Answers to questions about the U-31

Since 05-15-05

Bulletin 24

www.submarineresearch.com

Subj: Bulletin 24, Answers to questions about the U-31

Note: Submarine Research Center has a new and improved website, www.submarineresearch.com . We hope that you will check it out and participate in one or more of its research projects.

Note: A helpful submariner brought to my attention the fact that when dolphins are earned they never expire. Thus, the term "ex-submariner" doesn't make sense in terms of submarine veterans. His point is well taken and although he suggested the term, "retired submariner", the Center will henceforth use the term "submariner" to apply to all men who served or are serving in submarines.

Fuel Cell

Q: How does a fuel cell work?

A: A fuel cell is a chemical process resulting in a flow of electricity. They come in a variety of forms using a variety of chemicals including carbon monoxide, air, oxygen, hydrogen, phosphoric acid and molten carbonate. Most are designed to use air as the source of oxygen, but those used in submarines use oxygen in its liquid form stored in tanks. Conversion of chemical energy into electricity takes place silently, without combustion, thereby lending itself to stealth.

The U-31 uses a polymer electrolyte membrane which contains a solid polymer electrolyte. On one side of the PEM (anode)hydrogen is decomposed into its electrons and protons. The electrons are by-passed to the submarine's power supply in the form of electrical power, while the protons cross through the membrane. The electrons return to the cathode side of the membrane to reform with the protons to form water molecules. Water is the only by-product and it is ejected from the submarine at a temperature only slightly above ambient temperature.(photo 1,attachment page 1)

Oxygen is carried in two large external, insulated tanks in the upper superstructure. Hydrogen in crystalline form is contained in up to twenty eight tanks in the bottom of the boat.(photo 3, attachment page 1)

Q: What is a closed cycle engine? Is this the hydrogen peroxide engine used by the Germans in the Second World War?

A: In the early 1930's the German engineer, Helmuth Walter worked in Germaniawerft (Kiel). He designed an AIP engine using high-purity hydrogen peroxide as an oxidant. His engine used hydrogen peroxide from an onboard supply where it was decomposed using a permanganate catalyst into free oxygen and steam. The oxygen and steam were mixed with diesel fuel and combusted to drive a turbine which drove an electrical generator. At the end of the Second World War Germany installed scaled-down Walter turbines into seven Type XVII boats. None saw combat.

After the war Walter went to England where he worked at Vickers on improved AIP engines. Two boats were produced in the 1950's, HMS Explorer and HMS Excalibur both powered with advanced Walter turbines. While the boats achieved high underwater speeds the hydrogen peroxide was too volatile for operational use. The boats were known as HMS Exploder and HMS Excruciator.

Meanwhile, Sweden pioneered research in modern closed cycle engines. A reciprocating (diesel) engine greatly modified for closed cycle was used with oxygen (from LOX tanks) and mixed with argon to combust in cylinders producing rotational power. The exhaust gas was spray cooled and partially absorbed back into the intake side. Exhaust was cooled and discharged to sea. These engines have not experienced continued interest in view of more valued AIP systems.

The American submarine X-1 used a rudimentary AIP system in which oxygen for its small diesel engine was derived from Walter hydrogen peroxide decomposition injected into cylinders. In 1957 the hydrogen peroxide exploded blowing off the front of the submarine. Although no one was injured the AIP concept was abandoned.

Q: What is a Stirling Engine?

A: A Stirling Engine is an external combustion engine designed and built by Kockums of Sweden. It is a heat engine that, in general terms, is more or less independent of the type of fuel used. As long as a temperature difference can be created the engine will generate rotational power. It uses pure oxygen and a low-sulfur diesel fuel to react and produce heat.

Heat from an outside source is transferred to an enclosed quantity of working fluid or gas which is driven through a repeated sequence of thermodynamic expansion. The expanding gas pushes a piston (one or more) and is then drawn off by suction to a cooling chamber for subsequent recompression and reheating. (Photo 2, attachment page 1)

The Stirling concept is as complicated as the machinery and while three Swedish submarines have Stirlings installed in them they are considered by most European engineers to be inferior to the fuel cell as an AIP energy source.

Q: What is a skewback propeller?

A: The dictionary describes the word "skew" as follows: "to take a slanting or oblique course or direction, to swerve or twist." The skewback propeller (screw) is multi-bladed and bowl-shaped. That is, its outer diameter bends rearwards in reference to the submarine's axis. To better understand the skewback screw a look at screw design is helpful.

American submarines used four bladed double screws through the Second World War in order to accommodate stern torpedo tubes. The Albacore showed that a single, large screw provided better energy efficiency. Resistance to the single screw came from BuShips and Admiral Rickover. In 1955 the Admiral accepted the design benefits of a single screw and the Skipjack was fitted with a single seven bladed screw. She was launched in 1958, the first single screw submarine other than the Albacore since the B class submarine.

As we know, the object of screw design is to deliver as much thrust as possible per given unit of torque. However, this is modified by the concern to reduce cavitation noise emission as much as possible. The most effective propeller design attempts to accommodate the two contradictory concerns. The slower the propeller moves through the water the less is the cavitation. Thus, more blades moving at a slower RPM will maximize thrust and minimize cavitation. Since the outer diameter of each blade moves the fastest, cavitation is generated at these points. If the tips of the blades follow the leading edges (sweepback), cavitation is reduced. The screw is in a single, lateral plane perpendicular to the shipÕs axis and the trailing tips are swept back. (Photo 1, attachment page 2)

The U-31 screw takes this design one step further. It bends the blades rearward on a longitudinal axis giving the screw a bowl shaped appearance. This two-plane warping of the blades reduces cavitation to the minimum while not sacrificing thrust. The term "skewback" refers to the rearward bending of the propeller blades along the longitudinal plane.(Photo 2, attachment page 2)

Information on advanced U.S. screw design is classified. We invite comments from submariners who can share with us their knowledge without revealing classified information.

Q: How many crew members are there on the U-31 and what are their stations?

A: The boat's crew is comprised of nineteen enlisted and eight officers. They are berthed forward of the control room-attack center. To the rear of the control room-attack center is the machinery space. This space is not manned. The crew operates the boat from the three forward non-watertight compartments.

Since the Siemens motor is a slow-speed, infinite power setting motor the boat is "flown" through the water by manipulating controls similar to an airplane. The throttle delivers required turns in response to speed demands. The machinery spaces are automated using computers. Controls for engines, main power electrical switching and auxiliary machinery are all in the control room.

Enlisted rates are similar to those familiar to us with emphasis on electronics and computer science.

A Siemens 120 KW fuel cell module

The external combustion apparatus for the Sterling engine.

The fuel cell system and tank arrangement in U-31.

Los Angeles-class nuclear submarine showing its sweptback propeller

U-31 propeller showing sweepback in the lateral plane and skewback in the axial plane