How did U-boats of World War 1 work?

Question

How were they able to submerge?

Answer 1

U-boats in WWI submerged themselves the same way as modern submarines. Sea water is allowed into a big tank in the hull, and pushes the air in the tank into a compression flask. To surface again, the engines or batteries have to turn a pump which repressurises the air in the flask to expand into the tank and push the water out again.

Answer 2

U-boat is the anglicization of the German word U-Boot (help·info), itself an abbreviation of Unterseeboot (undersea boat). The primary targets of the U-boat campaigns in both world wars were the merchant convoys bringing supplies from Canada and the United States to Europe. Austrian submarines of World War I were also known as "U-boats".
The distinction between U-boat and submarine is common in English-language usage (where "U-Boat" is used to refer exclusively to the German naval submarines of the First and Second World Wars) but unknown in German, in which the term U-Boot refers to any submarine
German submarine U 9 (1910).At the start of World War I, Germany had twenty-nine U-boats; in the first ten weeks, five British cruisers had been lost to them. On 5 September 1914, HMS Pathfinder became the first warship to be sunk by a torpedo attack from the U-21. Later on in September, U 9 sank the obsolete British warships Aboukir, Cressy and Hogue (the "Live Bait Squadron").

For the first few months of the war, U-boat anti-commerce actions observed the current "prize rules" which governed the treatment of enemy civilian ships and their occupants. Surface commerce raiders were proving to be ineffective and on 4 February 1915, the Kaiser assented to the declaration of a war zone in the waters around the British Isles. This was cited as a retaliation for British minefields and shipping blockades. Under the instructions given to U-boat captains, they could sink merchant ships, potentially neutral ones, without prior warning. A statement by the U.S. Government, holding Germany "strictly accountable" for any loss of American lives, made no material difference.

On May 7, 1915, U 20 sank the liner RMS Lusitania with a single torpedo hit [1]. The sinking claimed 1,198 lives, 128 of them American civilians, including noted theatrical producer Charles Frohman and Alfred Vanderbilt, a member of the prestigious Vanderbilt family. The sinking shocked the Allies and their sympathisers because a unarmed civilian merchant vessel was attacked without any warning. According to the actual manifest, the Lusitania was carrying a small amount of non-explosive military cargo[2], making it a legitimate target under international law. However, this event turned American public opinion against Germany and was a factor in the United States' entry into the war nearly two years later.

The initial U.S. response was to threaten to sever diplomatic relations, which persuaded the Germans to re-impose restrictions on U-boat activity. The U.S. reiterated its objections to German submarine warfare whenever U.S. civilians died as a result of German attacks, which prompted the Germans to fully re-apply prize rules. This, however, removed the effectiveness of the U-boat fleet and the Germans consequently sought a decisive surface action, a strategy which culminated in the Battle of Jutland.

Although the Germans claimed victory at Jutland, the British Grand Fleet remained in control at sea. It was necessary to return to effective anti-commerce warfare by U-boats. Vice-Admiral Reinhard Scheer, Commander in Chief of the High Seas Fleet, pressed for all-out U-boat war, convinced that a high rate of shipping losses would force Britain to seek an early peace before the United States could react effectively.

The renewed German campaign was effective, sinking 1.4 million tons of shipping between October 1916 and January 1917. Despite this, the political situation demanded even greater pressure and Germany announced, on 31 January 1917, that its U-boats would engage in unrestricted submarine warfare from February 1. On 17 March 1917, German submarines sank three American merchant vessels (see First Battle of the Atlantic) and the U.S. declared war in April 1917.

In the end, the German strategy failed to destroy Allied shipping before U.S. manpower and materiel could be brought to bear in France. An armistice became effective on 11 November 1918.
At the end of World War I, as part of the Paris Peace Conference, 1919, the Treaty of Versailles restricted the total tonnage of the German fleet. The treaty also restricted the independent tonnage of ships and forbade the construction of submarines. Before the start of World War II, Germany started rebuilding U-boats and training crews, hiding these activities as "research" or other covers, so that when World War II started, Germany already had a few U-Boats ready for war.
During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which lasted the duration of the war. Germany had the largest submarine fleet in World War II, due to the Treaty of Versailles which limited the surface navy of Germany to six battleships (of less than 10,000 tonnes each), six cruisers and 12 destroyers. Although Winston Churchill, the United Kingdom's Prime Minister wrote "The only thing that really frightened me during the war was the U-Boat peril", evidence later accumulated showed that 98% of convoyed British ships in the first 28 months of the war crossed the Atlantic safely, and at no time was the U-boat force close to a successful blockade of the United Kingdom.
U-boat Pens in St Nazaire, FranceIn the early stages of the war, the U-boats were extremely effective in destroying Allied shipping, ranging from the Atlantic coast of the United States and Canada to the Gulf of Mexico, and from the Arctic to the West and Southern African coasts and even as far east as Penang. Because speed and range were severely limited underwater while running on battery power, U-boats were required to spend most of their time surfaced running on diesel engines, diving only when attacked or for rare daytime torpedo strikes. The most common U-boat attack during the early years of the war was conducted on the surface and at night. This period, before the Allied forces developed truly effective antisubmarine warfare (ASW) tactics, was referred to by German submariners as the "happy time."
The U-boat was essentially a sophisticated launch platform for its main weapon, the torpedo. World War II torpedoes suffered from many limitations: they could only be launched from periscope depth or while surfaced; until later on in the war they were completely unguided, travelling in a straight line until they either collided with an object or ran out of energy. Torpedoes were fitted with one of two types of fuses, one which detonated the warhead upon impact with a solid object and another which detonated magnetically, upon sensing a large metal object nearby. Ideally, when using magnetic torpedoes, the commander would know the target ship's draft (how low its keel sits in the water) and set the torpedo's depth so that it passed just underneath, detonating via the magnetic fuse. The explosion would create a temporary gas bubble underneath the suddenly unsupported keel, which would split as a result. In this way, even large or heavily-armored ships could be sunk or disabled with a single well-placed hit. In practice, however, both the depth-keeping equipment and magnetic fuses were notoriously unreliable in the early part of the war. Torpedoes would all too often run at an improper depth, detonate prematurely or even bounce harmlessly off the target's hull. Many commanders lost all faith in their torpedoes and the problems took some time to be fully rectified.
In the later years, Germany developed an acoustic homing torpedo. These were primarily designed to combat pursuing warships, which normally took the strategy of making erratic changes in speed and direction in order to make a hit with straight-running torpedoes nearly impossible. The acoustic torpedo was designed to run straight to an arming distance of 400 meters and then zero in on the loudest noise it heard. This sometimes turned out to be the U-boat itself, and at least two submarines were probably sunk by their own torpedoes. Additionally, it was found that the torpedoes were only effective against ships moving at at least 15 knots. U-boats also adopted "pattern-running" torpedoes which ran to a preset distance, then traveled in either a circular or ladder-like pattern. When fired at a convoy, this increased the probability of a hit in case the weapon missed its primary target.