German U-Boat Mechanics: Engineering And Functionality

The German U-boat, or undersea boat, was a naval submarine that Germany operated during the First and Second World Wars. U-boats were known for their unrestricted submarine warfare, attempting to disrupt merchant traffic towards the UK and force them out of the war.

U-boats were the first submarines to be used in war as substitutes for surface commerce raiders. They were highly effective, with the ability to travel 5,000 miles without refuelling, and could operate along the entire British coast. They were also highly destructive, sinking 8,000 merchant vessels and warships and killing tens of thousands.

U-boats were unique in their ability to submerge for brief periods, giving them an advantage when attacking or as a defensive measure. They were also highly manoeuvrable, with the ability to adjust their buoyancy to sink or float as needed.

U-boat buoyancy control

U-boats could adjust their buoyancy by filling or emptying their ballast tanks with water or compressed air. When the ballast tanks were empty, the U-boat had positive buoyancy and floated to the surface. When the ballast tanks were filled with water, the U-boat had negative buoyancy and sank. To surface again, compressed air was blown into the ballast tanks to displace the water. This allowed U-boats to control their buoyancy and reach neutral buoyancy, where they weighed the same as the water surrounding them, allowing them to stay at a specific depth.

U-boat buoyancy had to be constantly monitored and adjusted as it was affected by various factors such as fuel levels, ammunition usage, ocean depth, and water salinity. For example, when a torpedo was launched, it created a ton of positive buoyancy, so ballast water had to be immediately added to prevent the U-boat from rising to the surface. U-boat commanders preferred to maintain neutral buoyancy, as it allowed them to dive or surface rapidly without relying heavily on limited compressed air.

The process of diving and surfacing a U-boat was intricate and involved multiple steps. The bridge watch prepared the bridge, and the Petty Officer of the Watch went to the forward hydroplanes while the Officer of the Watch closed the conning tower hatch. The engine room personnel shut down the diesel engines, and the electric motor room personnel started the electric motors. The desired down angle was achieved, and the vent valve for the ballast tank was opened. The planes were then brought to a neutral position, and the quick diving tank was blown to release excess air. Once the desired depth was reached, the forward dive planes were adjusted to level the boat.

U-boats could also use snorkels, special pipes that allowed them to run their diesel engines while submerged at periscope depth, drawing outside air to operate the engines. This extended their range and allowed them to recharge their batteries.

U-boat propulsion

U-boats are propelled by diesel engines or electric motors. The former is the main propulsion system, with two diesel engines, one on each side of the U-boat. These engines consume outside air and were originally only operable on the surface. However, the development of the snorkel, a retractable pipe that supplies air to the diesel engines, meant that U-boats could run their diesel engines while submerged at periscope depth.

At greater depths, U-boats are propelled by electric motors using electricity drawn from giant internal batteries. When these batteries are depleted, they must be recharged by travelling on the surface or while snorkelling at periscope depth. The diesel engines are used to turn the electric motors so that they act as dynamos (generators) to recharge the batteries.

U-boat weaponry

U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. The term is an anglicised version of the German word "U-Boot", a shortening of "Unterseeboot" (under-sea boat). U-boats were most known for their unrestricted submarine warfare in both world wars, attempting to disrupt merchant traffic towards the UK and force the UK out of the war.

U-Boat Armament

U-boats were armed with torpedoes, mines, and deck guns. The main armament of the U-boat was the torpedo, with some models featuring rear torpedo tubes to allow for forward and backward targeting. Torpedoes could be launched from the surface or at periscope depth. Later in the war, acoustic homing torpedoes were developed, which could steer themselves towards the propeller noise of a target ship. U-boats could also lay mines from their torpedo tubes.

In addition to torpedoes, U-boats were equipped with deck guns. The German U-boats of Types I, VII, IX, and X had a very powerful secondary weapon in the form of a deck gun. Each boat had one in front of the conning tower, and with a good crew, they could fire 15-18 rounds per minute. Deck guns were often used to finish off damaged vessels or sink smaller ships. The gun typically had a crew of 3 to 5 and was commanded by the second watch officer.

The Type VII U-boats were equipped with the 8.8 cm gun, which fired a 12-14 kg round, with 250 rounds usually on board. The larger I, IX, and X U-boats accommodated the more powerful 10.5 cm/45 gun, with 110 rounds of ammunition typically carried, although some variants had 150 or 240 rounds.

At the beginning of World War II, the standard U-boat anti-aircraft (AA) weapon was a single 2 cm gun, known as the "Flak" from "Flugabwehrkanone". From 1941, the 3.7 cm gun SK C/30U became standard on Type IX boats. The conning tower was rebuilt several times to improve AA defence, eventually featuring two 2 cm C/38 guns on the bridge and an additional platform ("Wintergarten") with another two 2 cm guns.

U-Boat Propulsion

U-boats were powered by diesel engines and electric motors. The diesel engines were used on the surface and could also drive air compressors to feed air tanks needed for venting the ballast tanks. The electric motors were driven by batteries, which could be recharged by the diesel engines or while surfaced by running the electric motors as generators.

U-Boat Tactics

U-boat tactics in World War II initially focused on convoy battles, with U-boats operating as wolfpacks to great effect. However, improved convoy tactics and anti-submarine measures, such as high-frequency direction finding and the Hedgehog anti-submarine system, began to take a toll on the German U-boat force.

U-boat communication

Radio systems were used by the German Navy in both world wars to communicate with their U-boats. These systems were also used for communication between U-boats. During World War II, very low-frequency radio systems operating in the 15-25 KHz waveband were used to communicate with submerged submarines. These systems could reach a submarine that was up to 30m underwater, depending on the distance from the transmitter. However, U-boats could only receive messages and not send them this way, as transmitting required a huge antenna that was impractical for a submarine.

More common were radios in the high-frequency band, from ~1-30 MHz. The Telefunken E-437S, which operated at frequencies between 1.5-25 MHz, was the ship-to-shore system used by WWII-era boats. During World War II, VHF systems were used for close-range communication between surfaced subs or for communication with surface ships. With the exception of the VHF system, these all used Morse code, while the VHF waveband allowed for voice transmission.

To prevent anyone from eavesdropping, transmissions were encoded. During World War I, book codes were used, while in World War II, the famous Enigma machine was used. Subs also carried signal lamps and flags for communication on the surface, but these were line-of-sight limited.

However, these radio systems did give the Allies a way to track and locate U-boats. Radio Direction Finding (RDF) was developed during World War I as a way to track radio signals from U-boats and the German surface fleet. During the interwar years, this developed into High-Frequency Direction Finding (HF/DF or Huff-Duff), which could more accurately home in on a signal. These systems used several directional antennas to triangulate a U-boat's signals. To avoid this, some attempts were made to shorten the duration of signals, as longer signals were much easier to detect and track.

U-boat construction

Design and Development:

U-boats underwent significant design improvements between World War I and World War II. By using new steel alloys and adopting welding instead of riveting, U-boats in World War II had stronger hulls and could dive deeper. The power of diesel engines was increased, resulting in greater surface speeds. The introduction of fuel saddle tanks extended their range. U-boats also utilised a technique called "economical cruising," where only one diesel engine was used to drive both propeller shafts, improving fuel efficiency.

Armament and Technology:

U-boats were primarily armed with torpedoes, which could be launched from forward and sometimes rear torpedo tubes. Acoustic homing torpedoes, which could automatically steer towards a target by following propeller noise, were developed during World War II. U-boats were also equipped with deck guns for surface attacks. Technological advancements included the use of long- and short-wave transmitters for communication, as well as radar warning receivers and radar systems to detect enemy vessels.

Construction Process:

Production Numbers:

During World War I, Germany had a total of 343 submarines in service, with 199 units lost during the conflict. In the interwar period, submarine construction was restricted by the Treaty of Versailles, but Germany secretly continued development. By the start of World War II, Germany had 56 U-boats commissioned, with 46 operational and only 22 capable of Atlantic operations. During the war, U-boat production increased significantly, with mass-production techniques employed for the Type XXI and Type XXIII U-boats.

Strategic Importance:

U-boats played a crucial role in Germany's naval strategy, particularly in the Battle of the Atlantic. They aimed to disrupt Allied shipping lanes and force the UK out of the war by targeting merchant vessels and convoys. While U-boats achieved significant successes, improvements in Allied convoy systems, anti-submarine tactics, and radar technology ultimately turned the tide against them.

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