On April 7th, the first submarine propelled by a fuel cell made its maiden voyage. The Type 212 sub carries supplies of hydrogen and oxygen, which are combined in nine PEM (polymer electrolyte membrane) fuel cells, each of which produces up to 50 KW of electricity, and water as a waste product. This type of power plant generates no heat or noise and is probably the most difficult to detect sub in the world. There is also a diesel engine for surface cruising, and six torpedo tubes. The 1500 ton boat, the German U-31, can travel silently underwater for 15-20 days. Currently, only nuclear subs can stay under that long. Crew size for the Type 212 is 27. Max underwater speed is 36 kilometers per hour, and 21 surfaced. Germany is building four boats of this class. Greece and Korea have placed orders. While cheaper than nuclear propulsion, the U-31 is still expensive, costing about $330 million.