Toward diesel-electric power

World War I

By the eve of World War I all of the major navies included submarines in their fleets, but these craft were relatively small, were considered of questionable military value, and generally were intended for coastal operations. The most significant exception to the concept of coastal activity was the German Deutschland class of merchant U-boats, each 315 feet long with two large cargo compartments. These submarines could carry 700 tons of cargo at 12- to 13-knot speeds on the surface and at seven knots submerged. The Deutschland itself became the U-155 when fitted with torpedo tubes and deck guns, and, with seven similar submarines, it served in a combat role during the latter stages of the war. In comparison, the “standard” submarine of World War I measured slightly over 200 feet in length and displaced less than 1,000 tons on the surface.

The prewar submarines generally had been armed with self-propelled torpedoes for attacking enemy ships. During the war submarines also were fitted with deck guns. This permitted them to approach enemy merchant ships on the surface and signal them to stop for searching (an early war policy) and later to sink small or unarmed ships that did not warrant expenditure of torpedoes. Most war-built submarines had one and sometimes two guns of about three- or four-inch calibre; however, several later German submarines carried 150-millimetre guns (including the Deutschland class in military configuration).

An important armament variation was the submarine modified to lay mines during covert missions off an enemy’s harbours. The Germans constructed several specialized submarines with vertical mine tubes through their hulls; some U-boats carried 48 mines in addition to their torpedoes.

Also noteworthy was the development, during the war, of the concept of an antisubmarine submarine. British submarines sank 17 German U-boats during the conflict; the early submarine-versus-submarine successes led to British development of the R-class submarine intended specifically for this role. These were relatively small craft, 163 feet long and displacing 410 tons on the surface, with only one propeller (most contemporary submarines had two). Diesel engines could drive them at nine knots on the surface, but once submerged, large batteries permitted their electric motors to drive them underwater at the high speed of 15 knots for two hours. (Ten knots was a common speed for submerged submarines until after World War II.) Thus, they were both maneuverable and fast. Advanced underwater listening equipment (asdic, or sonar) was installed, and six forward torpedo tubes made them potent weapons. Although these submarines appeared too late to have any actual effect on the war, they pioneered a new concept in the development of the submarine.

All World War I-era submarines were propelled by diesels on the surface and by electric motors submerged, except for the British Swordfish and K class. These submarines, intended to operate as scouts for surface warships, required the high speeds then available only from steam turbines. The K-boats steamed at 23.5 knots on the surface, while electric motors gave them a 10-knot submerged speed.

World War II

Interest in submarines continued high within the world’s navies during the period between World Wars I and II. Britain, France, and Japan built improved types, and during this period the U.S. Navy built its first large long-range submarine, the Argonaut. Completed in 1928, it was 381 feet long, displaced 2,710 tons on the surface, was armed with two six-inch guns and four forward torpedo tubes, and could carry 60 mines. The Argonaut, the largest nonnuclear submarine ever built by the U.S. Navy, led to the highly successful Gato and Balao classes of U.S. submarines used in World War II.

During the 1930s the rejuvenated Soviet shipyards began producing large numbers of submarines, primarily coastal craft, in an attempt to make the Soviet Union a sea power without major expenditures for surface warships. But though the Soviet program achieved quantity, their ships were unsuitable for operations against the German Navy, their crews were poorly trained, and Soviet bases were blocked by ice much of the time.

World War II saw extensive submarine campaigns on all of the world’s oceans. In the Atlantic the principal German U-boat was the VII type, a relatively small but effective craft when properly employed. The Type VIIC variant was 220.25 feet long, displaced 769 tons on the surface, and was powered by diesel-electric machinery at a speed of 17 knots on the surface and 7.5 knots submerged. Armament consisted of one 90-millimetre deck gun, various antiaircraft guns, and five torpedo tubes, four forward and one aft. Either 14 torpedoes or 14 tube-launched mines were carried. Manned by a crew of 44, these submarines had a surface endurance of 6,500 miles at 12 knots, but, when they were submerged, their batteries would remain active a little less than a day at four knots.

The ultimate diesel-electric submarine evolved in the war was the German Type XXI, a 250-foot, 1,600-ton craft that could attain 17 1/2 knots submerged for more than an hour, could travel at six knots underwater for two days, or could “creep” at slower speeds for four days. These submarines were fitted with snorkel devices (see below), which made it unnecessary for them to surface fully to recharge their batteries after operating submerged. The Type XXI had an operating depth of 850 feet, more than twice what was then normal, and was armed with four 33-millimetre guns and six forward torpedo tubes (23 torpedoes carried). These properties made all earlier submarines obsolete. Existing Allied antisubmarine forces would have had serious trouble coping with these craft had the war continued past the spring of 1945.

A final German war design of particular interest was the Walter turbine propulsion plant. The need for oxygen for combustion had previously prevented the use of steam turbines or diesels while the submarine was submerged and air was at a premium. Hellmuth Walter, a German scientist, developed a turbine propulsion system using oxygen generated by hydrogen peroxide to operate the turbine while submerged. A simplified submarine, the V-80, built in 1940 and propelled by a Walter turbine system, could attain speeds of more than 26 knots submerged for a short period of time. After many delays, the first Walter-propelled Type XVII combat submarines were completed and could reach 25 knots underwater for brief periods, and a submerged run of 20 knots for 5 1/2 hours was achieved on trials. But these submarines, like the Type XXI, were not ready for full-scale operations when the war ended.

A notable German submarine development of World War II was the schnorchel device (anglicized by the U.S. Navy to “snorkel”). Its invention is credited to a Dutch officer, Lieutenant Jan J. Wichers, who in 1933 advanced the idea of a breathing tube to supply fresh air to a submarine’s diesel engines while it was running submerged. The Netherlands Navy began using snorkels in 1936, and some fell into German hands in 1940. With the advent of radar to detect surfaced submarines, the Germans fitted hundreds of U-boats with snorkels to permit the operation of diesels at periscope depth (to recharge batteries for underwater propulsion) with less of a possibility of detection by Allied radar-equipped ships and aircraft.

In the Pacific war the Japanese employed a large number of submarines of various sizes and types, including aircraft-carrying submarines, midget submarines, and “human torpedoes” carried by larger submarines. The Japanese I-201 class was a high-speed submarine, of 259 feet and 1,291 tons displacement, that had diesel propulsion for 15 knots on the surface; while underwater, large batteries and electric motors could drive the vessel at a speed of 19 knots for almost one hour. Each boat had two 25-millimetre guns and four forward torpedo tubes and carried ten torpedoes.

The highly successful U.S. submarine campaign in the Pacific war was waged mainly with the Gato- and Balao-class submarines. These were approximately 311.5 feet long, displaced 1,525 tons, and had diesel-electric machinery for 20-knot surface and nine-knot underwater speeds. The principal difference between the two designs was the 300-foot operating depth for the Gato class and 400-foot depth for the Balao boats. Manned by 65 to 70, these submarines had one or two five-inch deck guns plus smaller antiaircraft weapons and 10 torpedo tubes (six forward, four aft) and carried 24 torpedoes.

Postwar developments

After the war the Allies were quick to adopt advanced German submarine technology. The British built two peroxide turbine-propelled experimental submarines, but this concept lost favour because of the unstable properties of hydrogen peroxide and because of American success with nuclear propulsion. The Soviet Union began building modifications of the Type XXI submarine. Some 265 of these submarines, labeled Whiskey and Zulu class by NATO observers, were completed between 1950 and 1958, more submarines than built by all of the world’s other navies combined between 1945 and 1970. (In that period Soviet shipyards produced a total of 560 new submarines.)

The U.S. Navy studied German technology and converted 52 war-built submarines to the Guppy configuration (an acronym for “greater underwater propulsive power,” with the “y” added for phonetics). These submarines had their deck guns removed and streamlined conning towers fitted; larger batteries and a snorkel were installed; four torpedoes and, in some craft, one of the four diesel engines were removed. The result was an underwater speed of 15 knots and increased underwater endurance.

Although the major powers switched to nuclear power after World War II, the great bulk of the world’s navies have continued to buy—or in a few cases, build—submarines descended directly from the fast diesel-electric U-boats of the war. (Indeed, many of them were designed and built in West Germany.) Postwar diesel-electric submarines continue to be equipped with snorkels, but hunters have adopted improved radars that can detect even the small head of the snorkel, just as aircraft with more primitive radars could detect surfaced U-boats during World War II.

The main advances have been in weapons and sensors. Deck guns have been abandoned, in some cases for antiship missiles. Torpedoes, which can exceed 50 knots, either home onto their targets acoustically with self-contained sonar or are guided by electronic commands passed to them through a threadlike wire paid out behind the speeding projectile. In addition, many submarines are equipped with cruise missiles or antiship missiles for striking targets on land or on the sea surface. Submarine sonars, for detecting both surface ships and other submarines, have been enormously improved, and on the most advanced submarines the familiar periscope is being replaced by so-called photonic masts, or optronic masts. These are sensor systems that, like the periscope, project upward to the surface from the submarine’s sail; however, unlike the periscope, they relay optical, infrared, and radiowave information to the control room electronically, without the need for any hardware to pierce the submarine’s hull. The masts are directed by a simple joystick in the control room, and the data can be displayed on screens located anywhere on the submarine.

Maximum submerged speed, meanwhile, has increased only somewhat (to more than 20 knots) over the German Type XXI, and endurance at top speed is no greater than at the end of World War II. Improvements in the design of conventional lead-acid batteries have somewhat increased endurance at low speed. Many modern submarines, for example, can remain submerged (at about three knots) for as long as a week to 10 days. This is an important improvement, because during so long a period sea conditions can easily arise that would allow a submarine to escape or force submarine hunters on the surface to disperse, but the development of "air-independent propulsion" (AIP) using fuel cells has brought even greater improvement. Some AIP-capable submarines, equipped with fuel cells that use stored hydrogen and oxygen to generate electricity, are said to be able to operate at low speeds underwater for as long as a month.

For these reasons, diesel-electric submarines are still furtive but effective platforms, operating very quietly and conserving their energy for the postattack escape. Because their electric motors are quieter than nuclear units (and can even be shut off for a time), they are sometimes proposed as antisubmarine ambushers that would silently await their prey in areas through which enemy submarines are known to pass. AIP offers the possibility of other roles, such as operating in polar seas for long periods under ice, tracking coastal shipping in antiterrorist operations, or inserting special operations forces onto foreign shores. Modern diesel-electric submarines, AIP-capable or not, are thus affordable weapons platforms for many navies around the world that wish to defend their own coastal areas against all potential enemies, even nuclear powers.