The term U-boat is derived from the system used by the Kriegsmarine (German Navy) to name its submarines: U- followed by a number, where the U stood for Unterseeboot (literally, undersea boat), the German word for submarine. The main area of use for the U-boat was the Battle of the Atlantic, and its main objective was targeting the merchant convoys which were bringing supplies from the United States to Europe. During the early stages of the war, the U-boats were highly successful in this objective, and destroyed a great amount of Allied shipping. This intended effect of destructive power was prophesied by Kaiser Wilhelm II, issuing orders to some of the first U-Boat commanders during World War I, in February of 1917:
We will frighten the British flag off the face of the waters
and starve the British people until they, who have
refused peace, will kneel and plead for it.
The German admiral of the U-boats, Karl Doenitz, who helped design the system of cutting off Allied systemthe system of economic waralso advocated the U-boat technique known as the wolfpack, in which teams of U-boats would gang up on convoys and overpower the accompanying defense warships.
After the fall of France, the Kriegsmarine had immediate access to the Atlantic Ocean and the English Channel, which meant even more pressure for the RAF and RN. In addition, large fortified concrete ports to cover the U-boats were built, which could resist any successful bombing (this lasted through the entirety of the war). Most of the German fleet was moved to these protective bases; such strong air cover was another difficulty for the British. The structure, technology, and brutally-inventive engineering behind the U-boats were perhaps their biggest asset. Throughout the course of the war, the Kriegsmarine were constantly improving upon the design, producing many different types of U-boats as technology evolved. Landmarks in the development were the Type II, Type VII, and the Type IX U-boats.
The Type II was the second iteration of Germanys second-generation U-boats (they were stripped of all their U-boats after WWI, but in the 1920s and 30s rebuilt their fleet). It was designed as a coastal submarine, and its defining characteristic was its tiny sizeit was referred to as the Einbaum (dugout canoe) and had huge advantages of maneuverability over larger boats, as well as the ability to roam shallow waters and dive more quickly. They were effective within their intended role, but had limited cargo space and could only carry a couple torpedoes; they were a first step towards re-armament for the Germans, and provided the Kriegsmarine with an example to train and lay the foundation for larger boats to build upon.
Type II U-boat
Displacement: 279 to 329 tons (submerged)
Length: 140 ft; Beam: 13 ft; Crew: 25
Speed: 13 knots (surface), 7 knots (submerged)
Much more powerful than the Type II, the Type VII were more agile on the surface, carried larger crews, could hold 3 more torpedoes and had a more advanced mounted deck-gun. The second-generation Type VIIB boats were built with better fuel capacity, which added significant amounts of range, and increased speed. Two rudders increased agility. Though the real workhorse for the German effort was the Type VIIC, a larger and heavier boat which increased armaments. Perhaps the famous Type VIIC was the U-96, which had a feature-role in the movie Das Boot.
Type VIIB U-boat
Displacement: 753 to 857 tons (submerged)
Length: 218 ft; Beam: 20.25 ft; Crew: 44
Speed: 17.25 knots (surface), 8 knots (submerged)
The culmination of high speed, innovation, and heavy armament was the Type XXI, also known as the Elektroboots. The XXI was the first submarine design to operate entirely submerged underwater, instead of a system of repeated surfacingsa much less awkward mode of operation. Among the advancements was a hydraulic torpedo reload system that could reload all six torpedo tubes faster than the Type VIIs could reload a single tube. Better batter design made underwater range and speed enormous.
This method of constant improvement and implementation repeatedly increased the difficulties that American merchant marine and the RAF and RN were forced to face, adding to the general sense in the early stages of the war that the U-boats of the Nazi Navy would bring Germany eventual victory over the Allied Powers.
Type XXI Elekroboot
Displacement: 1819 to 2100 tons (submerged)
Length: 251 ft, Beam: 27 ft; Crew: 60
Speed: 15.6 knots (surface), 17.2 knots (submerged)
It should not be overlooked, of course, that all of these U-boat successes were before certain circumstances turned the tide of the U-boats and the German hopes in the war indefinitelyfor example, the breaking of the German Enigma code. In the end, the U-boat fleet suffered heavy amounts of loss, with 743 U-boats sunk and approximately 30,000 submariner casualties. U-boats were considered by much of the naval world as a poor-mans weapon: seek-and-destroy hunters of merchant ships used only by cowards. To read about some of the other naval vehicles used by the Germans in World War II, link to Boats.
U-234 is torpedoed by the USS Sutton
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The U-boats were obviously the main element to the Axis Powers submarine force. However, the Pacific theater of the war was one in which the sea was significant. As explained in the Allied Submarines section, Japans role in this theater was certainly one of defeat. However, beginning with Pearl Harbor, submarines served a large role in the Japanese Navy. For example, in a quirky example, the midget submarine was an interesting vehicle used in the attack on Pearl Harboralbeit unsuccessful. At a mere 79 ft length and height of 6 ft, the midget submarine was hypothetically intended for agile search-and-destroy missions. Click on the image below to link to the interesting story of the Japanese midget Ha-19.
 Image sources: left, www.spartacus.schoolnet.co.uk/ FWWgermanN.htm; right, http://online.mepl.co.uk/cgi-bin/dwsrun?PWACQ.DWO&dl=9&page=26
 Image source: www.hq.wwiionline.com/ profiles/uboats.shtml
 Image source: http://www.hq.wwiionline.com/profiles/uboats.shtml