than surface cruising. This “teardrop hull” would become the de facto standard for submarines starting in the 1960s. Completed in 1940, the V80 would carry out more than 100 trial dives in the Schlei inlet and off Hela in the Baltic, reaching a record-breaking underwater speed of 28 knots. However, this blistering performance came at a cost. The Walter propulsion system was exceptionally thirsty, consuming fuel at a rate 30 times greater than a diesel engine and giving the V80 a range of only 80 kilometres. It was also expensive to operate, with a single 6.5-hour trial run costing the modern equivalent of $200,000 in High-Test Peroxide alone.
The trials of V-80 were witnessed by Admirals Erich Raeder and Werner Fuchs, who, while impressed, were slow to approve further development. Finally, in 1942 Walter convinced Admiral Dönitz to approve the construction of a larger combat-capable Walterboot, the type XVII. Like the Type XXIII elektroboot, the 36-metre long, 300-ton Type XVII was a small coastal boat with a teardrop-shaped hull largely free of external projections. It was powered by two 4,900 horsepower Walter turbines with an auxiliary 210 horsepower diesel engine and a 76 horsepower electric motor for silent maneuvering. The use of aircraft-like controls reduced the number of crew required, while armament was two bow torpedo tubes with one reload each.
Four prototype Type XVIIAs were ordered, the first two, U-792 and U-793, being completed by Hamburg shipbuilder and aircraft manufacturer Blohm und Voss in October 1943. During trials in March 1944, U-793 attained an underwater speed of 22 knots with Admiral Dönitz aboard, while in June U-792 it reached 25 knots. However, the Type XVVIAs were plagued by numerous problems. The already thirsty Walter turbines were rendered even less efficient at depth by back pressure on the exhausts, while the boat’s lack of dive planes made it difficult to control at high speeds. Even worse, the length-to-beam ratio of the hull was found to be too small, resulting in excessively high drag. But perhaps the biggest flaw lay the heart of the system itself: hydrogen peroxide. Not only was HTP highly corrosive, dissolving human flesh on contact, but even the smallest speck of organic matter could cause it to decompose violently. This meant that even small leaks could – and did – cause severe injuries, fires, and explosions.
By the time the trials of the type A’s ended, the war was going badly for Germany, and Admiral Fuchs argued that the continued development of a new U-boat type would divert scarce resources from more proven types. But he was overruled by Dönitz, who approved the construction of 24 Type XVIIBs, which in bid to improve fuel efficiency were fitted with only a single 2,500 hp Walter turbine. This reduced the top speed to 21 knots but increased range to 113 nautical miles. A K Type submarine was also projected using a closed-cycle Diesel engines fed from cylinders of pure oxygen.
With the wartime situation rapidly deteriorating and Blohm und Voss struggling to complete orders of the Type XXI Elektroboot, the Kriegsmarine reduced the order from 24 to six boats. Of these, three were completed – U-1405, U-1406, and U-1407 – and three were still under construction when the war ended. None of the completed Type XVIIs saw action and all three – along with the prototype V-80 – were scuttled by their crews. At the Potsdam Conference in July 1945, where the major Allied leaders met to discuss the fate of postwar Europe, various German experimental submarines were allocated to the United States and Great Britain for evaluation, with the U.S Navy raising U-1406 and the Royal Navy raising U-1407 and capturing the partially-completed U-1408 and U-1410. While U-1406 was transported to the U.S. naval base at Portsmouth, New Hampshire, the Navy saw the Peroxide technology as too dangerous and expensive and never attempted to restore the submarine to working condition. The Walter turbine was removed and evaluated for several years before it and the hull were eventually scrapped.
The Royal Navy, by contrast, was considerably more enthusiastic about Peroxide propulsion, and set up an Admiralty Development Establishment at Barrow-in-Furness to further develop the technology. The research team included Dr. Hellmuth Walter himself and members of his original research group. Under Walter’s direction, the captured U-1407 was rebuilt as the research vessel HMS Meteorite, which was commissioned into the Royal Navy on September 25, 1945. Throughout 1946 Meteorite carried out a number of experimental trials, allowing the Admiralty to evaluate the performance and practicality of the Walter system. However, as during her original 1944 trials, Meteorite’s crew found her volatile, temperamental, and difficult to control, drolly describing her as being “75% safe.” Nonetheless the Admiralty was sufficiently impressed to approve the construction of two larger Peroxide-powered research vessels: HMS Explorer, commissioned in 1956; and HMS Excalibur, commissioned in 1958. Having served her purpose, HMS Meteorite was scrapped in 1949.
While ostensibly more advanced and streamlined than the old Type XVII U-boats, with retractable fittings like mooring bollards and sonar domes for higher underwater speed, HMS Explorer and Excalibur – known jokingly as the “Blonde Boats” since hydrogen peroxide is also used to bleach hair – proved no less troublesome to operate. The turbine compartment – left unmanned while underway – would often spew bright white flames, while on several occasions the boats suddenly filled with noxious fumes, forcing the crew to evacuate onto the deck. Even worse, the peroxide, stored in special plastic bags mounted outside the pressure hull, had an unnerving habit of spontanously exploding. Consequently, crews nicknamed the boats“HMS Exploder” and “HMS Excruciator.” Though the boats were capable of impressive underwater speeds of 25 knots, by the late 1950s it became clear that Peroxide-based propulsion was a dead-end technology, and the Blonde Boats served mainly as high-speed targets for more advanced submarine designs. The 1954 launch of U.S.S. Nautilus, the world’s first nuclear-powered submarine, ultimately sealed their fate. HMS Explorer was scrapped in 1965 and HMS Excalibur in 1968.
While Peroxide propulsion was short-lived in manned submarines, it found a much more successful application in torpedoes, a technology also pioneered by the Germans during WWII. In 1952, the UK introduced the Mk.12 “Fancy”, a 21-inch Peroxide-fuelled torpedo based on the wartime German Steinbut prototype. Though capable of speeds of up to 52 km/h, development was halted when, on June 16, 1955, a peroxide explosion led to the sinking of the S-Class submarine HMS Sidon and the death of 13 of her crew. Other navies have had greater success with HTP torpedoes, including Sweden and Russia. However even these are not without their problems, with the accidental explosion of an HTP-fuelled Type 65-76A torpedo being responsible for the loss of the Russian Oscar II-class submarine Kursk and all 118 of her crew on August 12, 2000. With most countries currently phasing out their HTP torpedoes, these will hopefully be the last victims of the century-long naval flirtation with the volatile power of hydrogen peroxide.
Expand for References
Zimmer, Phil, The Wonder of the Walter Boat, Warfare History Network, August 2021, https://warfarehistorynetwork.com/article/the-wonder-of-the-walter-boat/
1955-1970: Explorer Class, RN Subs, http://rnsubs.co.uk/boats/subs/explorer-class.html
The Walter U-Boats, U-boat.net, https://ift.tt/ExMAnZL
What Sank the Kursk Nuclear Submarine? Royal Society of Chemistry, https://edu.rsc.org/download?ac=508333
Stokes, P.R., Hydrogen Peroxide for Power and Propulsion, The Science Museum, London, January 14 1998, https://hydrogen-peroxide.us/history-Germany/hydrogen-peroxide-for-propulsion-and-power_PR_Stokes-1998.pdf
McDaniel, J.T., The Walter Submarine Propulsion System, https://www.jtmcdaniel.com/walter_turbine.html
Air Independent Propulsion, https://ocw.mit.edu/courses/2-611-marine-power-and-propulsion-fall-2006/bda3fdf7ba19c5f5e80436a9ccd07400_26in_usw_art_aip.pdf
Smith, Steven, Phantom of the Deep: Germany’s Underwater Wonder Weapon, History Net, July 11, 2017, https://ift.tt/yzubT17
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