J.T. McDaniel Official Website

The Walter Submarine Turbine

Basic Submarine Tactics in World War II

German submarine tactics in the Second World War were strongly influenced by experiences in the first. Captain (later Grand Admiral) Dönitz, the officer in charge of submarines during the entire war, had been a U-Boat commander during World War I. In the first war, submarines generally operated alone, mostly attacking targets of opportunity. Dönitz was determined to change this when war started again, and instituted a system of radio control from his headquarters intended to vector several U-boats onto convoys. Once engaged, the boats attacked independently, since coordination was much more difficult during the actual battle. The favored option was the night surface attack.

The use of surface tactics recognized a major limitation of submarines at that time. World War II submarines were really surface vessels. The hulls were designed with surface speed in mind. They would dive only to escape enemy warships, or in order to make an attack. Since the top submerged speed of a U-Boat was around 6~8 knots, and could not be sustained for more than about an hour under optimum conditions, it wasn't practical to pursue a surface target in a submerged boat. This limited speed also meant that it was harder for a U-Boat to maneuver to escape an attacker, and absolutely precluded trying to run away from one.

Allied advances in anti-submarine warfare, particularly the use of airplanes flying from escort carriers, and airborne radar, soon made surface operations extremely hazardous. Yet submarines, at least at first, had no real option but to spend a good deal of their time on the surface, since massive quantities of air were required for the operation of their diesel engines, which were used both for surface propulsion, and to charge the huge batteries that were used to operate the electric motors required for submerged propulsion.

Air Independent Propulsion for Submarines

The obvious solution to this problem was a propulsion system that didn't require outside air. The most prevalent modern air independent propulsion (AIP) system for submarines is nuclear power. This didn't become practical until the 1950s, however. The physics were known in time for World War II, but there were too many factors, from the need for highly-refined nuclear materials, to the complex mechanics of building a reactor that could both produce sufficient power, and also be compact enough to fit in a submarine, to allow such a sub to be built.

An interim solution was the adoption of the Schnorchel. This was a hollow mast, with a float valve at the top to exclude water should the head submerge, which allowed air to be drawn into the boat while it was submerged. The boat could then operate its diesels, while only the Schnorchel head remained exposed above water.

Still, the Schnorchel was, at best, only an emergency solution. (Albeit, one that is still in use for diesel/electric submarines.) The submarine had to remain close to the surface, and the Schnorchel head itself created a wake, and presented a target for enemy radar. Anti-radar coatings were tried, with some success, and radar detectors were also incorporated into the heads. While this reduced a submarine's vulnerability, something better was needed.

In the 1930s, Professor Hellmuth Walter began experimenting with hydrogen peroxide as a possible power source. By the early 1940s, Walter's research had progressed to the point where he was able to convince the Kriegsmarine to built some prototype submarines. By 1943, a Walter hydrogen peroxide turbine had been used to power an unarmed test U-Boat to a submerged speed of 26 knots. This was some 18 knots faster than the fastest conventional submarine of the period, and actually about 5 knots faster than the most common Allied escort vessels. (As 21 knots was, in any case, about the upper speed limit at which Asdic could be used, and few merchant vessels were faster than about 15 knots, it wasn't thought necessary to build faster escorts.)

Designs were drawn up for several types of Walter submarine. In the end, none ever became operational, though the design of the larger Typ XVIII Walter submarine provided the basis for the Typ XXI "elektroboot," which was almost ready for deployment at the end of the war.

Hull Design Integral

In addition to the actual Walter turbine, Professor Walter also designed the boats themselves. He recognized that conventional submarine hull designs, which were reasonably efficient while surfaced, were quite the opposite under water. In his designs he removed deck guns and other projections, which created drag. The hulls were streamlined, becoming more rounded. The conning towers were replaced by designs with a small cockpit, and smoothly-plated tops to reduce drag and, at the same time, the amount of noise the boat generated moving through the water. The result were designs that were actually faster when submerged than they were on the surface. They also tended to be larger than conventional designs, and slower diving as a result, but this disadvantage was compensated for by a design that made it unnecessary to operate on the surface except when entering and leaving harbour.

While no Walter boats actually entered combat service, the Typ XXI, which was the hull of the large Walter boat, with the huge Perhydrol tanks replaced by extra batteries, and conventionally powered, did enter service right at the end of the war. The one example that was in position to make an attack did not do so, since the cease fire order had been received, but her captain did make a mock attack on a British cruiser. The boat was able to do so with such a degree of stealth that the British initially refused to believe Captain Schnee's report until the logs of the cruiser and the U-boat were compared and the positions confirmed.

What Walter envisioned was a transition from a diving boat to a true submarine. His hull designs were sufficiently advanced that they became the basis of nearly all post-war submarine designs. The first nuclear boat, the U.S.S. Nautilus, was essentially an enlarged Walter hull with a nuclear powerplant. (Though a close look at the hull suggests that it may have actually owed more to the Japanese I-201 design, which was actually slightly faster than the German boats.)

The Walter Turbine

Walter achieved his remarkable results by using Perhydrol, a nearly pure hydrogen-peroxide solution, as an oxydizer. This was run through a catalysing system, which broke down the hydrogen peroxide (H₂O₂) into hydrogen and oxygen, in the process producing high pressure steam and oxygen at a very high temperature. The creation of the steam used up both of the hydrogen atoms and one of the oxygen atoms, leaving a free oxygen atom in the mixture. Since the temperature of the gases was hot enough to sustain combustion, diesel fuel was injected, which used the free oxygen atom. This increased both the heat and pressure of the steam. The steam was then used to power a turbine, which combined elements of both gas and Parsons (steam) turbine technology.

Unfortunately for the Kriegsmarine's submarine service—but fortunately for the Allies—the Walter system had nearly as many problems as benefits. The Perhydrol fuel was extremely corrosive, requiring the use of special fuel lines. Another problem, which was actually discovered by the Japanese, who used essentially the same system to power some torpedoes (including a prototype of the Kaiten "human torpedo" variant) was that, unlike conventional fuels, the Perhydrol required fuel lines without any right angle turns. The Perhydrol would sometimes "pile up" in the bends of such lines and spontaneously combust, with the obvious disastrous results.

Another drawback was that the Walter system was extremely thirsty. One reason for the larger hulls was simply that an enormous amount of fuel would have to be carried if the boats were to have sufficient range to be effective in combat.

The Typ XXVI Walter Submarine

The Typ XXVIw Walter boat depicted in With Honour in Battle was never actually built, though contracts were let and production of some sections begun. Thus, performance figures are only estimates, and probably a bit optimistic. For dramatic purposes, some liberties were also taken with the makeup of the crew. (I needed more officers, so that they could interact with each other; the real design called for only three, while I used seven, a number more in line with American crewing standards than German.)

There are also a few other variations between the submarine in the book and the actual design. These were mostly the result of errors in the references that were available at the time the manuscript was originally written, in the late 1970s, which apparently combined elements of two different Walter designs. For instance, available information at that time suggested that the 850-ton Walter boat possessed twin shafts and ten forward-firing torpedo tubes. In fact, the design called for a single shaft, and six of the tubes fired toward the rear of the boat, though with their breeches located in the same forward torpedo space as the four bow tubes. Also, unlike most other submarines of the period, the tower contained only an escape trunk, with the attack center located in the control room, or Zentrale. None of these technical differences affect the story, of course, and would probably only be noticed by U-boat scholars, but they have been corrected in the e-book edition, and I may also consider doing a new paperback edition at some point.

After the end of World War II, several navies continued development work on the Walter turbine system. In the end, all dropped the system as too dangerous. Most did adopt a number of Professor Walter's ideas in the area of hull design. The advent of nuclear propulsion in the 1950s rendered the further development of the Walter system unneccesary.

Curiously, in the last few years, there has been a renewed interest in air-independent propulsion for submarines. Modern developments, however, tend toward the use of Stirling Cycle (heat) engines and fuel cell technology. There was also a Russian AIP sub built during World War II, which used a conventional diesel engine that drew its oxygen supply from compressed gas bottles. Like the Walters, it was considerably hampered by capacity problems.