Key Point: The U-boat is back—and stealthy this time.
The German Navy was a pioneer in large-scale submarine warfare, its U-boats able to contest the United Kingdom’s superior navy in ways that German surface warships could not. While modern-day Germany no longer has the ocean-spanning naval ambitions of its predecessors, it has become a global leader in designing small, stealthy submarines that can effectively patrol littoral waters at a fraction of the cost of nuclear-powered submarines. The secret sauce in the new generation of German submarines is the use of hydrogen fuel cells for power, which allows submarines to operate nearly silently for weeks at a time without using expensive nuclear reactors.
During World War I and II, submarines were at their most vulnerable when their noisy, air-breathing diesel engines forced them surface to recharge batteries, exposing the boats to detection and attack. The Kriesgmarine built several experimental Type XVIIB submarines with an Air Independent Propulsion (AIP) system, using hydrogen peroxide fuel that theoretically enabled extended underwater endurance. In practice, the boats were considered dangerously unsafe and unreliable. Although the United Kingdom, Soviet Union and United States all experimented with AIP submarines after the war, development was abandoned in favor of higher-performing nuclear-powered submarines.
It was left to Sweden, in 1997, to deploy the first operational submarine using an AIP system, the stealthy Gotland-class boats that employed a heat-converting Stirling engine. German submarine developers were close on their heels with the Type 212 in 2002, which uses hydrogen fuel cells. Though more expensive and complicated to refuel compared to the Stirling, the German PEM hydrogen fuel cells benefit from greater power output (and thus higher speed), have no major moving parts that betray acoustic stealth, and do not impose limits on diving depth.