[1] Even before the Second World War the rocket designer Hellmuth Walter had been advocating the use of hydrogen peroxide (known as perhydrol) as a fuel.
In these cases the hydrogen peroxide was reduced chemically and the resulting gases used to spin a turbine at about 20,000 rpm, which was then geared to a propeller.
The battery capacity was increased significantly along with fuel stores, and the boat was designed to operate underwater for long periods.
After the war, tests carried out by the United States Navy on two captured Type XXIs showed they could outrun some ASW ships by going in the direction of heavy seas.
Another version suggests that the U-boat did make contact with HMS Norfolk of the Royal Navy, but did not fire as the captain had already received orders of surrender.
A number of boats were commissioned into Allied navies after the war for research purposes, and one into the Bundesmarine of post-war Germany.