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Prior to World War I, Krupp’s shipyard in Kiel, Germany, turned a maritime novelty into a deadly naval weapon.

World War I saw the first widespread use of four game-changing weapons: aircraft, tanks, machine guns and submarines. Of the four, subs were by far the most advanced system for their time and were remarkably similar to the boats with which the United States and Germany, particularly, entered the next world war. Had early 1940s airplanes been as similar to their 1918 counterparts, American pilots would have gone to Guadalcanal with fabric-covered open-cockpit biplanes rather than F4F Wildcats.

Like airplanes and moon rockets, submarines had been imagined, postulated and fictionalized for centuries by inventors, dreamers and spacey writers, though rarely did anyone build an actual sub. The few that did make it into the water were basically boats that sank (occasionally to resurface) and then stumbled about blindly at minor depths.

In 1897 Irish-American John Philip Holland—on his sixth try at building a submersible boat—took sub technology in a new direction. He launched in New York a cigar-shaped, 75-ton craft with a gasoline engine that ran both the boat and a generator on the surface and an electric motor that propelled the vessel while submerged, powered by batteries the engine had charged. The vessel was also fitted with diving planes, like an airplane’s elevators, allowing it to dive and surface just as a fish would. At the time, Holland’s main competitor, American naval architect Simon Lake, insisted subs should sink and then bob back to the surface in a level attitude, which never caught on.

By the time Germany’s Krupp Germaniawerft shipyard laid the keel of Forelle, the first true unterseeboot (“undersea boat”), or U-boat, in 1903, all the basic features of what would become the classic World War I submarine had been developed and tried by someone, somewhere, sometime. Electric motors for underwater propulsion; storage batteries and engine-driven generators to charge them; double hulls in which a pressure vessel was enclosed by a light, external streamlining shape; ballast tanks; dive planes; pneumatically expelled torpedoes—none was a secret.

Yet the Germans got it right from the start. U-1, commissioned in 1906, was a fully functional, no-excuses submarine. It was not a visionary leap by the German Imperial Navy, the Kaiserliche Marine, but the amalgamation of a half-century of innovation, the fortunate application of advanced German internal-combustion technology and the use of the Krupp shipyard in Kiel, which had been busily building submarines for the Russians. Ironically, the Germans based U-1 on an earlier design by French maritime engineer Maxime Laubeuf. Krupp later funded and built the experimental Forelle for the Imperial Russian Navy and sold three follow-on Laubeuf-type subs to Russia. Finally, in 1904 the Kaiserliche Marine, which had been buying small foreign subs of limited utility, ordered U-1.

The internal-combustion technology destined to be Germany’s major contribution to submarine development— and which set the precedent for submersibles until the advent of nuclear power plants in 1954—was the diesel engine. A major improvement over gasoline engines of the era, the diesel power plant provided excellent fuel efficiency and, therefore, greater range and eliminated the considerable danger (and sickening odor) of volatile gasoline fumes in an enclosed space shared with electric motors and batteries. The twinscrew U-1 was powered on the surface by two 200-hp Körting kerosene engines and while submerged by two battery-powered 100-hp electric motors.

Like nearly all submarines that would follow it, U-1 was double-hulled. The strong inner hull held pressure when the vessel submerged, while the thin outer hull held fuel and ballast tanks—the latter of which were flooded to submerge the boat and blown empty to surface—and provided a hydrodynamic shape for efficiency. An ancillary advantage was that engineers were able to place all of the structure needed to strengthen the inner pressure hull on the outside of the vessel rather than consume interior space.The outer hull also shielded the pressure hull from at least some depthcharge damage. A disadvantage, however, was that semiexternal fuel tanks often sprang leaks during depth-charge attacks, creating slicks that could betray a sub’s position.

U-1 had a single bow torpedo tube and carried three rounds—one “up the spout,” or in the tube, and two reloads. The torpedo was, of course, the development that made U-boats so effective. Essentially miniature, unmanned submarines, torpedoes are such fearsome weapons because they’re large enough to encapsulate an enormous amount of explosive energy yet compact enough to be carried by a relatively small and simple vehicle able to attack from well within the range of even the largest naval rifle.

The sub had a crew of 22 and could dive to a depth of almost 100 feet. Maximum surface speed was just short of 11 knots, with a surfaced range of 1,500 nautical miles at 10 knots. The boat’s submerged speed was said to be 8.7 knots, with a range of 50 nautical miles at 5 knots. And U-1 looked like a proper U-boat: Its small, streamlined conning tower sported a classic periscope, and an aggressive “beak” on the barrel-like hull cowled the torpedo tube’s muzzle. Nor did it hurt that Germany’s superior optics ensured the nation’s subs were fitted with the best periscopes the world would see for many decades.

Despite such innovations, U-1 never went to war. Submarine technology moved rapidly enough in the early 1900s that the pioneering vessel was obsolete by 1914 and was used solely for training. Rather than being the prototype of a class of identical boats, U-1 was the first of a rapidly improving series of submersibles soon equipped with proper diesel engines. One of U-1’s direct descendants, U-9, sank three British cruisers in less than an hour in September 1914 and in a couple of weeks killed more British sailors than Lord Nelson lost in all his battles.

In an era when nuclear subs dive as soon as they clear their homeport breakwater and don’t resurface for months, it’s sometimes forgotten that from World War I until the 1950s, underwater warships were not precisely submarines but submersibles— surface boats that dived briefly now and then but spent 99 percent of their time at the surface. Until 1944 and the first operational use of the snorkel—an intake/ exhaust tube that extended to the surface and allowed diesel propulsion and battery-charging while underwater—U-boats submerged only to attack or hide, and then not very deep.

So surface speed and seakeeping were a U-boat’s assets. After all, many of the vessels had to sail vast distances just to reach their patrol areas—not something you’d want to do at 5 knots. A World War I U-boat was basically a torpedo boat hull with a conning tower instead of a superstructure, atop and around the pressure vessel suspended below it. The Germans learned that if you did build a streamlined cigar with underwater speed in mind, your crew would be beat into insensibility on the surface by fierce pitching and rolling. U-boats’ large conning towers bristled with railings, ladders, jackstaffs and coamings. And their big deck guns, typically 86mm to 105mm, had high-drag teak decks and rigging for ease of surface operation.

Battery power didn’t last long running underwater, so U-boat captains had to calculate when and where to dive and best intersect their target within torpedo range. That’s why the simple tactic of zigzagging—frequent, irregular course alterations—was so effective for surface ships: A single zig or zag could nullify a U-boat commander’s careful calculations. When your top speed underwater is 5 or 6 knots, there’s no way to chase or maneuver to develop a new firing solution and sink a 10-knot freighter, much less a warship doing 20.

Thus, U-boats did their most fearsome work on the surface. Hollywood once had us believe a U-boat’s deck guns were for fending off furious destroyer attacks after a crippled sub was forced to surface, but the truth is that powerful deck ordnance was a far more effective and less expensive way to dispose of merchant ships, which were the World War I U-boat’s primary target. No sub commander would waste a torpedo—of which he had only a limited number—on a trudging collier or rusty banana boat.

One of the most remarkable World War I U-boats was Deutschland, named rather than U-numbered, initially, because it was an unarmed commercial vessel, crewed by the Norddeutscher Lloyd steamship line. It was the world’s first submarine freighter, an enormous, beamy, 1,875-ton (submerged) miniZeppelin. In 1916 Deutschland made two trips to the United States, first to Baltimore and then to New London, Conn., carrying hard-to-get chemicals and aniline dyes to the States and returning to Germany with a variety of strategic war materiel, mainly nickel, tin and rubber.

Deutschland was a blockade-runner. It cruised on the surface at 16 knots for most of the trip but submerged to pass unnoticed beneath the British-enforced North Atlantic blockade, which was otherwise quite successful at starving Germany of food and supplies. It also eluded a Royal Navy cordon off the U.S. East Coast. After Deutschland’s second cargo voyage, Germany converted the sub into U-155, sending it and six sister boats to war as long-range merchant raiders armed with particularly large-caliber deck guns.

U-155 wasn’t the only U-boat to call at an American port. U-53 popped into Newport, R.I., in fall 1916. For a few hours on October 7, its captain, Hans Rose, paid visits to U.S. naval officers and entertained visitors aboard U-53. Early the next morning, Rose sailed U-53 just out the harbor and quickly sank five Allied ships—three British, one Norwegian and one Dutch. All were in international waters, albeit within sight of the lightship Nantucket.

A special class of U-boats—UCs— were built as coastal minelayers, perhaps better termed minepoppers, since they popped the mines into place while submerged rather than laying them from the surface. While effective (U-boat mines sank nearly 3 million tons of shipping), they were deathtraps for their crews: Most were lost either to premature detonation of their own mines or from hitting other German mines in the fields they were servicing.

World War I U-boats were state of the art in every way but one: Naval experts who assessed the Kaiserliche Marine’s surrendered boats found that comfort, space and accommodations—”habitability,” they called it—were better on U.S. Navy subs of the era. But the fact remains, those comfy American boats sank not a single merchant ship during World War I. U-boats, on the other hand, sank 5,708 ships totaling about 11 million tons—roughly a quarter of all the world’s merchant shipping at the time (admittedly at a cost to the U-boat fleet of nearly half the 370-plus subs Germany built during World War I). U-35 alone accounted for 224 ships totaling more than 500,000 tons—most sunk by its deck gun. It averaged nine kills per patrol, making U-35 the highest-scoring submarine of either world war.

The source of the Kaiserliche Marine’s spectacular submarine successes was Germany’s unilateral decision early in 1915 to wage unrestricted submarine warfare. Until that time, various conventions and accepted principles of maritime combat ruled that a warship could only attack a noncombatant ship once the warship commander had irrefutably determined the merchant was carrying supplies to an enemy, and furthermore, that the commander must make provisions for the safety of its civilian crew. In fact, naval experts widely assumed early on that submarines would never be effective weapons against shipping, as they lacked the room to carry either prize crews (to sail a seized ship to a friendly port) or prisoners (if they sank one).

But Britain was starving Germany with its surface blockade, and the Kaiserliche Marine felt it had no recourse other than to return the favor— not by blockade, since Germany’s surface navy wasn’t powerful enough to do that, but by sinking the ships supplying Britain with food and war materiel.

As the U-boats couldn’t risk surfacing while carrying out the niceties of classic navy-versus-merchant confrontations—particularly since the introduction of heavily armed Royal Navy Q-ships, which masqueraded as tramps and trawlers specifically to lure enemy subs to the surface—Germany simply warned the world it would sink any ship (innocent, neutral or combatant) that entered a war zone.

The British were shocked. “Blockade and death by slow starvation are hallowed by use and wont,” one British naval commander oddly rationalized. “Speedy death by drowning is not.” The British despised submarines, one admiral famously calling them “underhand, unfair and damned unEnglish.” Said another Royal Navy admiral in 1914, “No country in this world would ever use such a vicious and petty form of warfare!”

Given a little more time, Germany’s U-boat plague conceivably could have forced the British to negotiate a peace. Instead, it helped force the United States into the conflict, sealing the Second Reich’s doom.

A popular misconception is that U-20’s May 7, 1915, sinking of the British liner RMS Lusitania with nearly 1,200 civilian casualties, including more than 120 Americans, was the major reason President Woodrow Wilson sent the American Expeditionary Force to Europe. But what actually pushed Wilson over the edge was the infamous January 1917 “Zimmerman Telegram,” intercepted and decoded by the British and gleefully shared with Washington.

German Foreign Minister Arthur Zimmerman had cabled his ambassador in Mexico City, alerting him that the unrestricted U-boat campaign was about to escalate, that it would subdue the British within six months as long as America stayed neutral, and that if America were to respond, the ambassador should seek an alliance: Mexico was to attack the United States, thus distracting it from the European war, and in return Germany would help Mexico reclaim Arizona, New Mexico and Texas. Zimmerman’s grasp of foreign relations apparently extended no farther than his bedside nightstand. As military historian Thomas Parrish writes in his excellent book The Submarine, Zimmerman didn’t seem to understand that “reintegrating Texas into Mexico would be as impossible as restoring the virginity of the Kaiserin.”

What finally beat the U-boats was the reinvention of the convoy system, whose history stretched back to the Spanish treasure fleets. At first glance, it might seem convoys would be self-defeating. After all, why collect all of a U-boat’s targets in one convenient flotilla? The truth is that spotting one 30-ship convoy in the immensity of the Atlantic was vastly more difficult than finding any one of 30 separate vessels scattered between North America and the British Isles. Convoys also presented a U-boat with target overload. A commander could get off only so many torpedo shots before the convoy was out of range or, more likely, its escorts put an end to further attacks. Convoy losses accounted for no more than 1 percent of all convoyed vessels, against 10 percent losses for ships sailing solo. Germany had lost the gamble it could starve England into submission before the Yanks arrived.

In 1919 postwar America began to crank out its first serious submersibles—the S-class, or S-boats, the first U.S. subs designed by the Navy rather than civilian inventors and contractors. The S-boats weren’t much good at first, but things began to look up when the Navy acquired several U-boats to reverse-engineer. Said the chief of the Navy’s Bureau of Engineering in 1927, “We find in general that departures from German practice [of U-boat construction] get us into trouble, and that trouble can generally be cured by strict adherence to German practice.”

In 1928, when the Navy began building its next sub class, it used as its prototype U-135, a boat already a decade old. The United States entered World War II with the resultant Gatoclass “fleet boats.” Imagine the U.S. Air Force capturing a 1939 Messerschmitt Bf 109 and in 1949 using it as the prototype of its next-generation fighter. That’s how advanced the Germans were in the undersea world.


For further reading, Stephan Wilkinson recommends: The Submarine: A History, by Thomas Parrish, and U-boats of the Kaiser’s Navy, by Gordon Williamson.

Originally published in the September 2010 issue of Military History. To subscribe, click here