The usual designation of German submarines, derived from “Unterseeboot.” Most U-boats were actually modified torpedo boats with some submerged capacity. There were several models or “Types” that had origins in World War I. Type II, IIb, and IIc were all small, prewar U-boats displacing 250–290 tons when surfaced. They had crews of 25 men but carried just six torpedoes. Additionally handicapped by a short, 18-day cruising range and lacking any deck gun, they were not suitable for ocean cruising and were mainly confined to wartime duty hugging the coast and prowling sea lanes of the North Sea. Types VII and VIIb displaced 626 and 753 surfaced tons respectively. Each had a crew of 44–50, a lot of men crammed into just 200 feet of boat that was only 20 feet wide. Type VII boats carried 11 torpedoes while the Type VIIb mounted 14. Four Type VIIs were fitted out as long-range attack/resupply boats capable of carrying 39 torpedoes each. Two were sent into distant Asian waters to hunt, taking advantage of the relative absence of Allied warships. Type IX boats were the mainstay of the German submarine fleet, though Type VIIs were its real workhorses. Over 250 feet long and hosting a crew of 48, the Type IX was larger but less manoeuvrable than the Type VII. A Type IX displaced 1,053 tons when surfaced and 1,153 tons submerged. It carried 22 torpedoes standard complement, fi red from four forward and two aft tubes. Deck armament varied, but usually comprised 105 mm and 37 mm guns. Type IXs had a surfaced range of 8,100 miles and a cruising speed of 18 knots. They could remain submerged for 65 miles at a constant 7.7 knots. Four prewar, experimental “U-cruisers” were designed at 750 tons each for crews of 110 men. They had a theoretical range of nearly 24,000 miles and carried 24 torpedoes. Commissioned in January 1939, none of these Type XI boats were completed. An early version of “stealth” technology was developed to conceal the conning towers of U-boats from enemy radar.
Various specialized U-boats were built or converted by Kriegsmarine yards, including minelayer boats sent to seed enemy and neutral waters. Several types of ad hoc supply boats and U-tankers were floated in late 1940. Some were captured Dutch or other enemy submarines. Other supply and fuel boats were put to sea after converting older German designs to new purposes. The Germans also commissioned purpose-built supply boats. These Type XIV boats were popularly called “Milchkühe” (“Milk Cows”). They lacked deck guns or torpedo tubes and so had no offensive capability. A Type XIV could carry four torpedoes for delivery to a hunter, but more often it used that space to transport extra fuel. They also supplied fresh food and a few small luxuries. Contact with on-station or patrolling hunter U-boats was made by radio. That exposed supply boats and hunters at the rendevous: intercepts of waypoint instructions and the great success of ULTRA code breakers permitted Western Allied navies and aircraft to specifically target high-value Milchkühe, along with the attack boats they exposed during resupply operations. From late 1942 even attack U-boats began to dispense with deck guns in favor of additional anti-aircraft guns to fend off an ever increasing threat from enemy aircraft. Over time, accumulation of anti-aircraft guns and armor on the bridge adversely affected submerged performance and stability while luring U-boat captains into the usually fatal error of trying to shoot it out with a diving and strafing enemy aircraft. In 1943 the U-boat arm experimented with seven anti-aircraft “flakboats” bristling with guns and ordered to fight back in a group defense. These instructions only worsened U-boat losses, as enemy aircraft group-attacked in response. The tactic of engaging aircraft was abandoned in preference for a return to rapid combat dives when any aircraft was spotted.
An all-out building program by German and captured shipyards was initiated immediately upon the change of Kriegsmarine command from Admiral Erich Raeder to Admiral Karl Dönitz in January 1943. Adolf Hitler was also persuaded by Dönitz to commission new classes of U-boats, including 10 experimental and highly costly “Walter boats,” hydrogen peroxide propulsion submarines named for engineer-designer Hellmuth Walter. None saw action and all were scuttled on Dönitz’s orders in May 1945. The main production effort aimed to launch 30–40 Type IXs per month until that older model could be replaced. The successors were two new “Elektroboote” The first, the Type XXI, was a long-range cruiser capable of more speed while submerged (12 knots) than surfaced (6 knots), with battle “sprint” or escape capability of 18 knots under water for a maximum of 90 minutes. Type XXIs were 50 percent larger than standard Type IXs, displacing 1,800 tons submerged. Most critically, they were true submarines that could stay submerged for extended periods using Schnorchel equipment. The second Elektroboote model was the Type XXIII. A coastal craft, it was not ocean-capable. It displaced just 260 tons and was intended to attack shipping no farther out than the British Isles or Mediterranean. In July 1943, Hitler ordered construction of 140 Type XXIs and 238 Type XXIIIs. From September, all new construction was confined to Elektroboote models. Just 61 Type XXIIIs were finished by the end of the war; only four made war patrols. Only 120 Type XXIs were built and just a single boat put to sea. It headed for Norway on April 30, 1945, the day Hitler killed himself. It never met or sank an Allied ship.
Dönitz was full of plans right to the end, envisioning new U-boat technologies that would not become available before 1947, and boasting to Hitler as late as March 1945 that his U-boats were now ready to relaunch and win the naval war. Yet, he rigidly discouraged experimentation and testing of new designs before and during the war. None of his plans, not even the new Elektroboote and Schnorkel technology whose development he helped delay, made any difference to the war at sea. The new boats and technologies did not prevent the OVERLORD invasion fleet or huge follow-on supply convoys from reaching Normandy, nor did they block the DRAGOON landings in southern France on August 15, 1944. Instead, dedication of high-grade German steel and scarce skilled labor to building fleets of U-boats that never saw action proved a significant drain on German tank, artillery, and anti-aircraft tube production. A total of 1,170 U-boats were commissioned during the war. The fleet peaked at 460 U-boats, but the last 200 were completed too late to train crews and none saw active service. A total of 739 U-boats were sunk by the enemy or otherwise lost at sea from 1939 to 1945; over 30,000 submariners were lost with them. By 1944 and 1945 life expectancy was reduced to the maiden war patrol of a new crewman. The U-boat arm was also deeply radicalized and nazified, as the “grey sharks” were employed by Dönitz and Hitler not to win the war at sea but to delay defeat on land by interfering with the Western Allied buildup to invasion and resupply thereafter
With Germany’s surrender imminent, Dönitz resisted Japanese blandishments to send his surviving U-boats into the Pacific. In any case, he did not have fuel to do so. Instead, he ordered Operation REGENBOGEN : the scuttling of the entire surviving U-boat fleet; 218 of his captains obeyed. As happened in 1918, the Royal Navy sought immediate destruction of any surviving U-boats. Of the boats that remained in German dry docks or whose captains surrendered rather than obey the Dönitz’s Götterdämmerung-like order, 30 were divided among the major Allied navies following the surrender. The rest were destroyed at sea, mainly by the British, from November 1945 through January 1946. The last commander of the U-boat arm, Admiral Hans von Friedeburg, committed suicide in late May 1945. Dönitz chose not to go down with his defeated fleet. He was arrested, tried, and convicted of war crimes by the Nuremberg Tribunal.