Thursday, 15 January 2015

Under Water - FaQ on Submarines

Naval Forces Mag shows: Sketch, Wilhelm Bauer, Brandtaucher, Submarine
"Brandtaucher", sketch by Wilhelm Bauer

How is a submarine propelled? How does the fuel cell system work? How is the crew catered for? These questions and many more are answered here.

THE dream of mankind to be able to dive under water 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, 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 under water? MTU submarine expert Arndt von Drathen and Dr. Ute Arriens of ThyssenKrupp Marine Systems (TKMS) provide the explanations.

How is a submarine propelled?
In a submarine with diesel-electric propulsion, a diesel engine drives a charging generator. The generator set 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.

ThyssenKrupp Marine System Class 212 A with MTU
ThyssenKrupp Type 212A submarines
are equipped with Air Independent Propulsion (AIP) systems. 

Where does the engine get its air from and where does the exhaust go?
The submarine’s charging generator can only be operated when the vessel is on the surface or at periscope depth because the engine needs air for fuel combustion. At periscope depth, 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. So that the submarine can remain undetected, however, it should only remain at periscope depth for as short a time 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 boost pressure system developed by MTU.

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.

How fast can a submarine travel?
On average, submarines with diesel-electric propulsion can travel at 20 knots (37 kph) submerged and about half as fast on the surface.

How long can a submarine stay submerged?
Submarines with diesel-electric propulsion generally have to surface every couple of days to run the charging generator and recharge the batteries. However, with a special fuel cell system, subs can remain under water for longer. The present record – set by an HDW Type 212A submarine – is 14 days. If a submarine is unable to surface, the regulations require that the crew is able to survive for at least six days.
Naval Forces Mag knows how long submarines can stay submerged.
How long can submarines stay submerged?

How does the fuel cell system for submarines work?
ThyssenKrupp Marine Systems is the only supplier to offer an air-independent 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 like the process of electrolysis that you learn about in school – only in reverse. Besides the electric current generated, the only waste product is pure water.

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 very many other systems required for sustaining life and navigating under water. High power is also an important consideration, of course, so that the batteries can be charged as quickly as possible. And so that the sub has a long operating range, economical fuel consumption is equally essential. More recently, compliance with current exhaust emission specifications 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 life-sustaining, 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, shipbuilder and supplier can occupy a number of years before the shipyard can start the actual process of building the boat, which likewise takes several years. The preparations for the development of the MTU Series 4000 submarine unit took two years and the development process proper up to delivery to the client will require five years.

How is a submarine built?
As is the general rule in shipbuilding, a submarine is built in sections. As the work progresses, the shipyard workers weld the sections together to build up the complete vessel. It is fascinating to see how all the bulkheads, decks and, in particular, the piping and ducting in the individual sections fit together so precisely when they are brought together. Because it operates under water, a submarine only has very few and very small openings to the outside. Therefore, anything that will not fit through the hatches of a sub – from the engine to the proverbial kitchen sink – has to be installed in the individual sections before they are assembled. This particular situation also has to be taken into account for the servicing and repair of all equipment installed on board. All spare parts have to be designed to fit through the hatches. So for MTU it is all the more important to supply an especially reliable and low-maintenance charging unit.

What is everyday life in a submarine like?
A submarine is in operation 24 hours a day, seven days a week. It is workplace, home and leisure space all at the same time too. In a sub, a lot of people are living together in a very confined space breathing an artificial atmosphere. For weeks on end they see no daylight and there is no day/night routine. Instead, the time is divided into four-hourly sections. Four hours on watch followed by four hours for eating, sleeping, showering or leisure. If ever there are problems, however, the crew members are immediately called back on duty even in their off-watch periods.

How is the crew accommodated?
Until a few years ago, crew members often had to share berths. Today, however, each crew member generally has his/her own bunk. It is not much wider than a person of average build and does not even offer enough overhead room to stretch your arms out. This means that more bunks can be fitted above one another and only a curtain provides the sleepers with a certain amount of screening from the general goings-on in the submarine. But because of the shift-working system in a submarine, there is never any peace and quiet. Only the captain has his/her own cabin so as to be able to work undisturbed on confidential documents.

How is the crew catered for?
At the beginning of a voyage, every inch of available space in the submarine is used to store food. The galley in a sub is very small. It is just about big enough for the ship’s cook to turn a pirouette. And providing a continual supply of meals for the other crew members means that the cook also works around the clock.

What might future submarines look like?
One of the next technological leaps for submarine applications will undoubtedly be the changeover in battery technology to lithium-based batteries. Compared with the lead-acid batteries generally used today, the maintenance work required by a lithium battery is minimal. What is more, a lithium battery can store many times more energy and be fully charged at any time. For submarines operating below the surface for long periods, that will substantially reduce their visibility in conventional diesel-electric mode. MTU Friedrichshafen has already taken this possibility into account and designed its new charging unit to work equally effectively with lead-acid or lithium batteries.
Copyright: MTU-Report, posted by Stefan Nitschke; Images: Thyssen Group Marine.