Type 205 coastal defense submarine

The German Type 205 was a class of compact diesel electric attack submarines was a continuance of the evolution from Type 201 developed by Howaldtswerke-Deutsche-Werft AG (HDW). The Type 201 U-Boats were the first post-war new construction, with development starting in 1955. During the construction of the 12 Type 201 U-Boats, additional military requirements resulted in a new Type 205, starting with the 4th boat of the building program, construction of which was already underway.

These submarines were used by both the German and Danish navies between 1967 and 2005. These single-hull vessels were optimized for use in the shallow Baltic Sea. Each boat was 144 feet long and displaced just over 500 tonnes when submerged. They carried a crew of just 22 and were armed with torpedoes and mines.

Any vessel made from magnetic materials has a magnetic signature. It decreases as the cube root of distance, and does not propagate like a wave. There are significant natural and man-made noise sources that can obscure the magnetic signature of a submarine, but magnetic detectors remain good sensors for close-in detection. There was early interest in the use of non-magnetic steels, used in the German Type 205 and 206 submarines, but non-magnetic steel has not been popular in more recent submarines for unknown reasons.

The hulls of the early Type 205 boats suffered from stress corrosion cracking, a problem which received much negative publicity at the time. An urgent program was undertaken to develop a new steel to overcame these problems. The new high-strength, non-magnetic, austenitic steel proved very satisfactory in service, thoroughly overcoming the doubts that were raised by those early problems.

Austenitic steel is a non-magnetic stainless steel that has enough nickel and chromium or manganese to retain austenite at room temperatures. Austenite is a solid solution of ferric carbide or carbon in iron. The presence of chrome and nickel throughout the heating and cooling process causes austenitic steel to take on certain identifiable properties which makes it more adaptable for some projects, with greater elasticity and good dynamic strength. Austenitic steel is extremely durable where exposure to the elements is concerned.

Thirteen boats were completed, including two for the Royal Danish Navy [Hdms Narhvalen and Hdms Nordkaperen]. The Danish boats were built in accordance with the German specifications and almost identical to the German type 205, but modified to meet special Danish demands. The two Danish submarines were built using magnetic steel due to the fact, that the anti magnetic steel used in the type 205 subs had shown not to satisfactory. The Type 207 Kobben Class is a version of the German Type 205 submarine, built in Germany, and customized for use by the Royal Norwegian Navy.

The Type 205A was an improved version that was distinguishable from the Type 205 by virtue of its larger conning tower and hull-mounted sonar. The Kobben class of submarines is based on Germany's Type 205. They served with the Royal Norwegian Navy from 1964 to 2001. The Type 206 class of submarines were based on the Type 205 class but featured a number of improvements, the most significant of which was the use of an improved non-magnetic steel for the construction of the hull.

By the end of the Cold War the effective combat strength of submarine forces was 18 type 206 submarines and 6 type 205 submarines. Construction of submarines of the new type 211 was postponed for the moment. There were plans for beginning overhaul and modernization of the first six type 206 submarines, which were designated 206A. Older submarines (type 205) that had been in the inventory since the late 1960s were gradually phased out of the fleet's effectives at the start of the 1990s. All six Type 205 submarines were replaced in the mid-1990s with the Type 212.

From 1987 to 1988, U11 was converted to a twin-hulled boat, which meant it got a second outer hull, which would protect it against practice torpedo hits in the area of the pressure body. After this conversion, it was possible to use U11 on national and international maneuvers as target. She was the only submarine target for submarines, surface ships and aircraft within NATO.

Modem conventionally powered submarines are generally powered by diesel-electric propulsion systems. Diesel engines are used on the surface or with a snorkel to charge batteries which are used to power electric motors for propulsion underwater. While underwater, oxygen is stored in high pressure tanks, and released slowly to sustain the crew. Carbon monoxide, and other undesirable chemicals, are scrubbed from the air with chemicals if air cannot be exchanged via the snorkel. Such submarines are very quiet on battery operation, but they must operate on or near the surface frequently in order to recharge batteries and replenish their breathing air supply, rendering them vulnerable to detection by acoustic (engine sounds) and non-acoustic (visual, radar, etc.) means. The indiscretion ratio is defined as the ratio of the time spent charging the submarine batteries (at the surface), to the time elapsed for the complete discharge/charge cycle of the batteries. In a typical diesel-electric submarine, the indiscretion ratio is about 10%, and increases rapidly with increasing speed.

Air Independent Propulsion (AIP) involves carrying supplies of fuel and oxidant which are reacted in an energy conversion device to ultimately produce electrical energy. This is used primarily for propulsion and to power all electrical equipment. Since this can be carried out underwater, it offers the potential to reduce the indiscretion ratio and to enhance submerged performance by extending the underwater range and endurance of the submarine at low patrol speeds (typically 4 to 5 knots).

An add-on section or "plug", containing the entire AIP system, involves cutting the body of the submarine in two, and welding in the insert section, which would typically be about 10 to 15 m long, for a hydrogen/oxygen fuel cell system with 100 MWh of stored reactants. At these endurances, the mass and volume of reactants primarily determine the size of the plug, with the size of the AIP energy converter and subsystems being of secondary importance. For the same fuel (eg diesel) and endurance, the higher oxygen requirement of heat engines compared to fuel cells would necessitate larger oxygen storage requirements, and therefore a longer plug. Nevertheless, different methods of hydrogen storage or generation can have a substantial effect on the resultant plug length, somewhat diminishing the oxygen advantage of fuel cells over other AIP systems.

Such an operation was successfully carried out with the German Type 205 submarine U1 on two occasions. In 1987 fuel cell AIP plant was installed, and underwent nine months of successful sea trials. Following decommissioning in 1992, closed cycle diesel AIP plant was installed prior to two months successful sea trials in early 1993.

The nine month sea trial in 1988-89 clearly demonstrated that the fuel cell propulsion system is modular and flexible with a high reliability, conferring operational and tactical advantages on submarine operation. Siemens developed a solid polymer electrolyte fuel cell system which promised better performance than that used in the U1.