Underwater

Man's dream of diving underwater for longer than the human body is capable of unassisted is at least as old as the dream of flying. Swimming like a fish below the surface of the sea, enclosed in a capsule, is something people attempted in ancient times and in the Middle Ages. In 1492, the Italian Roberto Valturio drew a sketch of a submarine that in its external shape very closely resembles modern-day submersibles. In 1851, the German inventor Wilhelm Bauer staged a trial of his ‘incendiary diver’. It was the first modern submarine and was built according to the inventor’s plans (left) at the Schweffel & Howaldt engineering works and iron foundry in Kiel. mtu started making engines for submarines in 1959, and they can now be found in service in all the world’s oceans. But how does a submarine move? How do you build one, and how does a diesel engine function underwater? mtu submarine expert Arndt von Drathen and Dr Ute Arriens from ThyssenKrupp Marine Systems provide the explanations.

Where does the engine get its air from and where does the exhaust go?

The submarine’s charging unit can only be operated when the vessel is on the surface or in snorkeling mode because the engine needs air for fuel combustion. In snorkeling mode, the sub is just a few meters below the surface and the air for combustion is drawn in through the snorkel, from where it passes to the engine in the engine room. If the submarine is to remain undetected, however, snorkel conditions can only be maintained for as short a period as possible. The exhaust is discharged under pressure below the surface. For every meter of water above the exhaust outlet, the engine has to generate an extra 100 mbar of exhaust pressure so that the water cannot run into the engine. That is only possible with a special charge air system developed by mtu.

How is a submarine propelled?

In a submarine with diesel-electric propulsion, a diesel engine drives a battery charging unit. The diesel genset acts as a battery charger and charges the batteries with electric current. That electric current powers the electric propulsion motor, which in turn drives the propeller. As well as submarines with diesel engines and fuel cells there are also atomic subs with a nuclear reactor for generating the power.

How does the fuel cell system for submarines work?

ThyssenKrupp Marine Systems is the only supplier to offer an airindependent fuel cell propulsion system and has successfully commissioned numerous installations. For this type of supplementary energy generator, the sub requires a supply of liquid oxygen and hydrogen on board. The two fuel components are fed into the fuel cell, which converts them into electricity. It is very much like the process of electrolysis as taught in schools – only in reverse. Besides the electric current generated, the only waste product is pure water.

How long can a submarine stay submerged?

Submarines with diesel-electric propulsion generally have to surface every couple of days to run the charging unit and recharge the batteries. However, with a special fuel cell system, subs can remain underwater for longer. The present record – set by an HDW Class 212A submarine – is 14 days. If a submarine is unable to surface, regulations require that the crew must be able to survive for at least six days.

How does the snorkel work in heavy seas?

In heavy seas a cap briefly closes off the snorkel to prevent water running through it into the submarine. In that short period, the volume of air inside the sub serves as a temporary reserve for supplying air to the engine.

What special requirements does a submarine diesel engine have to meet?

For operating in a submarine, an engine should at least be very quiet so that the sub is less audible. It should also be as small as possible to leave room for the many other systems required for sustaining life and navigating underwater. High power output is also an important consideration, of course, so that the batteries can be charged as quickly as possible. To ensure that the sub has a long operating range, economical fuel consumption is equally essential. More recently, compliance with current emissions standards has become an added requirement for submarine diesels.

How long does it take to build a submarine?

In contrast with surface vessels, submarines move in three dimensions in space, and as distinct from aircraft, they have to be able to maintain a static position in the water even without propulsive power. As well as that technological challenge, there are a large number of lifesustaining, communication and navigation systems to be included in the construction process, and everything has to be brought together in the smallest of spaces. So every submarine design has to be very carefully conceived and planned. The coordination work between client, shipyard and supplier can last a number of years before the actual construction process gets underway, which in Itself takes several years. Preparations for development of the mtu Series 4000 submarine genset took two years and the development process proper will require five years before the vessel is actually delivered to the customer.